1. LINUX System : Lecture 6 Shell Programming

2. 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.

3. 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

4. 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.

5. 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

6. 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.

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

8. 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

9. 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

10. 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

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

12. 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

13. 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

14. 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.

15. 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

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

17. 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

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

19. 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

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

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

22. 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

23. 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

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

25. 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)

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

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

28. 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

29. 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

30. 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

31. example
#!/bin/csh set j = ( ) foreach i ($j) echo $i Hello end
C Shell

32. 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

33. 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

34. 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

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

36. 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