LINUX System : Lecture 6 Shell Programming

1. LINUX System : Lecture 6Shell 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 • 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 #!/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 • 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 • $ 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 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 % expr 10 \* 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 [else commandlist-2] 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 ) commandlist end C Shell

21. switch switch ( str ) case string1: commandlist1 breaksw case string2: commandlist2 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 HarmfulEdsger 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 This is a line.

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: ” set num = $< 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 = (1 2 3 4 5) 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 endif echo “Validation is OK. We continue.” C Shell

34. example #! /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 continue 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 One Four Two Two Three Four Two One Three out.txt in.txt