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Wiley Linux Administrator Street Smarts: A Real World Guide to Linux Certification Skills

1. Zs 83484 book Page 9 Monday September 18 2006 8 58 AM Task 1 1 Use Basic Command Line Features 9 you can move from section to section of the documentation for a program Type info info to learn more about this system Both man pages and info pages are usually written in a terse style They re intended as ref erence tools not tutorials they frequently assume basic familiarity with the command or at least with Linux generally For more tutorial information you must look elsewhere such as this book or the Web The Linux Documentation Project http t dp org is a particularly relevant Web based resource for learning about various Linux topics Obtaining Superuser Privileges To fully administer a Linux system you must sometimes use the superuser account which goes by the name root Although you can log directly into the root account it s generally best to instead log into a regular user account and then acquire superuser privileges One way to do this is to use the su command whose name stands for switch user You can actually acquire any user s identify in this way by typing the target username after the com mand as in su hyde to acquire hyde s privileges If you omit a username root is assumed so typing su alone is equivalent to typing su root In any of these cases you must know the target user s password so only users who know the root password may acquire superuser privileges via su Once you re using
2. Als 83484 book Page 17 Monday September 18 2006 8 58 AM Task 1 3 Manage Links 17 TABLE 1 3 Examples of Symbolic Permissions with chmod continued Command Initial Permissions End Permissions chmod o rwx bigprogram rwxrwxr X rwx rwx chmod g u report tex rw r r rw rw r chmod g w o rw report tex rw rw rw rw r As with many other file manipulation commands chmod accepts a R option to operate recursively on an entire directory tree Criteria for Completion To complete this task you should have a new directory home project7 which contains a number of files copied from etc The copied files should be owned by an ordinary user on your system fred in the examples Task 1 3 Manage Links Native Linux filesystems have always supported a feature known as links A link is a way to refer to a file in one location from another location or to use multiple names for a single file Linux supports two types of links which are described shortly They re created and managed with the 1n command In this task you ll learn how to create and manage links Scenario To make it easier for users to access the files in the directory you created in Task 1 2 you want to create links to some of the files it contains To do so you ll create both types of links and in the process you ll learn how to remove and manage links Scope of Task Links are not difficult to manage although the differences between the two type
3. Before delving into the details of commands used to manage find and manipulate files you must be able to use basic command line features Linux supports several shells which are pro grams that accept typed commands and display their output Some shell commands are built into the shell but many others are actually external programs Knowing how to use your shell s features will enable to you to be more productive at the Linux command line The most common Linux shell is the Bourne Again Shell bash and it is the one described here Other shells such as tcsh and zsh support similar features although some details differ Scenario A user is experiencing problems with the whatis program a standard Linux script that the user frequently runs As the administrator you must check that this script exists and that it s not obvi ously corrupt In the process you ll use several important Linux command line features e Als 83484 book Page 3 Monday September 18 2006 8 58 AM Task 1 1 Use Basic Command Line Features 3 gt Actually correcting a problem such as the one described here is likely to gr require the use of Linux s package management tools These are described in Phase 3 Managing Software Scope of Task This task is fairly straightforward It requires little time and effort You might want to con tinue experimenting with bash after you ve completed this task though Duration This tas
4. Procedure Vi is a bit strange particularly if you re used to GUI text editors To use Vi you should first understand the three modes in which it operates Once you understand those modes you can begin learning about the text editing procedures Vi implements You must also know how to save files and exit from Vi Most Linux distributions actually ship with a variant of Vi known as Vim or Vi TE Improved As the name implies Vim supports more features than the original Vi does The information presented here applies to both Vi and Vim Most dis tributions that ship with Vim enable you to launch it by typing vi as if it were the original Vi Vi Modes At any given moment Vi is running in one of three modes Command mode This mode accepts commands which are usually entered as single letters For instance 1 and a both enter insert mode although in somewhat different ways as described shortly and o opens a line below the current one Ex mode To manipulate files including saving your current file and running outside pro grams you use ex mode You enter ex mode from command mode by typing a colon typ ically directly followed by the name of the ex mode command you want to use After you run the ex mode command Vi returns automatically to command mode Insert mode You enter text in insert mode Most keystrokes result in text appearing on the screen One important exception is the Esc key which exits from insert mode back to
5. PN VW 83484 book Page 1 Monday September 18 2006 8 58 AM Working on the Command Line 83484 book Page 2 Monday September 18 2006 8 58 AM Linux s tools for running programs and manipulating files are simple unglamorous and powerful Although you can use GUI tools to drag and drop files real Linux street smarts requires you to be able to manage your files from the command line by using odd sounding commands such as 1s cp In and mv In the first few tasks of this phase you ll learn to use some of the most basic of these tools Don t think that the basic nature of these commands is unimportant though like the foundation of a house these commands support more dramatic parts of the structure Specific tasks described in this phase include using basic command line features managing files directories and links finding files and editing files Later tasks in this phase go further examining streams pipes redirection the shell environ ment and shell scripts You ll also learn about managing accounts Most of the tasks in this phase assume you have both a normal user account and root access but you must also be able to create delete and otherwise manage normal user accounts for yourself or other users of a Linux system This phase maps to portions of the CompTIA Linux objectives 2 and 3 and to TE portions of the LPIC objectives 103 104 109 and 111 Task 1 1 Use Basic Command Line Features
6. s default shell and the comment field Even an encrypted password could be broken if it were readable so the shadow password system locks this sensitive data as well as additional account information in etc shadow which is readable only by root The format of the etc shadow file is sim ilar to that of etc passwd in that it consists of colon delimited fields The first two fields contain the username and encrypted password while remaining fields contain account aging and other advanced information Account management involves creating deleting or modifying the information in etc passwd and etc shadow In principle this can be done with a text editor and in fact some extraordinarily street savvy administrators work this way Most administrators though use command line or GUI tools to help manage the task These tools obviate the need to remember what fields hold what data and minimize the risk of encountering problems from typos such as accidentally deleting a colon The basic Linux account management tools are useradd userdel and usermod These tools add delete and modify existing user accounts respectively Linux also provides groupadd groupdel and groupmod tools to perform similar tasks with groups although some of the details of operation differ Adding User Accounts To add an account you use the useradd command At its simplest you can use this command followed by the username you want to use useradd tbrown
7. This task will take about an hour to complete Writing real world scripts can take anywhere from a minute or so up to several hours or even days depending on the complexity of the script and your own proficiency at script writing Setup To perform this task you must log into your computer as an ordinary user One of the scripts presented in this task requires root access to run Yowll also need root access if you want to copy any of these scripts to a system directory but this isn t necessary to complete the task You may use a text mode or GUI login although the first script launches X programs so you ll need an X session to test it The scripts themselves are executed from the command line but you ll edit them in an editor which may be a purely text based editor such as Vi or nano or a GUI editor such as KEdit Caveats As programs scripts can contain bugs Although the scripts presented in this task are fairly simple and are likely to be harmless even with typos bugs in scripts can cause infinite loops in which the script never terminates excessive CPU use accidental file deletions and other problems These problems can be more serious if a buggy script is run as root or even if a bug free script is used inappropriately by root Thus unless a script absolutely requires root access you should always develop and test scripts as a non root user Procedure The first thing to understand about scripts is how to create the
8. amp Aside from the first line that identifies it as a script the script looks just like the commands you might type to accomplish the task manually except for one fact The script lists the com plete paths to each program You may need to modify the path to kmai1 it s not always stored in usr bin This is usually not strictly necessary but listing the complete path ensures that the script will find the programs even if the PATH environment variable changes Also each program launch line in Listing 1 2 ends in an ampersand amp This character tells the shell to go on to the next line without waiting for the first to finish If you omit the ampersands in Listing 1 2 the effect will be that the first xterm will open but the second won t open until the first is closed Likewise KMail won t start until the second xterm terminates Although launching several programs from one script can save time in startup scripts and some other situations scripts are also frequently used to run a series of programs that manip ulate data in some way Such scripts typically do not include the ampersands at the ends of the commands because one command must run after another or may even rely on output from the first A comprehensive list of such commands is impossible because you can run any program gt Zl Y 83484 book Page 47 Monday September 18 2006 8 58 AM Task 1 9 Write Basic Scripts 47 you can install in Linux as a command e
9. as shown in Figure 1 1 Your default shell should be running in this window If your computer displays a GUI login prompt but you d prefer to work in a purely text mode session press Ctrl Alt F1 This key sequence switches to the first virtual console which normally holds a text mode session You can switch between several usually six text mode consoles by pressing Alt F1 through Alt F6 for each of the first six virtual consoles The X Window System Linux s GUI environment normally runs in virtual console 7 so pressing Alt F7 from a text mode session switches to X if it s running FIGURE 1 1 An xterm window enables you to type text mode commands in a GUI session rodsmith halrloprillalar rodsmith halrloprillalar Zs 83484 book Page 5 Monday September 18 2006 8 58 AM Task 1 1 Use Basic Command Line Features 5 For this task you should log in as an ordinary user In fact it s good practice to always log in as an ordinary user You can subsequently acquire superuser privileges from your ordinary user account if you need such access described shortly in Obtaining Superuser Privileges Logging in as an ordinary user and then obtaining superuser privileges is better than logging in directly as root because this two step process leaves a trace in the system log file of who acquired root privileges On systems with multiple administrators this can make it easier to track down who caused problems
10. create a home directory while m forces the system to create one Which behavior is the default varies from one system to another Default group You set the name or GID of the user s default group with the g default group option The default for this value varies from one distribution to another Default shell Set the name of the user s default login shell with the s shel option On most systems this defaults to bin bash Specify a UID The u UID parameter creates an account with the specified user ID value UID This value must be a positive integer and it is normally above 500 for user accounts System accounts typically have numbers below 100 The o option allows the number to be reused so that two usernames are associated with a single UID No user group In some distributions such as Red Hat the system creates a group with the same name as the specified username The n parameter disables this behavior This list of options isn t complete consult useradd s man page for more options As an example of some of these options in action suppose you want to place Trevor Brown s real name in the comment field and set his home directory to home2 trevor You could do so at account creation time by including appropriate parameters useradd c Trevor Brown d home2 trevor tbrown After typing this command or the simpler version shown earlier be sure to check for the existence of the home directory If it s not presen
11. s shared 1 home 1 shared chown 1 users home 1 shared When you use Listing 1 3 you need type only three things the script name with the desired username and the password twice For instance if the script is called mkuser you might use it like this mkuser sjones Changing password for user sjones New password Retype new password passwd all authentication tokens updated successful ly Most of the scripts programs operate silently unless they encounter problems so the inter action including typing the passwords which don t echo to the screen is a result of just the passwd command In effect Listing 1 3 s script replaces seven lines of commands with one Every one of those lines uses the username so by using this script you also reduce the chance of an error Another type of variable is assigned within scripts themselves for instance they can be set from the output of a command These variables are also identified by leading dollar signs but they re typically given names that at least begin with a letter such as Addr or Name These variables are the same as variables you can set at the command line as described earlier in Task 1 8 but unless you use the export command they don t become environment variables a 4 0 Zl Y 83484 book Page 49 Monday September 18 2006 8 58 AM Task 1 9 Write Basic Scripts 49 For instance consider Listing 1 4 which displays the current IP address of the
12. the prompt or is omitted Zs 83484 book Page 4 Monday September 18 2006 8 58 AM 4 Phase 1 Working on the Command Line Procedure To complete this task you must log into your account on the affected workstation and enter several Linux commands In the process you ll learn how to type and edit commands how to use command completion and how to use command history You ll also learn a few of the more common Linux commands Logging In The first task is logging into the Linux system From the console there are two ways to log in using a graphical user interface GUI login manager or using a text mode console In either case you type your username and then your password typically in response to a login or username prompt and a password prompt respectively Upon a successful text mode login you ll be greeted by a Linux command prompt and you can begin issuing Linux commands GUI logins though present you with a graphical desktop environment probably the K Desktop Environment KDE or the GNU Network Object Model Environment GNOME Although GUI desktop environments are convenient for end users they don t usually present you with a text shell by default To access one you must locate an appropriate option from the menuing system Look for entries called Shell Terminal or xterm If all else fails look for an option to run a command by name and type xterm in the dialog box This should launch an xterm window
13. ting as in machine4 example com 0 0 This value is set automati cally when you log in but you may change it if necessary You can run multiple X sessions on one computer in which case each one gets a different DISPLAY number for instance 0 0 for the first ses sion and 1 0 for the second EDITOR Some programs launch the program pointed to by this environment variable when they need to call a text editor for you to use Thus changing this variable to your favorite editor can help you work in Linux It s best to set this variable to a text mode editor though GUI editors might cause problems if they re called from a program that was launched from a text mode login ING grams in the current directory can be run This practice poses a security risk though because a miscreant could create a program with the name of some other program such as 1s and trick another user into running it by simply leaving it in a directory the victim frequents Even the root user may be vic timized in this way For this reason it s best to omit the current directory from the PATH variable especially for the superuser If it s really needed for ordi nary users put it at the end of the path ke The PATH variable often includes the current directory indicator so that pro N a 4 0 Zl Y 83484 book Page 44 Monday September 18 2006 8 58 AM 44 Phase 1 Working on the Command Line To learn what environment variables are set
14. Pay attention to the spaces and lack thereof particularly around the equal sign adding spaces around the equal sign will cause this command to fail If you want a space between the prompt and the point at which you begin typing be sure to include one at the end of the new prompt string The scenario presented earlier though specified that you want to include the time and date in the command prompt To do this you must include special strings that serve as stand ins for other data Specifically the current date can be denoted by d and the time can be denoted by Thus you can set your prompt to include the date and time by issuing the following command PS1 d The result might resemble the following although of course the date and time shown in the prompt will change with the real date and time Thu Jun 08 11 44 AM Zl Y 83484 book Page 40 Monday September 18 2006 8 58 AM 40 Phase 1 Working on the Command Line In most distributions the system is configured with default prompts that include your user name the computer s hostname and the current directory but details differ You can learn what your shell prompt is by typing echo PS1 This command displays the contents of the PS1 variable Consult the man page for bash for more information on common substitution strings including those found in your distribution s default shell prompt Setting the PS1 variable as just described has one import
15. Task 1 2 Specifically you use the x option to add execute permissions probably in conjunction with a to add these permissions for all users For instance to make a file called my script executable you d issue the fol lowing command chmod atx my script You ll then be able to execute the script by typing its name possibly preceded by to tell Linux to search in the current directory for the script If the script is one you run regularly you may want to move it to a location on your path such as usr local bin When you do that you won t have to type the complete path or move to the script s directory to execute it you can just type my script Using Commands One of the most basic features of shell scripts is the ability to run commands You can use both shell internal commands and external commands Most of the commands you type in a shell prompt are in fact external commands they re programs located in bin usr bin and other directories on your path You can run such programs as well as internal com mands by including their names in the script You can also specify parameters to such pro grams in a script For instance suppose you want to start a script that launches two xterm windows and the KMail mail reader program Listing 1 2 presents a shell script that accom plishes this goal Listing 1 2 A Simple Script That Launches Three Programs bin bash usr bin xterm amp usr bin xterm amp usr bin kmail
16. com mand mode a 4 0 Zl Y 83484 book Page 25 Monday September 18 2006 8 58 AM Task 1 5 Edit Files 25 If you re not sure what mode Vi is in press the Esc key This will return you P to command mode from which you can re enter insert mode if necessary Unfortunately terminology surrounding Vi modes is inconsistent at best Command mode is sometimes referred to as normal mode and insert mode is sometimes called edit mode or entry mode for instance Ex mode is often not described as a mode at all but as colon commands Basic Text Editing Procedures In this task the 1i10 conf entry for your kernel has been accidentally deleted so you must re create this entry Listing 1 1 shows the original 1i1o conf file used in this example If you re using a lilo conf file from your computer or that you found on the Internet it isn t likely to be identical so you may need to adapt some of the details in the following procedure in minor ways Alternatively you can type Listing 1 1 using a text editor with which you re already familiar and save it to a file on your disk Listing 1 1 Sample etc 1ilo conf File boot dev sda map boot map instal 1l boot boot b prompt default 1inux timeout 50 jmage boot vml inuz Jabel 1inux root dev sda6 read only g Don t try editing your real etc lilo conf file as a learning exercise a mis NING take could render your system unbootable the next time you type lilo You
17. command remove your hands from the keyboard proofread the command and be sure it s the correct command before you press the Enter key to finalize it All of these commands can be used by ordinary users with the partial exception of chown only root may use chown to change the primary owner of a file although ordinary users may use chown to change the group of a file These examples show root using the commands because the task is an administrative one that requires root privileges because of the locations of the files Procedure To perform this task you must create a shared directory copy files to the new directory remove extraneous files and set the ownership and permissions on the new directory and the files you ve copied These actions utilize some of the most important Linux file manipulation commands such as mkdir cp rm chown and chmod Creating a Shared Directory To create the shared directory use the mkdir command which creates makes a directory hence the command s name This command takes the name of the directory you want to create as an argument For instance to create a directory called home project7 you d type this mkdir home project7 Thereafter the home project7 directory should exist By default this directory is owned by the user who issued the mkdir command and has permissions based on the defaults for that user You can tell mkdir to create a directory with specific permissions by adding
18. computer on which it runs This script uses the variable ip which is extracted from the output of ifconfig using grep and cut commands The trailing backslash on the second line of the script indicates that the following line is a continuation of the preceding line When assigning a value to a vari able from the output of a command that command should be enclosed in back quote characters which appear on the same key as the tilde on most keyboards These are not ordinary sin gle quotes which appear on the same key as the regular quote character on most keyboards Listing 1 4 Script Demonstrating Assignment and Use of Variables bin sh ip ifconfig ethO grep inet cut f 2 d cut f 1 d echo Your IP address is ip Listing 1 4 relies on the networking command ifconfig which is described in more detail in Phase 6 You can type ifconfig by itself to see what its output includes The second line of Listing 1 4 uses grep and cut to isolate the IP address from the rest of the ifconfig output Scripts like Listing 1 4 which obtain information from running one or more commands are useful in configuring features that rely on system specific information or information that varies with time You might use a similar approach to obtain the current hostname using the hostname command the current time using date the total time the computer s been run ning using uptime free disk space using df and so on
19. new editor Making Your Changes Permanent Chances are you don t want to type a series of commands to adjust your environment variables every time you log into your Linux system or launch a new xterm window You can automate the process by modifying bash configuration files Table 1 4 summarizes the locations and names of these files TABLE 1 4 Common bash Configuration Files Type of File Login File Location Non Login File Location Global etc profile and files in etc etc bashrc or etc bash bashrc profile d User bash_login profile or bashre bash_profile The bash configuration files can be either global they apply to all users or user they apply to individual users and either login files they apply to login sessions such as those initiated from a text mode login prompt or non login files they apply to non login sessions such as those started in xterm windows The precise names used for these files varies from one distri bution to another The most common user configuration files are bashrc and profile so look for those files To make your changes permanent you must locate existing lines that set the environment variables you want to modify or add new lines Whatever the name or location bash configuration files are actually shell scripts as described in more detail in Task 1 9 For now you can simply add or modify existing export commands Type the commands in the shell script just as you d type
20. pro grams that don t open disk files though a 4 0 Als 83484 book Page 10 Monday September 18 2006 8 58 AM 10 Phase 1 Working on the Command Line i Don t shut down or reboot Linux by pressing the power or reset buttons on the ING computer s case Like most modern OSs Linux requires a controlled shutdown to avoid damaging the data on its hard disk If you want to shut down the system use a shutdown option at a GUI login prompt or type shutdown h now as root Replace h with r if you want to reboot the computer rather than shut it down As a practical matter if the computer has completely frozen you may need to perform an uncontrolled shutdown but you should try to avoid this practice whenever possible Criteria for Completion You have completed this task when yov ve verified the presence of the whatis command in usr bin and checked to see that it s a shell script that is a program that consists of text mode commands that a shell executes In the process you ll learn about file examination commands command completion command history the Linux man system and acquiring superuser access If a user were really having problems with the whatis command though it might be missing or corrupt in which case you d be unable to find it in usr bin or it might contain gibberish rather than text mode commands Task 1 2 Manage Files and Directories Many Linux features are implemented via files and directories T
21. the root account though you can use su to acquire any user s privileges without a password This can be a helpful problem solving tool since you can locate problems that are user specific Try using su to acquire root privileges Depending upon your system configuration chances are your prompt will change For instance a Fedora Core system shows the following prompts sally halrloprillalar su Password root halrloprillalar sally Logging Out Once you re done you should log out This is a particularly important security measure if your computer is in a public place If you re using a login shell that is if you logged into the shell via a login prompt you can log out by typing logout at the command prompt If you re using an xterm window or have used su to acquire another user s privileges though you should type exit to log out In fact exit will also work with login shells If you logged into an X session using a GUI login tool you should log out from that session Most desktop environments provide an obvious way to do that via their main menuing sys tems Typically you ll see a power button icon or a menu option titled log out or exit Some environments give you the option of logging out rebooting the computer or shutting down the computer Be sure to save files and exit from programs that open files before logging out of an X session You needn t worry about shutting down xterm windows or other
22. them at a command prompt If you need to add a line rather than modify an existing one be careful to keep it out of command structures such as if statements Your best bet is to add new lines to the very end of the file Try modifying an appropriate bash login script to change the shell prompt or default editor Log out log in again and test your changes If they didn t work restore the file to its original a 4 0 Zl Y 83484 book Page 42 Monday September 18 2006 8 58 AM 42 Phase 1 Working on the Command Line condition and try again with another file it may take a bit of trial and error to locate the correct file If you like you can change the global configuration file to affect all users but you must do so as root In addition to startup scripts bash provides shutdown or logout scripts The most com mon name for this script is bash_logout which is a user script Modifying this or similar scripts can be handy if you want to ensure that certain actions are taken when a user logs out You might use it to destroy sensitive temporary files for instance Exploring Other Environment Variables Linux supports many more environment variables than the PS1 and EDITOR variables described earlier In fact you can set any environment variable you want PHASE1 or GOBBLEDEGOOK for instance To be useful though an environment variable must be used by bash or by some other program Table 1 5 summarizes some common envir
23. user account on your Linux system You can and should perform this task as an ordinary user You may perform this task in either a text mode or a GUI login but in the latter case you must launch an xterm or similar command prompt window Caveats Performing this task as root poses the usual risks of accidentally damaging the installation If you badly corrupt your own bash configuration files it s conceivable but unlikely that you ll be unable to log in again If this happens use a root login to copy etc skel bashrc to your user home directory and change the ownership of the copied file to your normal user account Procedure To complete this task you ll change two environment variables First you ll do this tempo rarily so that your changes affect just a single login session You ll then make your changes permanent by editing a user configuration file At the end of this procedure I describe several other important environment variables which you might choose to change if you so desire Adjusting Your Shell Prompt The prompt in bash is controlled through the PS1 variable You set a variable by specifying the variable name minus the leading dollar sign an equal sign and the value to which you want to set the variable In most cases you should surround the value of the prompt by quotes For instance you can set your bash prompt to read Your command by typing the following PS1 Your command
24. users aka world permissions Each of these three classes of users con sumes three characters which denote the presence or absence of read write and execute permis sions If an r w or x character is present in the respective position the class has the relevant permission if a dash is present the class lacks that type of permission Table 1 1 summarizes some possible permissions and their uses however as there are 512 possible permissions Table 1 1 is incomplete Most of the 512 possible permissions are bizarre though Table 1 1 contains the most common permissions in practice Note that the leading character is sometimes omitted from the permission string as it s not really part of the permissions per se Read and write permissions are fairly self explanatory Execute permission identifies executable files that is program files Note that you can remove execute permission for some users to ensure that a program may only be run by certain users such as the program s owner TABLE 1 1 Example Permissions and Their Likely Uses Permission String Octal Code Meaning rwx rwx rwx 777 Read write and execute permissions for all users rwxr xr Xx 755 Read and execute permission for all users The file s owner also has write permission a 4 0 Als 83484 book Page 15 Monday September 18 2006 8 58 AM Task 1 2 Manage Files and Directories 15 TABLE 1 1 Example Permissions and Their Likely Uses continued Permissi
25. various other ways such as by permissions perm file size size and owner user The man page for find details these options consult it for more information Using locate The locate command is much less flexible than find but it s also much faster To use locate type the command name followed by the name of the file you want to find locate whatis The locate command works by searching a database that it maintains If this database is out of date typing updatedb as root will update it The locate command returns the names of all the files in the database whose names contain the string you specify Thus this command is likely to return the names of many files that merely contain the string you specify such as makewhatis and whatis 1 gz This can be a real problem if you re searching for a file with a short name The locate database also doesn t include every directory on the computer but it does include enough directories that it s likely to be a useful tool Many Linux distributions actually use a program called slocate rather than BQ locate The slocate program is a more security aware version of the pro gram it checks who s calling the program and adjusts its output to remove references to files to which the user shouldn t have access Distributions that use slocate typically create a link called locate so that you can call the pro gram using this more common name Using whereis The whereis program sear
26. This command creates an account called tbrown using defaults for various account param eters On most systems the user s home directory will be home tbrown the shell will be bin bash the comment field will be empty and the UID and GID will be assigned based on the lowest available numbers for both Most importantly the account s password will be dis abled more on that shortly Some systems give each user a unique GID and create an appropriate group STE to go with it as a default policy Other systems assign new users to an exist ing group typically users as a default policy gt 83484 book Page 31 Monday September 18 2006 8 58 AM Task 1 6 Manage Accounts 31 You can create an account with different defaults by placing appropriate parameters between useradd and the account name Alternatively you can create an account with the defaults and then use usermod to change them Some features you re particularly likely to want to adjust include Comment The c comment parameter passes the comment field for the user Some admin istrators store public information like a user s office or telephone number in this field Others store just the user s real name or no information at all Home directory You specify the account s home directory with the d home dir parameter This defaults to home username on most systems Do or do not create a home directory The M option forces the system to not automatically
27. When combined with conditional expressions described shortly variables become even more powerful because then your script can perform one action when one condition is met and another in some other case For instance a script that installs software could check free disk space and abort the installation if there s not enough disk space available One special type of variable was mentioned earlier in this chapter environment variables Environment variables are assigned and accessed just like shell script variables The difference is that the script or command that sets an environment variable uses the export command to make the value of the variable accessible to programs launched from the shell or shell script that made the assignment Environment variables are most often set in shell startup scripts but the scripts you use can access them For instance if your script calls X programs it might check for the presence of a valid DISPLAY environment variable and abort if it finds that this variable isn t set By convention environment variable names are all uppercase whereas non environment shell script variables are all lowercase or mixed case Using Conditional Expressions Scripting languages support several types of conditional expressions These enable a script to perform one of several actions contingent on some condition typically the value of a variable One common command that uses conditional expressions is i f which allows the sy
28. account management The passwd command is also critical in that it enables you to set a password on new accounts so that they re usable File manipulation com mands such as cp and rm help you manage the files that are associated with accounts Task 1 7 Use Streams Pipes and Redirection The command line tools introduced earlier enable you to interact with your Linux system at the command line typing commands and viewing their output Sometimes though you might want to do more with that output than simply view it for instance you might want to store it ina file or pass it as input to another program Fortunately Linux provides the means to do just that In Linux input and output operations are described as streams Standard input and output streams may be redirected to send output to or read input from a regular file or piped between two pro grams These capabilities provide a great deal of flexibility as you ll soon learn PX Part of the Unix philosophy to which Linux adheres is whenever possible to Ane do complex things by combining multiple simple tools Redirection and pipes help in this task by enabling simple programs to be combined together in chains each link feeding off of the output of the preceding link a 4 0 83484 book Page 34 Monday September 18 2006 8 58 AM 34 Phase 1 Working on the Command Line Scenario A user of a Linux computer you administer reports problems with network connections from
29. ach of these three commands has its own unique strengths and weaknesses and you should learn the basics of all three of them Zl Y 83484 book Page 21 Monday September 18 2006 8 58 AM Task 1 4 Find Files 21 Duration This task should take half an hour or an hour to complete Once you ve mastered these com mands you should be able to search for files in a matter of seconds although some of these commands may take several minutes to execute Setup No special setup is required Although the commands used in this task may be used by either root or ordinary users some of them work better when run as root because root may exam ine the contents of any directory whereas ordinary users may not Thus you should use su to acquire root privileges for this task Caveats If you run this task as root be careful what you type Although the search commands them selves are non destructive you should be sure not to mistype a command and make it some thing destructive Procedure To perform this task you ll search for the whatis program file using each of the three file location commands in turn In the process you ll learn the capabilities and limitations of each of these commands Using find The find utility implements a brute force approach to finding files This program finds files by searching through the specified directory tree checking filenames file creation dates and so on to locate the files that
30. am execute repeatedly or only upon certain conditions Programs written in the bash shell language are often referred to as scripts Compared to programs written in languages such as C C and Pascal bash scripts are easy to create but slow to execute This makes them good choices for simple programs and for throwaway programs that aren t likely to be used very often In fact many of Linux s key startup and sys tem configuration files are actually shell scripts Thus understanding how to create and mod ify scripts will help you administer a Linux system Scenario You find that you want to simplify or otherwise improve various tasks that you routinely per form so you want to create scripts to handle these tasks Specifically you want a script to launch a few X programs with one command a way to simplify account creation tasks a script that reports nothing but the computer s IP address a script that plays all the wav audio files in a directory and a script that copies files only if the destination file doesn t already exist Scope of Task Script writing is a very open ended activity scripts can perform a wide variety of tasks and there may be multiple ways of achieving the same goals within a script This particular task presents a few scripts that illustrate important scripting features gt Zl Y 83484 book Page 45 Monday September 18 2006 8 58 AM Task 1 9 Write Basic Scripts 45 Duration
31. and some Linux users are very attached to it Entire books have been written about Vi Consult one of these or a Vi Web page like http ww vim org for more information Saving Changes To save changes to a file type w from command mode This enters ex mode and runs the w ex mode command which writes the file using whatever filename you specified when you launched Vi Related commands enable other functions Edit new file The e command edits a new file For instance e etc inittab loads etc inittab for editing Vi won t load a new file unless the existing one has been saved since its last change or unless you follow e with an exclamation mark Include existing file The r command includes the contents of an old file in an existing one Execute an external command The ex mode command executes the external command that you specify For instance typing 1s runs 1s enabling you to see what files are present in the current directory Quit Use the q command to quit from the program As with e this command won t work unless changes have been saved or you append an exclamation mark to the command as in q You can combine ex commands such as these to perform multiple actions in sequence For instance typing wq writes changes and then quits from Vi Criteria for Completion To complete this task you must successfully edit a copy of your 1ilo conf file to add a new kernel You don t need to test the copy of the f
32. ant limitation The change is restricted to the current running instance of bash Any program launched from bash will inherit the default value of PS1 This is true even of bash itself With your altered prompt dis played try typing bash to launch another shell from the current one Your command prompt will be replaced by your default prompt Typing exit from the new shell will return you to the original one with the modified prompt To make a change that can be inherited by programs launched from your current session you must use the export command after setting the variable PS1l d export PS1 A variable that s been exported in this way is referred to as an environment variable as opposed to a simple variable Alternatively you can combine these commands on a single line export PS1 d After typing these commands if you launch a new instance of bash it will inherit the changed command prompt Even this change though won t affect new logins or new xterm windows to make your change truly permanent you must adjust configuration files as described shortly in Making Your Changes Permanent Setting a Program Specific Environment Variable Many environment variables affect non bash programs One of these that you might want to adjust is EDITOR Some Linux programs launch an external text editor for certain operations and these programs often consult the EDITOR environment variable to determine what pr
33. by DOS and Windows Thus if you use non Linux filesystems on removable disks or par titions shared across OSs you may not be able to create links on them Creating Hard Links The 1n command works much like the cp command type the command name the name of the current file and the link filename you wish to use In sample file fstab This command creates a hard link between the original sample file and the new fstab In Task 1 2 fstab was renamed samp1le fi le A hard link is a duplicate filename that refers to the original file Both filenames are equally valid and once a hard link is created either may be used with precisely the same effect You can tell how many hard links exist by examining the long output of 1s Is 1 total 9 drwxr xr x 22 fred users 1184 May 25 12 51 X11 rw r r 2 fred users 2260 May 25 12 51 fstab rw r r 2 fred users 2260 May 25 12 51 sample file The second column of this output shows for ordinary files the number of filenames that point to the file Ordinary files show 1 in this column files with a single hard link in addition to aa Zs 83484 book Page 19 Monday September 18 2006 8 58 AM Task 1 3 Manage Links 19 the original name show 2 and so on For directories the second column s number refers to the number of directories within the specified directory including pointers to the directory itself and its parent directory pP Current versions of Linux forbi
34. ches for files in a restricted set of locations such as standard binary file directories library directories and man page directories This tool does not search user directories or many other locations that are easily searched by find or locate The whereis utility is a quick way to find program executables and related files like documentation or con figuration files The whereis program returns filenames that begin with whatever you type as a search cri terion even if those filenames contain extensions This feature often turns up configuration files in etc man pages and similar files To use the program type whereis followed by the name of the program you want to locate For instance the following command locates whatis whereis whatis whatis usr bin whatis usr X11R6 bin whatis usr bin X11 whatis usr man manl whatis 1 gz usr share man manl1 whatis 1 gz The result shows the whatis executable usr bin whatis links to it in other directo ries and the man page for whatis including a link in a second location Your system might a 4 0 Zl Y 83484 book Page 23 Monday September 18 2006 8 58 AM Task 1 5 Edit Files 23 find hits in a slightly different set of directories than is shown here Don t be concerned about this The whereis program accepts several parameters that modify its behavior in various ways These are detailed in the program s man page Criteria for Completion To complete this task you
35. ctory name as a target with out a trailing slash though For instance if home project7 sample fi le were a directory the second command would copy etc fstab into that directory under the name fstab You can copy an entire directory tree by using one of the recursion options r and R cp R etc X11 home project7 This command copies the entire contents of the etc X11 directory including all its sub directories to home project7 Using r in place of R is likely to result in the same behavior but some versions of cp produce subtly different effects for these two commands For information on other cp options consult the man page for the command Removing Extraneous Files Now that you ve created the new project directory and placed some files in it you may want to do some housecleaning For this task you may want to first change into the directory in which you want to operate so that you don t need to type the complete path to each file cd home project7 X11 If you type 1s you ll see a list of files and directories Perhaps your project doesn t need access to the xorg conf file you can remove it with rm rm xorg conf MA Be sure you type this command from within your copied directory tree ING home project7 X11 If you type this command from the original etc X11 directory X is unlikely to work the next time you start it As with cp you can use the r or R option to recursively delete an entire direc
36. d making hard links to directories but this was s possible with some earlier versions of Linux Hard links to a single file may exist in two or more different directories however both directories must exist on the same filesystem partition or removable disk Because hard links are created by pointing two filenames at the same file data it makes no sense to create hard links across filesystems Creating Soft Links Soft links aka symbolic links are an alternative to hard links Instead of creating a duplicate directory entry that points directly to the same underlying filesystem data you are creating a new file that contains the filename of the target file As a consequence it s possible to point to files across filesystems To create a soft link you use the 1n command but pass it the s option In s sample file another link 1s 1 total 9 drwxr xr x 22 fred users 1184 May 25 12 51 X11 Irwxrwxrwx 1 fred users 11 May 26 15 30 another link gt sample file rw r r 2 fred users 2260 May 25 12 51 fstab rw r r 2 fred users 2260 May 25 12 51 sample file This example shows the result including how soft links appear in directory listings Note that the link count in the second column of the listing doesn t increase when a soft link is cre ated The soft link itself though includes the 1 file type code in the permissions string and shows the linked to file after the filename in a long listing Unlike hard links so
37. detail in Task 1 2 Examining the File Now that you know the command file is present you can examine it Three commands are very handy for examining text files cat This command s name is short for concatenate and it s used to merge two or more files together If it s passed just one filename though it copies the file to the screen This can be a good way to look at a short text file Try it on the whatis file by typing cat usr bin gt Als 83484 book Page 6 Monday September 18 2006 8 58 AM 6 Phase 1 Working on the Command Line whatis Unfortunately the whatis file is too long to fit on a standard 80x25 screen although it might fit on an extra large screen or resized xterm more This command is a simple pager it displays text a page that is a screenful at a time When you re done reading a page press the spacebar and more displays the next page less Unix and hence Linux programmers aren t afraid to display their sometimes quirky senses of humor The more program is limited for instance it doesn t let you page backward in a file Thus when it came time to improve more the authors of the improved program called it less asa joke You can use the keyboard s arrow keys and various other options to move back and forth in the file When you re done press the Q key to exit from the program Typeman less to learn more about less The man command is described in more detail shortly in G
38. ed to know anything about the format of the etc 1ilo conf file to perform this task for now the goal is simply to learn the basics of Vi You might want to know though that lilo conf controls the way Linux boots at least on computers that use the Linux Loader LILO boot loader Modifying this file therefore modifies the options that are available when you first boot the computer Duration This task should take half an hour or an hour to complete Once you re proficient with Vi you should be able to perform similar tasks in a matter of minutes gt Zl Y 83484 book Page 24 Monday September 18 2006 8 58 AM 24 Phase 1 Working on the Command Line Setup In principle you could use an emergency boot disk but you ll probably find it more convenient to use your regular Linux installation Log into your computer as an ordinary user You should then copy the etc lilo conf file to a safe temporary location such as your home directory cp etc lilo conf If your system lacks a 1ilo conf file locate one on the Web or enter the one presented shortly in a GUI text editor and save it in your home directory Caveats Do not try to perform this task as root and do not attempt to directly edit etc 1lilo conf Doing so is likely to damage your system Of course in a real emergency recovery situation you d need to perform these tasks as root but for practice purposes learning Vi as an ordi nary user is safer
39. er key causes a script to display the specified string mail Themail command can be used to send email from within a script Pass it the s subject parameter to specify a subject line and give it an email address as the last argument If it s used at the command line you would then type a message and terminate it with a Ctrl D If it s used from a script you might omit the message body entirely or pass it an external file as the message using input redirection You might want to use this command to send mail to the superuser about the actions of a startup script or a script that runs on an automated basis Many of these commands are extremely complex You can consult their man TE pages for more information Even if you have a full grasp of how to use some key external commands simply executing commands you might type at a command prompt is of limited utility Many administrative tasks require you to modify what you type at a command or even what commands you enter depending on information from other commands For this reason scripting languages include additional features to help you make your scripts useful Using Variables Variables can help you expand the utility of scripts A variable is a placeholder in a script for a value that will be determined when the script runs Variables values can be passed as param eters to scripts generated internally to the scripts or extracted from the scripts environments a 4 0 Z
40. es that are links in the original directory tree This behavior can result in an unexpected increase in the space required on your CD R media Some tools provide options that influence how they treat links so consult your tool s man page or other documentation if you run into link handling problems Criteria for Completion To complete this task you should create and delete links in the test directory you created in Task 1 2 You should create both hard links which are duplicate directory entries and soft links which are special files that point to other files by name Task 1 4 Find Files A complete Linux system is likely to contain thousands of files Although Linux uses a hier archical directory structure designed to place files in particular locations depending upon their types sometimes files get lost You might know that a file is present but be unable to locate it because you ve forgotten its location or because it s been accidentally moved In such cases knowledge of Linux s file location commands can be invaluable Scenario Returning to the scenario from Task 1 1 suppose you didn t find the whatis executable where you expected it in usr bin Your task now is to see if the file might be located somewhere else on the computer To do this you ll use several Linux commands that are designed to help you find files Scope of Task This task covers three Linux commands for locating files find locate and whereis E
41. etting Help MA All of these commands are intended to work on text files When fed non text ING files such as binary program files cat is likely to cause a lot of annoying beeping and may leave your screen displaying gibberish when you type new commands Typing reset should restore the screen to usability Both more and less cope a bit better with binary files but you re not likely to be able to learn much from them Recent versions of less are often smart enough to rec ognize certain common binary file types and pass them through a translator so you can see their contents in text form Working with Directories Whenever you re running a shell you re working in a specific directory When you refer to a file without providing a complete path to the file the shell works on the file in the current working directory Similar rules apply to many programs The cd command changes the cur rent working directory For instance typing cd home sally changes the current directory to home sally The tilde character is a useful shortcut it stands for your home directory so cd will have the same effect as cd home sal11y if your home directory is home sal ly Many Linux systems display the current working directory in their prompts In Figure 1 1 the current working directory is the home directory that character appears in the com mand prompt and will be replaced by other directory names if you use cd to change directo ries If y
42. fected Invoke an editor You can launch a full fledged editor to edit a command by pressing Ctrl X followed by Ctrl E The bash shell attempts to launch the editor defined by the FCEDIT or EDITOR environment variable or Emacs as a last resort Environment variables are described later in this phase These editing commands are just the most useful ones supported by bash consult its man page to learn about many more obscure editing features In practice you re likely to make heavy use of command and filename completion command history and perhaps a few editing features Try editing your previous command by pulling it out of the history and then changing it For instance you might add the 1 option to the command that did not use it or verify the presence of another file such as usr bin who Getting Help Linux provides a text based help system known as man This command s name is short for manual and its entries its man pages provide succinct summaries of what a command file or other feature does For instance to learn about man itself you would type man man The result is a description of the man command The man utility uses the less pager to display information When you re done press Q to exit from less and the man page it s displaying Some programs have moved away from man pages to info pages The basic purpose of info pages is the same as that for man pages but info pages use a hypertext format so that gt
43. ft links don t work quite exactly like the original link The time to access a soft link is minutely longer than the time to access the original file You can delete the soft link itself without affecting the linked to file but if you delete the original file the soft link will be broken it will point to nothing You can create soft links to directories In practice soft links are more common than hard links The fact that they can be created across filesystems and the fact that they can point to directories makes them more flexible You should be cautious though not to break soft links by deleting moving or renaming the original files Managing Links The mv rm and cp commands work on links just as they work on the original files Thus link management is just like ordinary file management for instance suppose you decide you only aa Zl Y 83484 book Page 20 Monday September 18 2006 8 58 AM 20 Phase 1 Working on the Command Line want the copied fstab file to be accessible under two names you can delete either of the two hard links with rm rm fstab This example deletes the second link if sample file had been deleted instead you d break the symbolic link another 1ink You should be aware that some file operations will do odd things with links and particu larly with symbolic links For instance if you create a CD R from files on your hard disk you might find that it contains duplicates of fil
44. he shell and its configuration it may beep If you press the Tab key again the system responds by displaying the possible comple tions You can then type another character or two and if you haven t completed the command or filename press the Tab key again to have the process repeat The most fundamental Linux commands have fairly short names cd pwd and so on Some other commands are much longer though such as traceroute or sane find scanner Filenames can also be quite lengthy up to 255 characters on many filesystems Thus command completion can save a lot of time It can also help you avoid typos To try out command completion type the preceding 1s command to verify the presence of the whatis program but press the Tab key once or twice after you type the wh portion of the whatis name Chances are you ll see a list of half a dozen or more possible completions Add the a from the whatis name and press the Tab key again Your system might then complete the whatis name or display a shorter list of possible completions depending upon whether or not other files with names beginning wha appear in your usr bin directory Using Command History Another helpful shell shortcut is the shell history The shell keeps a record of every command you type stored in bash_history in the case of bash If you ve typed a long command recently and want to use it again or use a minor variant of it you can pull the command out of the history The s
45. if an administrator makes a mistake pP The Linux system administration account is conventionally called root but i it s possible to configure Linux with aliases for this name Under any name this account is often referred to as the superuser account Verifying the Presence of a File To verify that a file is present use the 1s command This command s name stands for list it shows a list of files that match a file specification you provide Task 1 2 describes file speci fications in more detail For now check for the presence of the whatis file which is located in the usr bin directory 1s usr bin whatis usr bin whatis Used without any extra parameters 1s displays the names of all the matching files Because this example provides the complete name of a single file only that filename is displayed on the line immediately following the command s line If you type the name of a directory 1s displays the names of all the files in that directory Try this with the usr bin directory now but be prepared for copious output The usr bin directory holds the program files associated with many but not all Linux commands Like many Linux commands 1s accepts options that modify its actions One of the most important is 1 that s a lowercase L not a number 1 which generates a long listing Is 1 usr bin whatis rwxr xr x 1 root root 2409 Nov 19 2004 usr bin whatis I describe what this output means in more
46. ile but you should verify that it s been modified as you desired To do so use cat or less Task 1 6 Manage Accounts As a multi user OS Linux requires that users have accounts This requirement is part of Linux s security system so you shouldn t try to bypass account management or give it short shrift say by letting many people share an account Instead you should learn how to create delete and otherwise manage Linux accounts ITE Although these tools are perfectly capable of handling routine account main tenance tasks on small systems they differ from one distribution to another The text based tools described here are much more consistent across distri butions and are the tools tested on Linux certification exams lt a Most Linux distributions provide GUI tools for account management a 4 0 83484 book Page 29 Monday September 18 2006 8 58 AM Task 1 6 Manage Accounts 29 Scenario Personnel changes require you to add an account for a new user Trevor Brown and delete an account for an employee who s leaving Susan Jones Changes to the amount of free space on two disks also requires you to move another user s Dale Smith s files from the home2 to the home directory tree Scope of Task This task involves creating deleting and modifying accounts Each of these operations is fairly straightforward but you must understand the basics of the relevant commands and you must be aware of the con
47. implest way to do this is to press the up arrow key on your keyboard this brings up the previous command Pressing the up arrow key repeatedly moves through mul tiple commands so you can find the one you want If you overshoot press the down arrow key to move down the history The Ctrl P and Ctrl N keystrokes double for the up and down arrow keys respectively Another way to use the command history is to search through it Press Ctrl R to begin a backward reverse search which is what you probably want and begin typing characters that should be unique to the command you want to find The characters you type need not be the ones that begin the command they can exist anywhere in the command You can either keep typing until you find the correct command or after you ve typed a few characters press Ctrl R repeatedly until you find the one you want The Ctrl S keystroke works similarly but searches forward in the command history which might be handy if you ve used a backward search or the up arrow key to look back and have overshot In either event if you can t find the command you want or change your mind and want to terminate the search press Ctrl G to do so Try out these features now use the up arrow key to recall your previous command and press Ctrl R and a portion of that command to search for it gt Zs 83484 book Page 8 Monday September 18 2006 8 58 AM 8 Phase 1 Working on the Command Line Editing Com
48. in your particular session type env This command officially runs a program in a modified environment however when typed alone it displays the values of all the currently set environment variables The output of this command is likely to be quite long so you may want to pipe it through less or redirect the output to a file for later exam ination Some of the environment variables will have fairly suggestive names but others are likely to be obscure You can find the purpose of some in the man page for bash but others have meaning only to particular programs If you don t understand the purpose of an environment variable don t try to change it ignore it or if you re curious try looking it up using a Web search engine Criteria for Completion To complete this task you should have successfully tested modifications to at least two envi ronment variables PS1 to set the shell prompt and EDITOR to set the default editor You should have made both transient changes by setting the variable and using the export com mand on a command line and more permanent changes by editing a bash configuration file Task 1 9 Write Basic Scripts The bash and other Linux command prompt shells are more than simple command line tools they re powerful programming languages You can string together commands to have them execute one after another add variables to improve flexibility and use program control state ments to have parts of the progr
49. k will take about half an hour to complete Once you re familiar with the commands using each one takes just seconds Setup You must have access to a working Linux system For the specific examples shown in this task your system should be configured to give you a bash shell by default and the whatis program must be installed Both of these conditions are usually true If your Linux computer is not cur rently turned on do so now and wait for it to finish booting Caveats Most Linux systems configure bash as the default shell for all users If your account is con figured to use tcsh or some other shell some of the commands and procedures described here won t work Even with bash a few details differ from one account or computer to another Most commonly the form of the command prompt varies from one system to another and sometimes even from one account to another For simplicity and brevity this book denotes user shell commands with a leading dollar sign which is often used as part of real bash prompts as in 1s This line means to type 1s at a user command prompt Some systems use a prompt that ends in a greater than sign gt instead of a dollar sign Most Linux systems provide a different prompt often terminating in a hash mark for the superuser root account When a command must be typed as root this book shows com mand lines beginning with this symbol Is When short commands appear inside paragraph text
50. ks Pipes and redirection can be combined in complex ways and obscure variants may be used For instance the amp 1 and amp 2 strings refer to standard output and standard error respectively This fact enables you to pipe standard error without also piping standard output netstat p 2 gt amp 1 gt dev null text2gif gt error message gif a 4 0 Zl Y 83484 book Page 38 Monday September 18 2006 8 58 AM 38 Phase 1 Working on the Command Line This example relies on subtle order effects On the command line standard error is redirected to standard output before standard output is redirected to dev nu11 however bash interprets these redirections in the opposite order Thus this command will send nothing as input to text2gif if you reverse the order of the two redirection operators Also when redirecting to amp 1 or amp 2 you should include no space between the redirection operator and its destination Criteria for Completion This task demonstrated the use of pipe and redirection operators for standard input standard output and standard error You should now be able to apply these operators in working with Linux commands that generate textual output or expect textual input A few commands such as text2gif generate binary output on standard output and rely on you to be able to redirect or pipe it appropriately Task 1 8 Manage the Shell Environment Linux shells are simply programs and like many program
51. l Y 83484 book Page 48 Monday September 18 2006 8 58 AM 48 Phase 1 Working on the Command Line Variables that are passed to the script are frequently called parameters They re represented by a dollar sign followed by a number from 0 up 0 stands for the name of the script 1 is the first parameter to the script 2 is the second parameter and so on To understand how this might be useful consider the task of adding a user As described earlier in Task 1 6 cre ating an account for a new user typically involves running at least two commands useradd and passwd You might also need to run additional site specific commands such as com mands that create unusual user owned directories aside from the user s home directory As an example of how a script with a parameter variable can help in such situations con sider Listing 1 3 This script creates an account and changes the account s password you ll be prompted to enter the password when you run the script It creates a directory in the shared directory tree corresponding to the account and it sets a symbolic link to that directory from the new user s home directory It also adjusts ownership and permissions in a way that may be useful depending on your system s ownership and permissions policies Listing 1 3 A Script That Reduces Account Creation Tedium bin sh useradd m 1 passwd 1 mkdir p shared 1 chown 1 users shared 1 chmod 775 shared 1 In
52. m You use a text editor to type commands then modify the file permissions on the resulting file so that it s executable Once you know how to perform these basic operations you can proceed to create scripts that meet the objectives of this task Beginning a Shell Script Shell scripts are plain text files so you create them in text editors A shell script begins with a line that identifies the shell that s used to run it such as the following bin sh The first two characters are a special code that tells the Linux kernel that this is a script and to use the rest of the line as a pathname to the program that s to interpret the script Shell scripting languages use a hash mark as a comment character so the script utility itself ignores this line although the kernel doesn t On most systems bin sh is a symbolic link that points to bin bash but it could point to some other shell Specifying the script as using bin sh guarantees that any Linux system will have a shell program to run the script but if the script uses any features specific to a particular shell you should specify that shell instead for instance use bin bash or bin tcsh instead of bin sh gt Zl Y 83484 book Page 46 Monday September 18 2006 8 58 AM 46 Phase 1 Working on the Command Line When you re done writing the shell script you should modify it so that it s executable You do this with the chmod command as described in
53. m look for configuration files or as a default location in which to store files a 4 0 Zl Y 83484 book Page 43 Monday September 18 2006 8 58 AM Task 1 8 Manage the Shell Environment 43 TABLE 1 5 Common Environment Variables and Their Meanings continued Variable Name Explanation LD_LIBRARY_PATH A few programs use this environment variable to indicate directories in which library files may be found It works much like PATH PS1 This is the default prompt in bash It generally includes variables of its own such as u for the username h for the hostname and W for the current working directory This value is frequently set in etc profile but it is often overridden by users TERM This variable is the name of the current terminal type To move a text mode cursor and display text effects for programs like text mode editors Linux has to know what commands the terminal sup ports The TERM environment variable specifies the terminal in use This information is combined with data from additional files to pro vide terminal specific code information TERM is normally set auto matically at login but in some cases you may need to change it DISPLAY This variable identifies the display used by X It s usually 0 0 which means the first numbered from 0 display on the current computer When you use X in a networked environment though this value may be preceded by the name of the computer at which you re sit
54. mands Frequently after finding a command in the history you want to edit it The bash shell like many shells provides editing features modeled after those of the Emacs editor Move within the line Press Ctrl A or Ctrl E to move the cursor to the start or end of the line respectively The left and right arrow keys will move within the line a character at a time Ctrl B and Ctrl F will do the same moving backward and forward within a line Pressing Ctrl plus the left or right arrow keys will move backward or forward a word at a time as will press ing Esc and then B or F Delete text Pressing Ctrl D or the Delete key deletes the character under the cursor while pressing the Backspace key deletes the character to the left of the cursor Pressing Ctrl K deletes all text from the cursor to the end of the line Pressing Ctrl X and then Backspace deletes all the text from the cursor to the beginning of the line Transpose text Pressing Ctrl T transposes the character before the cursor with the character under the cursor Pressing Esc and then T transposes the two words immediately before or under the cursor Change case Pressing Esc and then U converts text from the cursor to the end of the word to uppercase Pressing Esc and then L converts text from the cursor to the end of the word to low ercase Pressing Esc and then C converts the letter under the cursor or the first letter of the next word to uppercase leaving the rest of the word unaf
55. match the specified criteria Because of this method of operation find tends to be slow but it s very flexible and is very likely to succeed assuming the file for which you re searching exists To search the entire computer for the whatis program file by name type the following command find name whatis This command specifies the path to search meaning the entire directory tree and the cri teria to use for the search name whatis to search for a file whose name is whatis Because this command searches the entire directory tree it s likely to take a long time to complete perhaps several minutes This command is guaranteed to find any file meeting the specified cri teria though at least assuming you have permission to read the directory in which it resides and don t make a mistake when specifying the criteria The output of the find command is a list of files that match the specified criteria one file per line If you re not certain of the exact filename you can use wildcards in the filename specifica tion as in name what to search for any file whose name begins with what Using quotes around the search specification ensures that the shell will pass the wildcards to the find com mand rather than try to expand the wildcard itself a 4 0 Zl Y 83484 book Page 22 Monday September 18 2006 8 58 AM 22 Phase 1 Working on the Command Line In addition to the name criterion you can search for files in
56. might put your test 1ilo conf file in your home directory for this exercise The first step to using Vi is to launch it and have it load the file In this example type vi 1ilo conf while in the directory holding the file The result should resemble Figure 1 2 which shows Vi running in a Konsole window The tildes down the left side of the display indicate the end of the file The bottom line shows the status of the last command an implicit file load command because you specified a filename when launching the program TK 83484 book Page 26 Monday September 18 2006 8 58 AM 26 Phase 1 Working on the Command Line FIGURE 1 2 The last line of a Vi display is a status line that shows messages from the program Shell Konsole ot dev sda ap boot map install boot boot b timeout 50 image boot vmlinuz label linux root dev sda6 read only lilo conf 10 lines 162 characters Shell No 2 Shell No 3 Adding a new entry to 1i 10 conf involves duplicating the lines beginning with the image line and modifying the duplicates Therefore the first editing task is to duplicate these four lines To do this follow these steps 1 Move the cursor to the beginning of the image line by using the down arrow key you should see the cursor resting on the 7 The h j k and keys can also be used in place of the left down up and right A TE arrow keys This is a holdover from the days before all keyboards had arr
57. motely via logins using protocols such as the Secure Shell SSH In such cases standard input standard output and standard error are all directed over the remote login protocol and so ultimately correspond to a keyboard and monitor on the remote computer Redirecting Output To begin the task you want to investigate the network connections maintained by Firefox To do so you ll use the netstat command which displays information on all the network con nections maintained by the computer You ll use the p option to netstat so as to display the program names netstat p When you type this command though chances are you ll see so much output scroll past that you won t be able to read it all before most of it disappears off the top of the screen One way around this problem is to redirect standard output to a file You do this with the redirection a 4 0 Zl Y 83484 book Page 36 Monday September 18 2006 8 58 AM 36 Phase 1 Working on the Command Line operator gt which you place after the command and before the name of a file that is to receive the output netstat p gt net connections txt The file net connections txt now contains the output of the netstat p command with one exception described shortly You can then open net connections txt in a text editor or view it with a program such as less in order to study its contents in greater detail Do so now and search for references to the Firefox brow
58. move a file from one partition or removable disk to another though mv must copy the file and then delete the original This operation is likely to take longer partic ularly with large files Setting File and Directory Ownership In Linux all files and directories have owners These owners are Linux accounts such as your user account or root Files and directories are also tied to Linux groups which are collections of accounts By default mkdir creates directories that are owned by the user who issued the command and with group ownership by that user s default group After you create a directory as root you can adjust the directory s ownership by using the chown command chown fred users home project7 Po This example assumes the presence of the fred account and the users group OTE You may need to adjust it for your system This command gives ownership of the home project7 directory to the user fred and the group users You may separate the username and group name with either a colon as shown in this example or a dot You may apply the chown command to both directories and files If you omit the username chown changes the group of the file or directory without changing the a 4 0 Als 83484 book Page 14 Monday September 18 2006 8 58 AM 14 Phase 1 Working on the Command Line file s main owner This is effectively the same as the chgrp command which works much like chown but accepts a group name alone
59. n commands The keyword function may optionally precede the function name In either event the function is called by name as if it were an ordinary internal or external command Functions are very useful in helping to create modular scripts For instance if your script needs to perform half a dozen distinct computations you might place each computation in a function and then call them all in sequence Listing 1 6 demonstrates the use of functions in a simple program that copies a file but aborts with an error message if the target file already exists This script accepts a target and a destination filename and must pass those filenames to the functions Listing 1 6 A Script Demonstrating the Use of Functions bin bash doit cp 1 2 function check if s 2 then echo Target file exists Exiting exit fi check 1 2 doit 1 2 4 0 Zl Y 83484 book Page 52 Monday September 18 2006 8 58 AM 52 Phase 1 Working on the Command Line If you enter Listing 1 6 and call it safercp you might use it like this assuming the file original txt exists and dest txt does not safercp original txt dest txt safercp original txt dest txt Target file exists Exiting The first run of the command succeeded because dest txt did not exist When the com mand was run a second time though the destination file did exist so the program terminated with the error message Note that the functions are no
60. nput and output involve files A program can read or write data from or to a disk file but even the keyboard and screen are treated much like files These devices cor respond to three separate streams Standard input This stream often abbreviated stdin corresponds to the keyboard When a program wants input from the user it opens standard input and reads data from it as if it were reading data from a disk file Standard output This stream is often referred to as stdout and it corresponds to the text mode display which can mean a text mode console the contents of an xterm or similar win dow or a remote login session s display Programs write data to standard output as if it were a file and the characters so written then appear on the screen Standard error Ordinarily this stream often abbreviated stderr is the same physical device as standard output however it s used by programs to display error messages rather than ordi nary output The reason it s treated separately is so that it may be redirected separately so that you can interact normally with a program while sending error message to a file or so that you can redirect normal output to a file while still seeing error messages on the screen If you use a Linux computer at the console that is using the keyboard and monitor that are attached directly to the computer these streams correspond to the computer s own key board and display Linux systems can also be used re
61. nput redirection operator points left toward the command name whereas the output redirection operators point right toward the filename of the file you want to create Piping Data between Programs The preceding example showed text2gif operating on a file that was created by another pro gram This approach is common in Linux so common in fact that a variant of the redirection operator exists to simplify matters This tool is known as a pipe or a pipeline and it s a way to send standard output from one program directly into another without using any on disk file The symbol for a pipe is a vertical bar located above the Enter key on the same key that holds the backslash character on most keyboards You place the pipe character between the commands For instance instead of saving the output of netstat p ina file and then examining that file with less you can pipe the result directly into less netstat p less Another common use of a pipe is to send the results of a lengthy command through grep which searches for lines containing a particular string that you specify For instance to search for the lines in the netstat output that refer to Firefox you might issue the following command netstat p grep firefox You can create a pipeline containing multiple commands and even redirect the output of the final command netstat p grep firefox text2gif gt netstat output gif Additional Pipe and Redirection Tric
62. o gram to use First check what program your account is configured to use by default echo EDITOR usr bin vi I Although the EDITOR variable is frequently defined it isn t guaranteed to be JTE If a variable isn t set displaying it with echo will show an empty line as output Of course your system might not be configured to use Vi as shown in this example Another test is to use a utility that consults the EDITOR environment variable One such tool is crontab type crontab e and you should see a probably empty file appear in your default editor Exit from this editor without making any changes to the file aa Zl Y 83484 book Page 41 Monday September 18 2006 8 58 AM Task 1 8 Manage the Shell Environment 41 Suppose you don t want to use Vi as your default editor though perhaps you prefer nano You can adjust your default editor by changing the environment variable export EDITOR usr bin nano Mh Check that usr bin nano exists before typing this command If it doesn t ING exist locate the file s true location or substitute another editor such as usr bin emacs or usr bin jed You must use the export command to set the environment variable not simply a variable for the current bash session since EDITOR is used by programs other than bash Once the environment variable has been set test it by typing crontab e again You should see the same probably empty file you saw before appear in your
63. o perform some operations you may need to move rename delete or otherwise modify files and directories Thus an understanding of the commands used to accomplish these jobs is necessary for effective use of Linux This task gives you practice with these commands Scenario A new project is starting at your place of work To prepare you must create a directory to hold files that are to be accessible to all the members of this project who are already members of the users group You must also populate this new directory with a few files which for pur pose of this exercise you ll copy from the etc directory Scope of Task This task requires creating copying and managing the permissions of both files and directories Duration This task should take about half an hour to complete Once you ve learned the task you should be able to perform similar tasks in just a few minutes a 4 0 Zs 83484 book Page 11 Monday September 18 2006 8 58 AM Task 1 2 Manage Files and Directories 11 Setup You need perform no special setup to do this task just log into your computer and acquire root privileges Caveats Because this task is performed as root and uses powerful file manipulation commands a potential for devastating errors exists Certain typos when entered as root can obliterate an entire Linux installation Be very careful when performing this task or any other task that requires root privileges When you type a
64. o run it from X however the diagnostic commands must be run from the command line either in an xterm or similar window or from a separate text mode login To prepare for this task you should log into your Linux system as an ordinary user in GUI mode Once logged in launch the Mozilla Firefox Web browser by locating it from the menu system or by typing firefox in an xterm window Once Firefox is running browse to a Web site any external Web site will do You should then launch an xterm or similar window to obtain a command prompt or press Alt Ctrl F1 to switch to text mode and log in as an ordi nary user to get a command prompt Caveats If you don t have or don t want to use Firefox you may use any other Web browser such as Konqueror Opera or even the text mode lynx If you do you must change the references to the browser as appropriate in the following procedure o e Zl Y 83484 book Page 35 Monday September 18 2006 8 58 AM Task 1 7 Use Streams Pipes and Redirection 35 This task may be performed as an ordinary user If you run it as root you run the usual risks of performing tasks as root Procedure This task investigates three practical techniques redirecting output redirecting input and pip ing data between programs All three techniques rely on the fact that Linux uses input and out put streams so you should first understand a bit of theory Understanding Streams To Linux programs i
65. on String Octal Code Meaning rwxr x 750 Read and execute permission for the owner and group The file s owner also has write permission Users who are not the file s owner or members of the group have no access to the file rwx 700 Read write and execute permissions for the file s owner only all others have no access rw rw rw 666 Read and write permissions for all users No execute permissions to anybody rw rw r 664 Read and write permissions to the owner and group Read only permission to all others rw rw 660 Read and write permissions to the owner and group No world permissions rw r r 644 Read and write permissions to the owner Read only permission to all others rw r 640 Read and write permissions to the owner and read only permission to the group No permission to others rw 600 Read and write permissions to the owner No permis sion to anybody else r 400 Read permission to the owner No permission to any body else The second column in Table 1 1 provides an octal base 8 code corresponding to the per mission string Each cluster of three permission bits can be represented as a 3 bit number which in turn can be represented as a single octal number from 0 to 7 Read permission cor responds to 4 write permission corresponds to 2 and execute permission corresponds to 1 Add the permissions you want to obtain the corresponding octal digit To change permissions y
66. onment variables and their uses This table is not comprehensive though in principle any program may use its own unique environment vari ables If you read in a program s documentation that it uses particular environment variables you can set them using the techniques described earlier TABLE 1 5 Common Environment Variables and Their Meanings Variable Name Explanation USER This is your current username It s a variable that s maintained by the system SHELL This variable holds the path to the current command shell PWD This is the present working directory This environment variable is maintained by the system Programs may use it to search for files when you don t provide a complete pathname HOSTNAME This is the current TCP IP hostname of the computer PATH This is an unusually important environment variable It sets the path for a session the path is a colon delimited list of directories in which Linux searches for executable programs when you type a program name For instance if PATH is bin usr bin and you type 1s Linux looks for an executable program called 1s in bin and then in usr bin If the command you type isn t on the path Linux responds with acommand not found error The PATH variable is typically built up in several configuration files such as etc profile and the bashrc file in the user s home directory HOME This variable points to your home directory Some programs use it to help the
67. ou use the chmod command Permissions are sometimes called the file s mode so chmod is short for change mode This command takes a mode expressed either in octal form or as a set of symbolic codes The octal form is easier to understand although many newcomers find the octal representation confusing chmod 660 home project7 xorg conf a 4 0 Zl y 83484 book Page 16 Monday September 18 2006 8 58 AM 16 Phase 1 Working on the Command Line This command grants rw rw permissions to the xorg conf file in home project7 The symbolic form of the command represents a series of changes to permissions using the codes summarized in Table 1 2 TABLE 1 2 Codes Used in Symbolic Modes Permission Change Permission to Set Code Meaning TypeCode Meaning Modify Code Meaning u owner add r read g group remove w write o world set equal x execute to a all X execute only if file is directory or already has execute permission s SUID or SGID t sticky bit u existing owner s permissions g existing group permissions o existing world permissions Using these codes works best if you know the current mode and want to change it by adding or removing certain specific permissions Table 1 3 summarizes some examples TABLE 1 3 Examples of Symbolic Permissions with chmod Command Initial Permissions End Permissions chmod a x bigprogram rw r r rwxr xr x chmod ug rw report tex r rw rw a 4 0
68. our prompt doesn t include this information and you want to know what directory you re working in type pwd This command s name stands for print working directory it dis plays the current working directory s name P Unlike DOS and Windows Linux doesn t use drive identifier letters such as i C All directories reside in a single unified directory tree Removable disks and multiple disk partitions are mounted within that tree as described in Phase 5 Managing Partitions and Filesystems Another important differ ence between Linux and Windows is that Linux uses a forward slash to separate directories whereas Windows uses a backslash for this purpose a 4 0 Zs 83484 book Page 7 Monday September 18 2006 8 58 AM Task 1 1 Use Basic Command Line Features 7 Using Command Completion Many users find typing commands to be tedious and error prone For this reason Linux shells include various tools that can help speed up operations The first of these is command com pletion Type part of a command or as an option to a command part of a filename and then press the Tab key The shell tries to fill in the rest of the command or the filename If just one command or filename matches the characters you ve typed so far the shell fills it in and places a space after it If the characters you ve typed don t uniquely identify a command or filename the shell fills in what it can and then stops Depending on t
69. out to begin customizing this second stanza 2 Up until now you ve operated Vi in command mode There are several commands that you can use to enter insert mode At this point the most appropriate is R which enters insert mode so that it is configured for text replacement rather than insertion If you prefer to insert text rather than overwrite it you could use i or a the latter advances the cursor one space which is sometimes useful at the end of a line For the purpose of these instruc tions type R to enter insert mode You should see REPLACE appear in the status line 3 Type the name of a new Linux kernel For the purpose of this example let s say it s called bzImage 2 6 13 so that s what you d type This entry should replace vmlinuz 4 Use the arrow keys to move the cursor to the start of linux on the next line You must replace this label so that your new entry has its own label Type a new label such as mykerne This label should replace the existing 1inux label Exit from insert mode by pressing the Esc key Save the file and quit by typing wq This is an ex mode command that writes changes and then exits quits from the editor The ZZ command is equivalent to wq Many additional commands are available that you might want to use in some situations Here are some of the highlights Case changes Suppose you need to change the case of a word in a file Instead of entering insert mode and retyping the wo
70. ow keys 2 You must now yank four lines of text This term is used much as copy is used in most text editors you copy the text to a buffer from which you can later paste it back into the file To yank text you use the yy command preceded by the number of lines you want to yank Thus type 4yy do not press the Enter key though Vi responds with the message 4 lines yanked on its bottom status line The dd command works much like yy but it deletes the lines as well as copying them to a buffer Both yy and dd are special cases of the y and d commands respectively which yank or delete text in amounts specified by the next character as in dw to delete the next word 3 Move the cursor to the last line of the file by using the arrow keys Type p again without pressing the Enter key Vi pastes the contents of the buffer starting on the line after the cursor The file should now have two identical image stanzas The cursor should be resting at the start of the second one If you want to paste the text into the document starting on the line before the cursor use an uppercase P command o Zl Y 83484 book Page 27 Monday September 18 2006 8 58 AM Task 1 5 Edit Files 27 Now that you ve duplicated the necessary lines you must modify one copy to point to the kernel whose entry was accidentally deleted To do so follow these steps 1 Move the cursor to the v in vmlinuz on the second image line You re ab
71. ptimal but might be acceptable if you choose a unique and good password for each account Phase 7 describes passwords in more detail Deleting User Accounts You can delete an account with userdel which works much like useradd userdel sjones This command deletes the sjones account It does not however delete the user s home directory or mail spool much less other files that may be owned by the user elsewhere on the computer You must manually delete or otherwise deal with these files You can then give the backup to the user if appropriate or store it yourself in case another user such as this individual s replacement in your organization needs the files A Before deleting an account back it up to tape CD R or some other medium P You can pass the r option to have userdel delete the user s home directory and mail spool This option won t delete other files the user may own elsewhere on the computer though To locate those files use find described in Task 1 4 with its uid n option to search for files owned by UID n If you search for files before deleting the account you can use find s user username option to search by username Modifying User Accounts What if an account already exists and you want to change it in some way You can modify the account with usermod which takes most of the same options as useradd Another important usermod option is 1 name which alters the username associated with the accoun
72. rd you can use the tilde key in command mode to change the case Position the cursor on the first character you want to change and press repeatedly until the task is done Undo To undo any change type u in command mode Opening text In command mode typing o opens text that is it inserts a new line immedi ately below the current one and enters insert mode on that line Searches To search forward for text in a file type in command mode followed immedi ately by the text you want to locate Typing will search backward rather than forward Changes The c command changes text from within command mode You invoke it much as you do the d or y commands as in cw to change the next word or cc to change an entire line Go to a line The G key brings you to a line that you specify The H key homes the cur sor that is it moves the cursor to the top line of the screen The L key brings the key to the bottom line of the screen Global replacement To replace all occurrences of one string by another type s original replacement where original is the original string and replacement is its replacement Change to a starting line number comma and ending line number to perform this change on just a small range of lines a 4 0 Zl Y 83484 book Page 28 Monday September 18 2006 8 58 AM 28 Phase 1 Working on the Command Line There s a great deal more depth to Vi than is presented here the editor is quite capable
73. ror mes sages you don t care about you might type whine 2 gt dev nul1 to run it and discard its error messages w A common trick is to redirect standard output or standard error to dev nul1 Redirecting Input Many Linux programs are designed to accept input from files whose names you specify on the command line Some programs though are designed to accept keyboard input via standard input What if you want to provide fixed input from a file to such programs though The answer is to use standard input redirection via the input redirection operator lt As an example suppose you wanted to convert the error messages txt file to a graphics format You might want to use the text2gif program part of the giflib package but it a 4 0 Zl Y 83484 book Page 37 Monday September 18 2006 8 58 AM Task 1 7 Use Streams Pipes and Redirection 37 requires you to enter text either as a command line option or via standard input Thus you d redirect standard input from the error messages txt file You d also have to redirect stan dard output to save the result in a file text2gif lt error messages txt gt error graphics gif You can use a graphics program such as the GIMP to view the error graphics gif file to verify that it contains the correct image Pa When using the redirection operators you start a command line with the pro TE gram name and then point the operators toward their destinations the i
74. s the details of their operation can be customized You can change the command prompt set the directories in which the shell searches for program files and so on These features are adjusted via environment variables which you can set on the command line or in configuration files Other programs can also use environment variables so in some cases setting an environment variable in a bash configu ration file can affect programs launched from bash Scenario You dislike your current bash prompt and want to change it to something that includes the date and time You also need to set an environment variable that tells various programs what editor to use when calling an external text editor You want to test these changes and then make them permanent by modifying your bash configuration file Scope of Task To perform this task you ll need to type a few commands at the command line and edit a con figuration file Consult Task 1 5 for information on using Vi for file editing or use your favor ite text based or GUI editor Duration This task will take about half an hour to complete Once you know how to change environ ment variables you ll be able to do so in under a minute but it may take longer than that to determine what environment variable needs changing gt Zl Y 83484 book Page 39 Monday September 18 2006 8 58 AM Task 1 8 Manage the Shell Environment 39 Setup To prepare for the task log into your
75. s of links Linux supports can be confusing to new Linux users and administrators This task will step you through the two types of links and provide tips on how to manage them Duration This task should take about half an hour to complete Once you ve learned the task you should be able to perform similar tasks in just a few minutes gt Zs 83484 book Page 18 Monday September 18 2006 8 58 AM 18 Phase 1 Working on the Command Line Setup You need perform no special setup to perform this task just log into your computer as the user who owns the files in the home project7 directory and type cd home project7 to change into that directory Be sure to complete Task 1 2 before starting this task Caveats If you perform this task as an ordinary user the risk to the computer is minimal If you opt to perform this task as root though you might accidentally delete or corrupt important sys tem files particularly if you perform these steps in the wrong directory Procedure The 1n command creates links so it s the most important command to know when it comes to link management Other link related tasks can be performed using ordinary Linux file manipulation commands such as mv cp and rm P Links require support in the underlying filesystem Although all Linux native TE filesystems support links they aren t supported in some non Linux filesys tems such as the File Allocation Table FAT filesystem used
76. sequences of making mistakes when manipulating user accounts Duration This task should take about half an hour to complete Once you re familiar with these tasks you can create delete or modify accounts in a few seconds to a few minutes depending upon the precise operations you need to perform Setup You should log into your Linux system and then use su to acquire superuser privileges Alterna tively you may log in directly as root although using su is preferable as described in Task 1 1 You may perform this task from a text mode login or within an xterm window from a GUI login For this task it is assumed that two accounts already exist on the computer sjones and dsmi th If your computer lacks these accounts they aren t standard you can create them yourself by fol lowing the instructions for creating the first account but change the usernames Caveats Account maintenance operations are potentially risky You might accidentally delete or mod ify the wrong account Even account creation poses risks particularly when you use advanced options you might accidentally give two accounts the same user ID UID codes which would make them essentially interchangeable Thus you should be particularly cautious when using these commands Because these commands require root access to work the usual caveats con cerning working as root also apply This task assumes that you re working on a Linux system that uses a local acco
77. ser These may be called fi refox bin Don t worry about what these references mean though the point is to familiarize your self with redirection not the output of netstat When you ran the netstat p command with redirection as an ordinary user chances are you saw a message appear on the screen to the effect that not all processes would be displayed This message was directed to standard error It too can be redirected but you must use the standard error redirection 2 gt operator rather than the standard output redirection operator netstat p 2 gt error messages txt If you examine the error messages txt file you ll see that it contains only the warning about not all processes being displayed standard output appears on the screen You can redi rect both standard output and standard error by using the amp gt redirection operator netstat p amp gt net connections txt All of these redirection operators overwrite whatever file you provide as an argument net connections txt or error messages txt in these examples When redirecting standard output or standard error but not both you can append to an existing file rather than over write it by adding a second greater than symbol netstat p gt gt net connections txt netstat p 2 gt gt error messages txt P This file is a device that s connected to nothing it s used when you want to get rid of data For instance if the whine program is generating er
78. should use the three file location commands to locate thewhati s pro gram Although the output from the locate program is likely to be quite long you should verify that the three programs all return the same key file which is likely to be usr bin whatis Task 1 5 Edit Files A great deal of Linux system administration involves editing files In Linux most configura tion files are ordinary text files and changing how the system functions involves editing these files Thus you should be proficient with at least one text editor in Linux Although you can use a fancy GUI text editor if you like one editor that s particularly important is Vi This edi tor is a simple text mode editor and it s important because it s a very lightweight editor that s accessible from most basic emergency systems Thus even if you prefer another editor you may be forced to use Vi in certain emergency recovery situations Scenario An accidental change to the etc 1i1o conf file has rendered a Linux system unbootable To recover you must boot using an emergency disk and edit this file using Vi For the purpose of this exercise of course you won t edit the real etc 1ilo conf file and you needn t even boot from an emergency disk although you can if you want to Instead you ll make a copy of etc lilo conf and edit the copy Scope of Task This task involves reviewing the basics of the Vi editor and trying out Vi editing tasks You will not ne
79. ss fully If command returns an error code the additional commands won t be run Conditional expressions are sometimes used in loops as well Loops are structures that tell the script to perform the same task repeatedly until some condition is met or until some con dition is no longer met For instance Listing 1 5 shows a loop that plays all the wav audio files in a directory Listing 1 5 A Script That Executes a Command on Every Matching File in a Directory bin bash for d in Is wav do play d done The for loop as used here executes once for every item in the list generated by 1s wav Each of those items filenames is assigned in turn to the d variable and so is passed to the play command Another type of loop is the while loop which executes for as long as its condition is true The basic form of this loop type is like this gt Zl Y 83484 book Page 51 Monday September 18 2006 8 58 AM Task 1 9 Write Basic Scripts 51 while condition do commands done The until loop is similar in form but it continues execution for as long as its condition is false that is until the condition becomes true Using Functions A function is a part of a script that performs a specific sub task and that can be called by name from other parts of the script Functions are defined by placing parentheses after the function name and enclosing the lines that make up the function within curly braces myf
80. stem to take one of two actions depending on whether some condition is true The i f keyword s conditional expression appears in brackets after the if keyword and can take many forms For instance f file is true if fi le exists and is a regular file s file is true if fi 1e exists and has a size greater than 0 and stringl string2 is true if the two strings have the same values a 4 0 Zl Y 83484 book Page 50 Monday September 18 2006 8 58 AM 50 Phase 1 Working on the Command Line To better understand the use of conditionals consider the following code fragment if s tmp tempstuff then echo tmp tempstuff found aborting exit fi This fragment causes the script to exit if the file tmp tempstuff is present The then key word marks the beginning of a series of lines that execute only if the conditional is true and fi if backwards marks the end of the if block Such code might be useful if the script cre ates and then later deletes this file since its presence indicates that a previous run of the script didn t succeed or is still working An alternative form for a conditional expression uses the test keyword rather than square brackets around the conditional if test s tmp tempstuff You can also test a command s return value by using the command as the condition if command then additional commands fi In this example the addi tional commands will be run only if command completes succe
81. t To change the home directory of Dale Smith s dsmith s account from home2 dsmith to home dsmith you d type the following command usermod d home dsmith a 4 0 Zl Y 83484 book Page 33 Monday September 18 2006 8 58 AM Task 1 7 Use Streams Pipes and Redirection 33 g Don t modify a user account when that user is logged in certain changes are ING likely to wreak havoc with work the user is doing Wait for the user to log out or if a change must be implemented immediately ask the user to log out Changing the account s home directory won t move the files in the directory To do that you must use the cp command In this case the a option to cp will copy the entire directory tree and preserve ownership permissions and other file characteristics You ll then delete the original directory cp a home2 dsmith home dsmith rm r home2 dsmith To be 100 percent safe though you might want to check that the new directory contains all the files it should before deleting the old one If possible wait for the user to log in and use the account before deleting the old directory Criteria for Completion To complete this task you should have created a new account tbrown deleted an old account sjones and changed the home directory location of a third account dsmith including mov ing its files The three commands used to perform this task useradd userdel and usermod are at the core of Linux
82. t you must create it yourself and change its ownership including its group mkdir home2 trevor chown tbrown users home2 trevor Alternatively and preferably you could add the m option to the useradd command This option has the advantage that the system copies a starting set of files from etc skel These files include things such as bash configuration files The useradd command won t ordinarily set a starting password for the account There is a p option to do this but it requires a pre encrypted password The best way to deal with this issue is to create new accounts in the presence of their users you can then as root use o 4 0 Zl Y 83484 book Page 32 Monday September 18 2006 8 58 AM 32 Phase 1 Working on the Command Line passwd to begin the password changing process and allow the users to type their own desired passwords passwd tbrown New UNIX password Retype new UNIX password passwd password updated successful ly You would type passwd tbrown then let the user type a password twice The system doesn t echo the password for security reasons Alternatively you can set the password yourself to some random value and find a way to com municate this value to the user The trouble with this approach is that the communication could be intercepted or copied leading to an immediate security breach Users might also leave their pass words set at the value you give them which is non o
83. t run directly and in the order in which they appear in the script They re run only when called in the main body of the script which in Listing 1 6 con sists of just two lines each corresponding to one function call Criteria for Completion To complete this task you must have created five scripts to perform the five tasks outlined ear lier I recommend that you try some simple variants on these tasks so that you create scripts that perform other tasks for instance you might modify Listing 1 5 so that it displays graph ics files in a directory rather than plays audio files
84. the m mode option between mkdir and the directory name Another method of adjusting permissions is described shortly in Setting File and Directory Permissions Ordinarily mkdir doesn t create directories in the path up to the final directory specified for instance if home didn t exist the preceding command would return an error message Adding the p or parents option though causes mkdir to create intervening directories This can be handy but it also means that if you mistype a directory name say hom instead of home mkdir will merrily create a new directory tree named after your typo a 4 0 Zs 83484 book Page 12 Monday September 18 2006 8 58 AM 12 Phase 1 Working on the Command Line Copying Files Linux s file copying command is cp which is short for copy In its most basic form cp copies a single file from one location to another cp etc fstab home project7 This command copies the etc fstab file to the home project7 directory you ll find the copy there under the same name as the original You can rename the copy as you make it by specifying a filename along with the target directory cp etc fstab home project7 sample file For clarity ve shown the target directory alone with a trailing slash in the first of these examples This indicates that project7 is a directory not a file and will result in an error message if home project7 doesn t exist Linux will accept a dire
85. the Mozilla Firefox browser As part of your diagnosis of this problem you want to use some diagnostic commands that produce copious output In order to do this more easily you will pipe the output streams from these programs into other programs and redirect the output into files that you can peruse later This task uses advanced network diagnostic commands These commands are JTE described in more detail in Phase 6 For now don t be too concerned with what these commands do just concentrate on their input and output streams the redirection of these streams and piping these streams between programs Scope of Task This task demonstrates several tools and techniques that you re likely to use quite heavily as a Linux user and system administrator Each of these commands and techniques is quite pow erful and so can take some time to master but once you ve mastered them they ll become sec ond nature Duration This task should take half an hour or so to complete Once you understand streams pipes and redirection you should be able to use these tools and techniques as a matter of course in a wide variety of commands most of which will take just seconds to type Setup You need a working Linux computer with a network connection and the Mozilla Firefox Web browser You may use another Web browser instead of Firefox but you must then modify the examples appropriately Firefox is an X based GUI Web browser so you ll need t
86. tory tree rm r mwm aa Zs 83484 book Page 13 Monday September 18 2006 8 58 AM Task 1 2 Manage Files and Directories 13 Depending upon your configuration you might or might not be prompted before rm deletes each individual file If you re prompted and don t want to be you can add the f option if you re not prompted but you do want to be you can add the i option gt The contents of etc X11 vary somewhat from one system to another Thus Br you might need to modify these examples on your system Moving and Renaming Files Linux uses a single command to handle both the move and rename operations mv To use this command type it followed by the current name of a file and then the new name or location of the file For instance to rename the home project7 X11 chooser sh file to home project7 X11 chooser you d type this mv home project7 X11 chooser sh home project7 X11 chooser If the target name for the file is a directory mv moves the file to that directory without renaming the file If the target name is a file in a directory other than the original directory mv moves and renames the file You can specify more than one source file but in that case the tar get must be a directory If the source and destination locations for the file are on the same partition mv does its work by rewriting directory entries thus it can operate quite quickly even if it s operating on a large file If you
87. unt data base Linux systems on LANs often refer to another computer for account management On such systems you would ordinarily modify accounts on the password server system instead of on the individual workstations and servers Procedure Before delving into account management tools you should understand the fundamentals of how Linux manages its passwords With a basic understanding in hand you can proceed to the three parts of this task adding an account deleting an account and modifying an account o 4 0 Zl Y 83484 book Page 30 Monday September 18 2006 8 58 AM 30 Phase 1 Working on the Command Line Understanding the Basics of Account Management Linux and Unix systems have traditionally stored account information in the etc passwd file This file consists of a series of lines each of which represents a single account as in dsmith x 512 100 Dale Smith home2 dsmith bin bash This line consists of a series of colon delimited fields that contain the account s username password UID number group ID GID number a free form comment home directory and default shell Note that in this example the password field contains a single x This is a code that means the password is encoded in another file etc shadow Most Linux systems today use this shadow password system as a way of increasing security the etc passwd file must be world readable to enable programs to access information such as the user
88. ven another script The following list includes a few commands that are commonly used in scripts Normal file manipulation commands The file manipulation commands such as 1s mv cp and rm are often used in scripts You can use these commands to help automate repetitive file maintenance tasks grep This command is described earlier in Piping Data between Programs in Task 1 7 It locates files that contain specific strings find Where grep searches for patterns within the contents of files find does so based on file names ownership and similar characteristics This command is described earlier in Task 1 4 cut This command extracts text from fields in a file It s frequently used to extract variable information from a file whose contents are highly patterned To use it you pass it one or more options that control what it cuts followed by one or more filenames For instance users home directories appear in the sixth colon delimited field of the etc passwd file You could there fore include cut f 6 d etc passwd in a script to extract this information sed This program provides many of the capabilities of a conventional text editor but via commands that can be typed at a command prompt or entered in a script echo Sometimes a script must provide a message to the user echo is the tool to accomplish this goal You can pass various options to echo or just a string to be shown to the user For instance echo Press the Ent
89. without a dot or colon If you omit the colon or dot and the group name chown changes only the primary owner of the file or directory The chown command has several options consult its man page for details One of the most useful options though is R which performs a recursive change to a directory and all the files and subdirectories it contains chown R fred users home project7 Setting File and Directory Permissions Earlier in Verifying the Presence of a File in Task 1 1 I described the long form of the 1s com mand 1s 1 which shows additional information on a file Specifically the output looks like this Is 1 usr bin whatis rwxr xr x 1 root root 2409 Nov 19 2004 usr bin whatis The columns in this output are the permissions the number of links to the file described in the next task the owner the group the file size in bytes the file creation date and the file name The permissions string can be perplexing at first It consists of 10 characters The first of these characters is a code for the file type A dash denotes a normal file while various characters stand for special file types Most important d refers to a directory and 1 refers to a symbolic link Other codes include c and b for character and block devices respectively used to access hardware via device files in dev The remaining nine characters in the permissions string represent permissions for the file s owner group and all other

Wiley Linux Administrator Street Smarts: A Real World Guide to Linux Certification Skills

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