Shell Programming: Introduction

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Shell Scripts

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• A shell script (or shell program) is a series of Unix commands placed in an ASCII text file.

• Each shell (ksh, csh, sh, ...) provides mechanisms for control (e.g., statements like if, while, for, ...).

• Unix commands + shell variables + control mechanisms = powerful programming language.

• See Chapter 4 (Nutshell) or Chapter 12 (Sobell) for ksh information.

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Executing Shell Scripts

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• Explicitly choosing shell to use:

      $ sh MyScript
      $ csh MyOtherScript
      $ ksh MyThirdScript
  

• Implicit selection of shell for script:
  • Suppose you have a script named "MyScript".
  • Make the first line of the script start with:

        #!/usr/local/bin/ksh
    

    (You could specify any command after "#!" to act as the "interpreter" for the commands in the script, and you can also give arguments.)
  • Use chmod to make it executable (recall general format
    "chmod [ugoa][+-][rwx] filename"):

        $ chmod a+x MyScript
    

  • Then you can execute MyScript like a regular command:

        $ MyScript
    

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A Simple Example

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• Creating a simple shell script (you could do this with vi):

      $ cat > gohome
      #!/usr/local/bin/ksh
      cd ~
      ls -F
      ^D
  

• Change file permissions:

      $ chmod a+x gohome
  

• Run it:

      $ gohome
  

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Remember Shell Variables?

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• A shell variable can have any name of the form [A-Za-z][A-Za-z0-9_]*.

• For ksh:
  • variable=value
    • Notice that there are no spaces on either side of the equals sign.

  • export variable=value
    • Using export makes the value visible to child processes.

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Using Variables

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• $ person=Terry

• To retrieve the value, preceed the variable name with a dollar sign ($):

      $ echo person
      person
      $ echo $person
      Terry
  

• Unsetting (erasing) variables:

      $ unset person
  

  This is not the same as "person=''", which leaves the variable defined, but with the null value.

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Some Basic Commands

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• For output, use "print" or "echo":

      $ print "Hello World"
      Hello World
  

• At the end of your script, use "return N" to give a specific return value, as in this script:

      #!/usr/local/bin/ksh
      print "Hello World"
      return 0
  

• You can use the built-in variable $? to access the exit status of the previously run command. If the above script were HelloScript:

      $ HelloScript
      $ print $?
      0
  

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If Statements

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• The basic form for if statements in ksh is:

      if condition
      then
          statements
      [elif condition
      then
          statements]
      [else
          statements]
      fi
  

• The then/else/elif/fi keywords take the place of braces ({})---braces mean something special to the shell!

• Examples of condition are (assuming integer a b):

      (( a == 10 ))
      (( b < 20 ))
      [[ $person = steve ]]
      [[ $person != todd ]]
      (( (a < 10) || (a > 100) ))
      [[ ($person != steve ) && ($person != todd ) ]]
  

• The elif or else parts can be omitted.

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Example If Statement

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    if [[ $person = steve ]]
    then
        print $person is on the sixth floor.
    elif [[ $person = todd ]]
    then
	print $person is on the fifth floor.
    elif [[ $person = markus ]]
    then
	print $person is on the fifth floor.
    else
        print "Who are you talking about?"
    fi

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While Statements

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• Basic form:

      while condition
      do
          statements
      done
  

• condition has the same syntax as for if statements.

• You can use break or continue or return inside a loop with the same meaning as in C.

• Example:

      #!/usr/local/bin/ksh
      # Report type of executable file anywhere in search path.
      #
      path=$PATH
      dir=${path%%:*}
      while [[ -n $path ]]; do
        if [[ -x $dir/$1 && ! -d $dir/$1 ]]; then
  	 file $dir/$1
  	 return
        fi
        path=${path#*:}
        dir=${path%%:*}
      done
      print "File not found."
      return 1
  

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Case Selection

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• The basic form of the case statement is:

      case expression in
          pattern1 )
  	    statements ;;
  	pattern2 )
  	    statements ;;
  	.
  	.
  	.
      esac
  

• The statements corresponding to the first pattern matching the expression are executed, after which the case statement terminates.

• The expression is usually some variable's value.

• The patterns can be plain strings, or they can be Korn shell patterns using *, ?, !, [], etc. (like file-matching patterns).

• A pattern can consist of several patterns separated by | (logical or), and the pattern can also be written as (pattern).

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Sample Case

---

    case $person in
        steve)
	    print "He's on the sixth floor." ;;
	todd | markus)
	    print "He's on the fifth floor." ;;
	*)
	    print I do not know $person. ;;
    esac

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For Loops

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• A for loop can be used to iterate over all items in an array or list.

• Basic form:

      for var [in list]
      do
          statements
      done
  

• If in list is omitted in a script, the list is assumed to be $* --- all the arguments to the script.

• Examples:

      for name in Jack Jill John Jane Dick
      do
        print "Next person is $name."
      done
  
      integer count=0
      for arg in $@
      do
        let count='count+1'
        print "argument $count: $arg"
      done
  
      for i in *.for
      do
        mv $i ${i%.for}.f
        print ${i%.for}.f
      done