Shell Programming

Shell Programming

What is a shell program? • After logging onto the system a prompt for input appears which is generated by a Command String Interpreter program called the shell. The shell: • interprets the input, • takes appropriate action, and • Finally prompts for more input. • The shell can be used either • interactively - enter commands at the command prompt, or • as an interpreter to execute a shell script • Note: Shell scripts are dynamically interpreted, NOT compiled.

Common Shells • C-Shell - csh • The default on teaching systems • Good for interactive systems • Inferior programmable features • Bourne Shell - bsh or sh - also restricted shell - bsh • Sophisticated pattern matching and file name substitution • Korn Shell • Backwards compatible with Bourne Shell • Regular expression substitution • emacs editing mode • Born Again Shell - BASH • Thomas C-Shell - tcsh • Based on C-Shell • Additional ability to use emacs to edit the command line • Word completion & spelling correction

Shell Scripts • For example, a simple C Shell Script in file name list.csh #! /bin/csh ls -al | more • In the above script the first line signals that the file list.csh contains a C Shell Script (csh). • To execute the above script we can either execute it with csh: • csh list.csh • or you can execute it directly from the command line but you must first change the permission of the file list.csh using the chmod command so that it has execute privileges • (chmod +x list.csh).

General Shell Scripts • If the first line is just a # then the script is interpreted by a C shell: # This is a sample C shell script.· echo -n the date today isdate # output today’s date. If the first line is in the form #! PathName, then the executable program PathName is used to interpret the script. · #! /bin/ksh· # This is a sample Korn shell.· echo -n the date today isdate # output today’s date. · If neither of the above rules apply, then the script is interpreted by a Bourne shell. echo -n the date today is· date # output today’s date. # This would be interpreted by the Bourne shell.

C Shell Commands • Variables • The C Shell offers a command "Set" to assign a value to a variable. • For example: set myname= Fred set myname = "Fred Bloggs“ set age=20 A $ sign operator is used to recall the variable values. For example: echo $myname will display Fred Bloggs on the screen A @ sign can be used to assign the integer constant values. For example: @myage=20 @age1=10 @age2=20 @age=$age1+$age2 echo $age

List variables set programming_languages= (C Java) echo $programming _languages C Java set files=*.* set colors=(red blue green) echo $colors[2] blue

Input/Output Commands • Echo command displays information on the screen. • For example: echo I am here will display "I am here" on the screen. $< command reads data from the keyboard For example: set myname = $< waits for data ( name) EXAMPLE write a script that reads name from the keyboard and display on the screen. #!/bin/csh set myname = $< echo $myname

Control Structures • The C Shell offers three loop structures foreach, while and repeat and 3 branching statements if, switch and goto. • foreach .. end • foreach variable name(list) commands end

EXAMPLE foreach color (red yellow green) echo one color is $color end set files = *.csh foreach filename=($files) echo $filename cat $filename end while .. End while (expression) commands end EXAMPLE set count=1 while ($count <= 2) echo $count @ count++ end

repeat repeat (expression) command repeat 3 echo hello

if Statement • The if statement has the following formats: 1) if (exp) command 2) if (exp) then commands endif 3) If (exp) then commands else commands endif

Example set myname = ali If ($myname == ali) then echo "my name is ali" else echo "my name is not ali" endif

switch .. case .. endsw switch (expr) case pattern1: list Breaksw case pattern2: ... Breaksw default: defaultlist Breaksw endsw

Example echo -n "Do you want to delete test.c?“ set answer = $< If ($answer == "yes") then rm test.c else If ($answer =~ a*) then echo -n "Do you really mean that?" set answer = $< If ($answer == "yes") then echo "rm *.*" endif endif

Arithmetic Expressions • ! logical negation • * / % multiplication, division, remainder • + -|| && etc., • Example: set num1=5*6 set num2=6 If (num1>20 && num2==6) then echo $num1 endif @ num1=5*6 set num2=6 if (num1>20 && num2==6) then echo $num1 endif

Aliases • alias dir ‘ls -a’ • unalias dir • Useful Aliases • cd..cd .. • rmrm -i (force interactive mode) • hhistory

Goto goto name ... name:

Example - Menu #!/bin/tcsh echo menu test program set stop = 0 while ($stop == 0) echo 1 : date and time echo 2 : who is logged on echo 3 : exit echo -n 'your choice ' set reply = $< switch ($reply) case 1 : date breaksw case 2 : who breaksw case 3 : set stop=1 breaksw default: echo illegal choice breaksw endsw end

Process Control in the C-Shell • Commands can be invoked in sequence using ; vi file.f ; job2 -? ; job3 • Commands can be invoked in parallel using | ps aux | grep igor | awk “{print $2}” • Commands can be collected into a group (sub-shell) using () (echo Status ; date ; who) > user_log • Commands can be logically related using && or || pc -o myprog.myprog && myprog (invoke pc; if successful, run myprog) pc -c myprog.p || vi myprog.p (invoke pc; if unsuccessful, invoke vi)

Cont. • Commands are loaded into the background using &matlab <input >& errors & warning if background job running at logout • Can keep background job running after logout: nohup matlab <input >& errors & Next login: ps aux | grep your_username if process details are printed, process still alive!

Job control commands: jobs # list jobs ^Z # stops current foreground job· · ^C # abandons current foreground job bg # restarts highest stopped job in # background mode· · bg %2 # restarts job [2] in background mode fg %ls # restarts job ls in foreground mode kill %1 # terminate job· stop %1 # stop job

Example % jobs % ls -R / >& /dev/null ...^Z Stopped % jobs –l [1] + 8646 Stopped ls -R / >& /dev/null % bg[1] ls -R / >& /dev/null & % jobs[1] Running ls -R / >& /dev/null % fg %ls ls -R / >& /dev/null ^Z Stopped % jobs [1] + Stopped ls -R / >& /dev/null % find / -name '*.*' -print >& /dev/null ^Z Stopped

% jobs [1] - Stopped ls -R / >& /dev/null [2] + Stopped find / -name *.* -print >& /dev/null % bg %2 [2] find / -name *.* -print >& /dev/null & % bg %1 [1] ls -R / >& /dev/null & % jobs [1] Running ls -R / >& /dev/null [2] + Running find / -name *.* -print >& /dev/null % kill %fi [2] Terminated find / -name *.* -print >& /dev/nul l% stop %1 % jobs [1] + Stopped (signal) ls -R / >& /dev/null % find / -name '*.*' -print >& /dev/null & [2] 8651 % jobs [1] + Stopped (signal) ls -R / >& /dev/null [2] - Running find / -name *.* -print >& /dev/null % kill %1 % jobs [1] Terminated ls -R / >& /dev/null [2] + Running find / -name *.* -print >& /dev/null % fg %2 find / -name *.* -print >& /dev/null ^C

Shell Programming