1. Shell Programming

2. 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.

3. 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

4. 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).

5. 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.

6. 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.
echo $myname
will display Fred Bloggs on the screen
sign can be used to assign the integer constant values.
@myage=20
@age1=10
@age2=20
@age=$age1+$age2
echo $age

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

8. 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
set myname = $<
waits for data ( name)
EXAMPLE
write a script that reads name from the keyboard and display on the screen.
#!/bin/csh
echo $myname

9. 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

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

11. repeat
repeat (expression)
command
repeat 3
echo hello

12. 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     
endif

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

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

15. 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

16. 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
if (num1>20 && num2==6)  then  
echo $num1

17. Aliases alias dir ‘ls -a’ unalias dir Useful Aliases cd.. cd ..
rm rm -i (force interactive mode)
h history

18. Goto
goto name
...
name:

19. 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
case 3 :
set stop=1       
default:
echo illegal choice  
breaksw  
endsw
end

20. 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)

21. 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!

22. 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

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

24. % 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 &
[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
[1] + Stopped (signal) ls -R / >& /dev/null
% find / -name '*.*' -print >& /dev/null &
[2] 8651
[2] - Running find / -name *.* -print >& /dev/null
% kill %1
[1] Terminated ls -R / >& /dev/null
% fg %2
find / -name *.* -print >& /dev/null ^C