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1. o h move one character left o 1 move one character right yes letter 1 moves you right o j move one character down o k move one character up o Backspace move back one character e by Line o O zero move to the beginning of a line o move to the end ofa line o Return first character of next line e by Word o w forward by word o b backward by word o eendof word e by Screen o CTRL f forward one screen ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 15 o CTRL b backward one screen Searching e text search forward for text e text search backward for text e n repeat previous search e N repeat previous search in opposite direction Undoing Changes and Recovery see also section on Saving and Exiting e u undo last change e U restore current line Saving and Exiting e ZZ quit vi and write the file if changes were made e wwrite file e w file save a copy to file e q quit file e q quit file and discard edits e e return to version of current file at time of last write this command and the one previous are useful if you make a serious mistake Note that when you use a command that begins with a colon or you do a search with or the cursor jumps to a command line as shown in the following screen After editing this document I hit Ese to go back into Command Mode and then I typed w Now when I hit Return my changes will be written to dog txt 3 LSOUNIX1 Of x File E2. 1 2 Free ports of most of the shells are available on the Internet Throughout the text we will use parenthetical notes such as this to describe historical notes and to compare different implementations of the UNIX System Often the reason for a particular implementation technique becomes clear when the historical reasons are described 5 5 vi Editor gt Introduction The default editor that comes with the UNIX operating system is called vi visual editor Alternate editors for UNIX environments include pico and emacs a product of GNU gt Working NOTE Both UNIX and vi are case sensitive Be sure not to use a capital letter in place of a lowercase letter the results will not be what you expect ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 13 To start type vi filename at the command line and you ll see a screen like this I typed vi dog txt to create a new file It works the same way with an existing file 3 LSOUNIX1 Pale ES File Edit Transfer Fonts Options Macro View Window Help 3 Sess 2 nhs n Ra be eee ee gt Editing Modes The most important thing to know about Vi and the most confusing is that it has two modes Command Modeand Insert Mode In Command Mode you can invoke editing commands move the cursor save or exit the file invoke the shell or enter Insert Mode In Insert Mode you can insert new text By default vi starts in Command Mode gt Insert M
3. a command line interpreter that reads user input and executes commands Once we log in some system information messages are typically displayed and then we can type commands to the shell program The user input to a shell is normally from the terminal an interactive shell or sometimes from a file called a shell script The common shells in use are summarized in Figure 1 2 Figure 1 2 Common shells used on UNIX systems FreeBSD Mac OS X Name Path 5 2 1 Linux 2 4 22 10 3 Solaris 9 Bourne shell bin sh link to bash link to bash Bourne again shell bin bash optional C shell bin csh link to tesh link to tcsh link to tcsh Korn shell bin ksh TENEX C shell bin tcsh a G The Bourne shell developed by Steve Bourne at Bell Labs has been in use since Version 7 and is provided with almost every UNIX system in existence The control flow constructs of the Bourne shell are reminiscent of Algol 68 The C shell developed by Bill Joy at Berkeley is provided with all the BSD releases Additionally the C shell was provided by AT amp T with System V 386 Release 3 2 and is also in System V Release 4 SVR4 We ll have more to say about these different versions of the UNIX System in the next chapter The C shell was built on the 6th Edition shell not the Bourne shell Its control flow looks more like the C language and it supports additional features that weren t provided by the Bourne shell job control a history mechanism and
4. bytes per block are contained in this directory gt File Permissions UNIX follows a three tiered file protection system that determines a file s access rights It is displayed in the following format Filetype owner rwx groupowner rwx others rwx For Example rwxr xr 1 kumar metal 20500 may 10 19 21 chap02 rwxr xr owner user group owner others The first group has all three permissions The file is readable writable and executable by the owner of the file The second group has a hyphen in the middle slot which indicates the absence of write permission by the group owner of the file The third group has the write and execute bits absent This set of permissions is applicable to others W isfor WRITE If this postion has a w in it then the file h can be changed or deleted by the corresponding user or group to which this set of symbols applies User Group or Others N rwx a i I isfor READ If this position has an r in it then the file can be read by the corre X isfor EXECUTE lf this postion has sponding userid or group to an x in it then the file is considered which this t of symbols applies executable and may be executed by the User Group or Others user or group to which this set of symbols applies User Group or Others The chmod Command We use the chmod command to change the access mode of a file This command comes in many flavors but we ll be talking
5. each of which will be copied into the destination directory Section 2 6 will discuss efficient specification of source files using wildcard characters To copy entire directories including their contents use a recursive copy S cp rd source directories destination directory ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 9 e mv move rename mvis used to rename files directories and or move them from one directory into another Exactly one source and one destination must be specified S mv source destination If destination is an existing directory the new name for source whether it be a file or a directory will be destination source If source and destinationare both files source is renamed destination N B if destinationis an existing file it will be destroyed and overwritten by source you can use the ioption if you would like to be asked for confirmation before a file is overwritten in this way e rm remove delete S rm target file s removes the specified files Unlike other operating systems it is almost impossible to recover a deleted file unless you have a backup there is no recycle bin so use this command with care If you would like to be asked before files are deleted use the i option S rm i myfile rm remove myfile rm can also be used to delete directories along with all of their contents including any subdirectories they contain To do thi
6. features such as allowing users to scroll backwards and forwards through the displayed file less not a standard utility however and may not be present on all UNIX systems e Specifying multiple filenames Multiple filenames can be specified using special pattern matching characters The rules are e matches any single character in that position in the filename gt matches zero or more characters in the filename A on its own will match all files matches all files with containing a e Characters enclosed in square brackets and will match any filename that has one of those characters in that position e A list of comma separated strings enclosed in curly braces and will be expanded as a Cartesian product with the surrounding characters For example matches all three character filenames Pell matches any five character filenames with ell in the middle he matches any filename beginning with he m z a l matches any filename that begins with a letter from m to z and ends in a letter from a to I 5 usr bin lib file expands to usr bin file usr lib file bin file and lib file Note that the UNIX shell performs these expansions including any filename matching on a command s arguments before the command is executed nae led a ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 11 5 4 Concept of Shells A shell is
7. r write w and execute x and the three users categories are the user who owns the file users in the group that the file belongs to and other users the general public An r w or x character means the corresponding permission is oresenit a means it is absent o links refers to the number of filesystem links pointing to the file directory see the discussion on hard soft links in the next section o Owner is usually the user who created the file or directory o group denotes a collection of users who are allowed to access the file according to the group access rights specified in the permissions field o Size is the length of a file or the number of bytes used by the operating system to store the list of files in a directory o dateis the date when the file or directory was last modified written to The u option display the time when the file was last accessed read o name isthe name of the file or directory Is supports more options To find out what they are type S man Is man is the online UNIX user manual and you can use it to get help with commands and find out about what options are supported It has quite a terse style which is often not that helpful so some users prefer to the use the non standard info utility if it is installed ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 8 S info Is 4 e cd change current working directory S cd pa
8. released 2001 Linux 2 4 is released 5 1 1 Unix is available in many flavors like e AIX Advanced IBM Unix e HP UX Hewlett Packard Unix e MINIX Minimal Unix e SCO UNIX e SOLARIS e XENIX e SUNOS e LINUX 5 1 2 Features Of Unix gt Portability As Unix has been re written in C hence it can run on machines from microcomputers to mainframe computers gt Security Unix is a very secure operating system Without the proper username and passwords it is not possible to work on Unix gt Background Processing Unix employs a technique of Background Processing wherein jobs tasks are executed in the background with the minimal interaction from the user ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 2 gt gt 5 1 3 Pipes Using the concept of pipes a Unix user administrator can easily link and work with multiple commands at the same time Redirection Tools These tools allow data to be re directed between files as per the requirement of the user Software Development Tools Unix supports a wide variety of tools using which the user could create and work on different programs viz C Language on the Unix Operating system Unix supports any language that has an interpreter or compiler Stable and Reliable Unix is a very reliable and stable Operating System It is less prone to crashes Easy to write programs Its easy to write programs in Unix as it hides machine architectur
9. the created process The created process itself is called the child of the creating process A process can have multiple child processes but a child process can have only one parent process Every process has a unique process ID or PID The PID is an integer that is assigned by the kernel when the process is created The process with PID O is a special kernel process called the swapper or sometimes called the scheduler which implements the concurrent execution of multiple processes on a single CPU as mentioned above PID 1 is also a special process called init which initializes the system and makes it ready for use by users init is considered the parent of all other processes since it creates them e UNIX command ps The ps command displays active processes The syntax for the ps command is ps options Some useful options i NE a___ iia al processos on emia ih he exception of group andes re oislaealproesses OOO a oessa onis OOO OSS Examples ps ef ps aux Examples ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 23 Typing ps alone would list the current running processes Below is an example of the output that would be generated by the ps command PID TTY TIME CMD 6874 pts 9 0 00 6877 pts 9 0 01 418 field UID PID PPID C STIME TTY TIME CMD description pts 9 0 00 ksh csh csh the user who owns the process the process id a
10. Contents 5 Gee sieeeeemrer ae eer ee titer nerretnrtre teeter E 1 5 1 Introduction to Unix Linux operating SYStEMS ccccccccccceeeeeeeseees 1 5 1 1 Unix is available in many flavors like ccccccceeesseeeseeeeeeees 2 5 1 2 Features Of UNIX ee E T ek 2 Silo UNIX Architecture eeessseesesssssssssesssessseeererersseseereeereeeeeeerereene 3 5 1 4 UNIX fundamentals sssssssssssssssssssssssssssssssssssssssssessees 4 5 2 Concept of file system ssssssycsdnensvandecrwnciineesea nip ountanouantvnnvatineuvenines 5 5 3 Directory and File Handling COMMANAS sccceccceeeeesseeseeeeeees 6 5 3 1 Anatomy of a UNIX Command cccssseseeeeeeceeeeeesseaeeeseeeees 6 54 Concepto Shells arene E Ea 12 5 5 VEJITO tosusetaitatnndsatoincttanstaiiecteinibtataensitatainditatseasieteumiitaneeaibesdainatias 13 gt Edun Modes erene E E 14 5 6 Basic file attributes in Unix chmod COMMANGA cccceeeeeseeeeeeeeee 18 5 7 concept of process working with ps command cccceceeseeeeeeees 21 Some useful options saisswsssnrcrcsersacscassnnicavadverniasetiucsnivanneindeannessccimmeniouieres 23 EXIMPIES a a E E E N 23 5 UnitV 5 1 Introduction to Unix Linux operating systems All operating systems provide services for programs they run Typical services include executing a new program opening a file reading a file allocating a region of memory getting the current time of day and so on Unix is
11. a very popular multi user multitasking time sharing operating system Unix has become the operating system of choice for various engineering and scientific applications The need for Unix could be determined by the different categories of application it suffices viz networking programming multimedia high performance computing to name a few ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 1 Examples of modern UNIX operating systems include IRIX from SGI Solaris from Sun Tru64 from Compaq and Linux from the Free Software community Even though these different flavors of UNIX have unique characteristics and come from different sources they all work alike in a number of fundamental ways Evolution of UNIX 1965 AT amp T GE IBM and Project MAC join together to develop a time sharing system named MULTICS Multiplexed Information and Computing Service 1969 AT amp T Bell Labs drops out of MULTICS project Ken Thompson writes first version of UNICS on a PDP 7 machine UNICS is pun on MULTICS and stands for Uniplexed Information and Computing Services UNICS is changed to UNIX 1973 Re written in high level language C thus making it machine independent 1977 1981 Unix was widely available at minimal cost and became popular for scientific applications 1982 Unix System Ill is released 1984 85 Unix System V is released 1992 93 4 4 BSD is released 1994 Linux 1 0 is
12. command line editing The Korn shell is considered a successor to the Bourne shell and was first provided with SVR4 The Korn shell developed by David Korn at Bell Labs runs on most UNIX systems but before SVR4 was usually an extra cost add on so it is not as widespread as the other two shells It is upward compatible with the Bourne shell and includes those features that made the C shell popular job control command line editing and so on The Bourne again shell is the GNU shell provided with all Linux systems It was designed to be POSIX conformant while still remaining compatible with the Bourne shell It supports features from both the C shell and the Korn shell The TENEX C shell is an enhanced version of the C shell It borrows several features such as command completion from the TENEX operating system ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 12 developed in 1972 at Bolt Beranek and Newman The TENEX C shell adds many features to the C shell and is often used as a replacement for the C shell Linux uses the Bourne again shell for its default shell In fact bin sh is a link to bin bash The default user shell in FreeBSD and Mac OS X is the TENEX C shell but they use the Bourne shell for their administrative shell scripts because the C shell s programming language is notoriously difficult to use Solaris having its heritage in both BSD and System V provides all the shells shown in Figure
13. dit Transfer Fonts Options Macro View Window Help ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 16 Tip If you forget which mode you are in hit Ese to make sure you are in Command Mode You can always re enter Insert Mode and you won t mess up your typing Quick reference for different commands used in vi editor Inserting and typing text a insert text and enter input mode Sa append text to end of line ESC re enter command mode J join lines Cursor movement h left J down k up 1 right beginning of line end of line 1 G top of document G end of document lt n gt G go to line lt n gt k page forward SB page backward W word forwards b word backwards Deleting and moving text Backspace delete character before cursor only works in insert mode X delete character under cursor dw delete word dd delete line restore with p or P lt n gt dd delete n lines ds delete to end of line dG delete to end of file yy yank copy line restore with p or P lt n gt yy yank copy lt n gt lines ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 17 Search and replace s lt search string gt lt replace string gt g Miscellaneous u undo W save file wWq save file and quit ZZ save file and quit ig quit without saving 5 6 Basic file attributes in Unix chmod command gt Basic File Attributes The UNIX
14. doc memo joe refers to the file or directory joe in the directory memo in the directory doc which must be a directory within the working directory From looking just at this pathname we know that both doc and memo have to be directories but we can t tell whether joe is a file or a directory The pathname usr lib lint is an absolute pathname that refers to the file or directory lint in the directory lib in the directory usr which is in the root directory gt Home Directory When we log in the working directory is set to our home directory Our home directory is obtained from our entry in the password file 5 3 Directory and File Handling Commands 5 3 1 Anatomy of a UNIX Command UNIX commands are executed by typing them at the prompt and pressing enter The figure below illustrates the anatomy of a simple unix command ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 6 options usually preceded by a dash ee a oN ls al scratch Gaa U 4 J ge ne command name argument quite often a filename or directory This section describes some of the more important directory and file handling commands e pwd print current working directory pwd displays the full absolute path to the your current location in the filesystem So S pwd usr bin implies that usr bin is the current working directory e Is list directory Is lists the contents of a directory If
15. e from the user Hierarchical File System Unix employs a hierarchical file system which is easy to implement and maintain Shells Unix has different types of shells viz Bourne C Korn etc Communication Unix has commands which allow communication between different users connected to the system UNIX Architecture In a strict sense an operating system can be defined as the software that controls the hardware resources of the computer and provides an environment under which programs can run Generally we call this software the kernel since it is relatively small and resides at the core of the environment Figure 1 1 shows a diagram of the UNIX System architecture Figure 1 1 Architecture of the UNIX operating system applications library routines The interface to the kernel is a layer of software called the system calls the shaded portion in Figure 1 1 Libraries of common functions are built on top of the system call interface but applications are free to use both The shell is a special application that provides an interface for running other applications ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 3 In a broad sense an operating system is the kernel and all the other software that makes a computer useful and gives the computer its personality This other software includes system utilities applications shells libraries of common functions and so on For example Linux is th
16. e kernel used by the GNU operating system Some people refer to this as the GNU Linux operating system but it is more commonly referred to as simply Linux Although this usage may not be correct in a strict sense it is understandable given the dual meaning of the phrase operating system It also has the advantage of being more succinct 5 1 4 UNIX fundamentals UNIX has been around for a long time over 30 years It predates the concept of the personal computer As such it was designed from the ground up to be a multi user shared networked operating environment UNIX has concepts such as Users Groups Permissions and Network Shared Resources such as files printers other computer systems etc built in to the core of its design This makes UNIX a uniquely powerful and flexible operating system Along with this power and flexibility comes some unique concepts that make UNIX what it is These concepts are relatively simple and should be understood to take full advantage of the operating system e Users In order to make use of a UNIX system you must first log in This requires a user account which consists of o Username This is your login name and is how you are identified to the system itself and to other users of the system o Password Along with your username your password grants you access to the system Don t forget or lose your password If you write your password down keep it in a safe place o Default group The default gr
17. file system allows the user to access other files not belonging to them and without infringing on security A file has a number of attributes properties that are stored in the inode There are seven attributes of all files in the current directory and they are 1 File type and Permissions Links Ownership Group ownership File size Last Modification date and time File name The file type and its permissions are associated with each file Links indicate the number of file names maintained by the system This does not mean that there are so many copies of the file File is created by the owner Every user is attached to a group owner File size in bytes is displayed Last modification time is the next field If you change only the permissions or ownership of the file the modification time remains unchanged In the last field it displays the file name S command is used to obtain a list of all filenames in the current directory For example ls Will display gt NNUBWHN total 72 rw r r 1 kumar metal 19514 may 10 13 45 chapO1 rw r r 1 kumar metal 4174 may 10 15 01 chap02 ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 18 rw rw rw 1 kumar metal 84 feb 12 12 30 dept Ist rw r r 1 kumar metal 9156 mar 12 1999 genie sh drwxr xr x 2 kumar metal 512 may 9 10 31 helpdir drwxr xr x 2 kumar metal 512 may 9 09 57 progs The total line tells how many blocks usually 1024
18. for the group for topsecret int We do this by leaving the permissions part of the command empty Before rw r topsecret inf Command chmod g topsecret inf After S sSs5 topsecret inf Open up publicity htmi for reading and writing by anyone Before rw r r publicity html Command chmod og rw publicity html After rw rw rw publicity html Other example Before rw r r publicity html Command chmod u rwx g rx o x publicity html After rwxr x x publicity html 5 7 concept of process working with ps command e UNIX Processes While a program can be defined as an executable file a process is an instance of a program that is being executed by the operating system Some operating systems use the term task instead of process Operating systems that are capable of executing more than one task process at a time are called multi tasking systems The unix kernel which is what the operating system is called provides many access points through which an active process can obtain services from the kernel These are called system calls The standard unix C library provides a C interface to each system call as a result the actual system calls appear as normal C functions to the programmer ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 21 A process consists of several components the executable program code itself referred to as the code or sometimes the tex
19. he filenames that form a pathname described next and the null character terminates a pathname Nevertheless it s good practice to restrict the characters in a filename to a subset of the normal printing characters We restrict the characters because if we use some of the shell s special characters in the filename we have to use the shell s quoting mechanism to reference the filename and this can get complicated gt Two filenames are automatically created whenever a new directory is created called dot and called dot dot Dot refers to the current directory and dot dot refers to the parent directory In the root directory dot dot is the same as dot gt Almost all commercial UNIX file systems support at least 255 character filenames gt Pathname A sequence of one or more filenames separated by slashes and optionally starting with a slash forms a pathname A pathname that begins with a slash is called an absolute pathname otherwise it s called a relative pathname Relative pathnames refer to files relative to the current directory The name for the root of the file system is a special case absolute pathname that has no filename component gt Working Directory Every process has a working directory sometimes called the current working directory This is the directory from which all relative pathnames are interpreted A process can change its working directory with the chdir function For example the relative pathname
20. includes among other possibilities editing and sharing files running software or using printers Even the contents of a UNIX system s display can be manipulated remotely The actions that an individual user is able to perform remotely is defined by the permissions assigned to that user or any group to which the user belongs for each of these activities Some of these things will be discussed in greater detail later on 5 2 Concept of file system Files and Directories File System The UNIX file system is a hierarchical arrangement of directories and files Everything starts in the directory called root whose name is the single character A directory is a file that contains directory entries Logically we can think of each directory entry as containing a filename along with a structure of information describing the attributes of the file The attributes of a file are such things as type of file regular file or directory the size of the file the owner of the file permissions for the file whether other users may access this file and when the file was last modified The stat and fstat functions return a structure of information containing all the attributes of a file ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 5 gt Filename The names in a directory are called filenames The only two characters that cannot appear in a filename are the slash character and the null character The slash separates t
21. no target directory is given then the contents of the current working directory are displayed So if the current working directory is S Is bin dev home mnt share usr var boot etc lib proc sbin tmp vol Actually ls doesn t show youallthe entries in a directory files and directories that begin with a dot are hidden this includes the directories and which are always present The reason for this is that files that begin with a usually contain important configuration information and should not be changed under normal circumstances If you want to see all files ls supports the a option Is a 4 Even this listing is not that helpful there are no hints to properties such as the size type and ownership of files just their names To see more detailed information use the l option long listing which can be combined with the a option as follows S Is a 4 or equivalently ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 7 S Is al 4 Each line of the output looks like this permissions owner type links size where o typeis a single character which is either d directory ordinary file l symbolic link b block oriented device or c character oriented device o permissionsis a set of characters describing access rights There are 9 permission characters describing 3 access types given to 3 user categories The three access types are read
22. ode Pretty straightforward in Insert Mode you can insert text Use the Backspace key to correct errors as you type Before you can do anything in Insert Mode you have to get there and there are several ways to do that e a append new text after cursor e A append new text at end of line e c begin change operation e C change to end of line e iinsert new text before cursor e Tinsert new text at beginning of line e o open anew line below current line e Oopen anew line above current line e R begin overwriting text ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 14 e s substitute a character e S substitute entire line To get back to Command Mode hit the Esc key Command Mode Command mode is a little more complex other than entering Insert Mode you can Move Delete Search Change and Save Commands can be applied to multiple objects For example dd deletes the current line If I type 5dd it deletes 5 lines Changing Text cw change word ce change line r replace character R replace text beginning at cursor Deleting Text dd delete current line D delete the remainder of the line dw delete word dG delete to the end of the file x delete current cursor position X delete back one character Copying and Moving Text e yy copy yank current line e ye copy to end of word e p paste yanked text deleted text can also be pasted Cursor Movement in Command Mode e by Character
23. oup that your username belongs to see Groups below o Contact info So that system administrators and other users can contact you if necessary o Home directory A directory or folder assigned to your username This grants you access to disk storage This is where you will keep your files and data ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 4 o Default shell The program which manages your login and command line sessions covered in detail later e Groups A UNIX group is a collection of users i e a list of usernames Groups provide a mechanism to assign permissions see below to a list of users all at once For our purposes group associations are typically based on which research group or area of study a user is affiliated with Each user can belong to more than one group e Permissions Everything in UNIX is owned by both a user and a group The simplest example of this would be files but this concept is not limited only to files By manipulating permissions the user who owns a file can define which other users and groups can read or modify that file In this way users can secure sensitive files from prying eyes and keep themselves or others from accidentially deleting important data e Shared Resources UNIX is a networked operating environment at its core As such nearly everything that one can access on the local system can also be accessed via the network from remote systems This
24. primarily about one of them chmod who permissions filename ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 19 This gives who the specified permissions for a given filename who The who is a list of letters that specifies whom you re going to be giving permissions to These may be specified in any order Letter Meaning u The user who owns the file this means you g The group the file belongs to o The other users a lall of the above an abbreviation for ugo permissions Of course the permissions are the same letters that you see in the directory listing Permission to read the file Permission to write or delete the file x Permission to execute the file or in the case of a directory search it e chmod Examples Let s change some of the permissions as we discussed a couple of pages ago Here s the way our files are now Is l will give rwxr xr x joe acctg archive sh rw rw r joe acctg orgchart gif rw rw r joe acctg personnel txt rw r r joe acctg publicity html drwxrwxr x joe acctg sales rw r joe acctg topsecret inf rwxr xr x joe acctg wordmatic First let s prevent outsiders from executing archive sh ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 20 Before rwxr xr x archive sh Command chmod o r archive sh After rwxr xr archive sh Take away all permissions
25. s use the r option To avoidrm from asking any questions or giving errors e g if the file doesn t exist you used the f force option Extreme care needs to be taken when using this option consider what would happen if a system administrator was trying to delete user will s home directory and accidentally typed S rm rf home will instead of rm rf home will e cat catenate type S cat target file s displays the contents of target file s on the screen one after the other You can also use it to create files from keyboard input as follows gt is the output redirection operator which will be discussed in the next chapter S cat gt hello txt hello world 4 ctrl d S Is hello txt hello txt S cat hello txt ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 10 hello world e more and less catenate with pause S more target file s displays the contents of target file s on the screen pausing at the end of each screenful and asking the user to press a key useful for long files It also incorporates a searching facility press and then type a phrase that you want to look for You can also use more to break up the output of commands that produce more than one screenful of output as follows is the pipe operator which will be discussed in the next chapter Is more less is just like more except that has a few extra
26. t portion of the process the data on which the program will execute the resources required for the execution such as memory workspace and access to various files and information about the state of the process The data portion contains items such as program variables and their values Among the resources required for execution is memory space which can be divided into two types heap and stack process number program counter a registers memory limits list of open files in the kernel not accessible directly to the user process user context The heap is a portion of memory allocated dynamically as needed at runtime for the use of the process Whenever the malloc or calloc functions are used in C for instance they reserve space in heap memory The stack portion of memory on the other hand is used by the process to aid the invocation of functions Every time a function is called the process reserves a portion of stack memory to store the values of parameters ECS 039 OBJECT ORIENTED SYSTEMS AND C Notes prepared by Diwakar Yagyasen 22 passed to the functions as well as for results returned by the functions and the local variables used within the functions The stack is also where space for all declared data types and structures is reserved at compile time All processes in unix exist in a hierarchy of parent child relationships Any process that creates or spawns another process becomes the parent of
27. th changes your current working directory to path which can be an absolute or a relative path One of the most common relative paths to use is i e the parent directory of the current directory Used without any target directory cd resets your current working directory to your home directory useful if you get lost If you change into a directory and you subsequently want to return to your original directory use cd e mkdir make directory S mkdir directory creates a subdirectory called directoryin the current working directory You can only create subdirectories in a directory if you have write permission on that directory e rmdir remove directory 7 S rmdir directory removes the subdirectory directory from the current working directory You can only remove subdirectories if they are completely empty i e of all entries besides the and directories e cp copy cp is used to make copies of files or entire directories To copy files use S cp source file s destination where source file s and destination specify the source and destination of the copy respectively The behavior of cp depends on whether the destination is a file or a directory If the destination is a file only one source file is allowed and cp makes a new file called destination that has the same contents as the source file If the destination is a directory many source files can be specified
28. unique identifier assigned to each process the parent process id the process that spawned the current process this field is obsolete the start time for the current process the controlling terminal for the current process the amount of CPU time accumulated by the current process the command used to invoke the process Use ps ef to get a full listing of all processes on the system Since there are generally many processes to be listed you ll want to pipe the output into a pager such as more ps ef more UID root root root root root daemon PID PPID He OO Oo SO ECS 039 OBJECT ORIENTED SYSTEMS AND C STIME ITY TIME Sep Sep Sep Sep Sep Sep 18 18 18 18 18 18 LES EES YO OO ES 0 54 00 he 00 00 oo n oO oO 17 CMD sched fetc init pageout fsflush usr lib saf sac t 300 usr lib nfs statd Notes prepared by Diwakar Yagyasen 24
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