LINUX System : Lecture 6 Shell Programming

Extension of Functionality UNIX is designed so that users can extend the functionality To build new tools easily and efficiently To customize the shell and user interface. To string together a series of Unix commands to create new functionality. To create custom commands that do exactly what we want.

Shell Command Interpreter that turns text that you type (at the command line) in to actions User Interface: take the command from user Shell Programming We often want to do a number of commands together And bundle them up into one new command. Just like a batch file in MS-DOS

Shell scripts Any collection of shell commands can be stored in a file called a shell script. Scripts have variables and flow control statements like other programming languages.

Popular Shells sh Bourne Shell ksh Korn Shell bash Bourne-Again Shell csh,tcsh C Shell (for this course) Shell scripts among those shells are slightly different

shell script Creating a simple shell script Run a shell script A shell script is a file that contains commands that the shell can execute. Any commands you enter in response to a shell prompt. A utility A compiled program Another shell script Control flow commands Run a shell script Enter the script filename on the command line The shell interprets and execute the commands one after another Why shell script? Simply and quickly initiate a complex series of tasks or a repetitive procedure.

Shell script example C Shell #!/bin/csh echo “Current Time - `date`” echo I am `whoami` C Shell

Invoking scripts There are two ways to launch scripts: 1) Direct interpretation csh scriptfile [args …] 2) Indirect interpretation The first line of the file must be #!/bin/csh and the file must be executable (permission). C Shell

Shell Variables Environment Variables (Global) environment variable Used to provide information to programs (Global) environment variable New programs and shells inherit environment variables from their parent shell (Local) shell variable Used only by that shell Not passed to other processes

Environment Variables “env” or “printenv” command Display current environment variables DISPLAY The graphical display to use, e.g. nyssa:0.0 EDITOR The path to your default editor, e.g. /usr/bin/vi GROUP Your login group, e.g. staff HOME Path to your home directory, e.g. /home/frank HOST The hostname of your system, e.g. nyssa IFS Internal field separators, usually any white space (defaults to tab, space and <newline>) LOGNAME The name you login with, e.g. frank PATH Paths to be searched for commands, e.g. /usr/bin:/usr/ucb:/usr/local/bin PS1 The primary prompt string, Bourne shell only (defaults to $) PS2 The secondary prompt string, Bourne shell only (defaults to >) SHELL The login shell you're using, e.g. /usr/bin/csh TERM Your terminal type, e.g. xterm USER Your username, e.g. frank

Set Shell Variables Mostly set automatically when log in setenv set $ setenv NAME value # in C Shell set $ set name = value # in C Shell

Variables To set variables: set X [= value] # processed as a string To unset variables : unset X Variable contents are accessed using ‘$’: echo $PATH C Shell

Array C Shell To create lists: set Y = (abc 1 123) To set a list element: set Y[2] = 3 To view a list element: echo $Y[2] To count the number of variable elements: echo $#Y set fname = prog1 rm ${fname}.c C Shell

Built-in Variables $user -- who am I? $path -- my execution path (list of directories to be searched for executables) $term -- what kind of terminal I am using $status -- a numeric variable, usually used to retun error codes $prompt -- what I am currently using for a prompt $shell -- which shell am I using (usu. either /bin/csh or /bin/sh) % set Will display the variable lists.

Arithmetic (@) command C shell provides arithmetic operaters must be used with the arithmetic (@) command Arithmetic command works only with integers. set count = 5 @ count += 2 echo $count 90

Shell Arithmetic expr op1 math-operator op2 Example % expr 1 + 3 % set A = `expr 3 + $B`

Command arguments A shell script to swap files: #! /bin/csh –f set tmp = $argv[1] cp $argv[2] $argv[1] cp $tmp $argv[2] Arguments : $argv The number of arguments to a script: $#argv -f option says we want fast startup (no read .cshrc) . C Shell

if-then-else if ( expr ) simple-command if ( expr ) then commandlist-1 endif C Shell

if-then-else cont’d An example: if ($#argv != 2) then echo “we need two parameters!“ else set name1 = $argv[1] set name2 = $argv[2] endif C Shell

Loops while ( expr ) commandlist end foreach var ( worddlist ) C Shell

switch C Shell switch ( str ) case string1: commandlist1 breaksw default commandlist endsw C Shell

goto (Considered harmful!) To jump unconditionally: goto label A label is a line such as: label: The classic paper on why not to use goto: Go To Statement Considered Harmful Edsger W. Dijkstra, CACM, March 1968 C Shell

shift command Moves the values in the parameters toward the beginning of the parameter list #!/bin/csh –f echo “There are” $#argv “parameters\n” while ($#argv > 0) echo –n “$argv[1] “ shift end echo “\n” echo “There are now” $#argv “parameters” echo “end of script” C Shell

Input Reading Line by Line % set x = $< This is a line. % echo $x

File Operators -e file : True if file exists -r file : True if file is readable -l file : True if file exists and is a symbolic link -w file : True if file exists and is writable -x file : True if file exists and is executable -o file : True if the user owns it -f file : True if the file exists and is a regular file -d file : True if the file exists and is a directory -s file : True if file exists and has a size greater than zero -z file : True if file length is zero (empty)

Logical operator ! : NEGATE && : logical AND || : logical OR Ex) if (! -e somefile) then # does not exist

Debugging %csh –vx somescript args -v : vervose -x : echoes the commands after all substitutions are made -n : syntax check. No execution

example #!/bin/csh if (-e $argv[1]) then echo $argv[1] exists else echo $argv[1] does not exist and cannot be opened endif # rest of script here C Shell

example #!/bin/csh set sum = 0 echo –n “Enter a number: ” set num = $< while ($num != “”) @ sum += $num echo –n “Enter the next number: ” end echo “\nThe sum of the number is : $sum” C Shell

Guidelines Shell script is better than C program if the problem can be solved by using UNIX commands Why script? Easier to create and modify Easy to debug Good thing to do Use redirection and pipe Do validity check (argument number , type) Check existence of files and directories Display error messages

example #!/bin/csh set j = (1 2 3 4 5) foreach i ($j) echo $i Hello end C Shell

example #!/bin/csh set ary = `cat ary.dat` echo “The whole array : $ary” echo “The number of elements : $#ary” echo “The first element: $ary[1]” echo “The last element: $ary[$#ary]” C Shell

Numeric validation example #!/bin/csh echo $argv[2] > temp grep ‘^[0-9]*$’ temp > /dev/null if ($status != 0) then echo “Month argument is not numeric” exit 1 endif if ($argv[2] < 1 || $argv[2] > 12) then echo “Month argument must be <1…12>” exit 2 echo “Validation is OK. We continue.” C Shell

example C Shell #! /bin/csh -f foreach name ($argv) if ( -f $name ) then echo -n "delete the file '${name}' (y/n/q)?" else echo -n "delete the entire dir '${name}' (y/n/q)? " endif set ans = $< # $< means “read a line” switch ($ans) case n: continue case q: exit case y: rm -rf $name endsw end: C Shell

Exercise 1 Write a shell script that displays the number of files and directories in a given directory format % ./fd_count.csh directory_name

Exercise 2 Write a shell script that removes duplicate words from an input text file. Format % remove_dup.csh in.txt out.txt Four Two One Three Four Two One Three out.txt in.txt