Department of Engineering

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Constructions

The shell has loop and choice constructs. These are described in the shell's manual page (type man bash on Linux or MacOS). Here are some examples. Type them into a file to see what they do, or copy-paste the programs onto the command line. Note that after a # the rest of the line isn't executed. These comment lines are worth reading.

        # Example 0 : While loop. Keeping looping while i is less than 10
        # The first line creates a variable. Note that to read a 
        # variable you need to put a '$' before its name
           i=0
           while [ $i -lt 10 ]  
           do
             echo i is $i
             let i=$i+1
           done

        # Example 1 : While loop.
        # This script keeps printing the date. 'true' is a command
        # that does nothing except return a true value.
        # Use ^C (Ctrl-C) to stop it.
           while true 
           do
             echo "date is"
             date
           done

        # example 2: For Loop.
        # Do a letter, word and line count of all the files in
        # the current directory. 
        # The `*' below is expanded to a list of files. The 
        # variable `file' successively takes the value of 
        # these filenames. Preceding a variable name by `$' 
        # gives its value.
           for file in *
           do
             echo "wc $file gives"
             wc $file 
           done

        # Example 3: If.
        # like the above, but doesn't try to run wc on directories
           for file in *
           do
             if [ ! -d $file ] #ie: if $file isn't a directory
             then
               echo "wc $file gives"
               wc $file 
             else
               echo "$file is a directory"
             fi
           done

         # Example 4 : Case - a multiple if
         # Move to the user's home directory
         cd
         for file in .?*
         do
            #Now check for some common filenames.
            case $file in
              .kshrc) echo "You have a Korn Shell set-up file";;  
              .bashrc) echo "You have a Bash Shell set-up file";;
              .Xdefaults) echo "You have an X resource file";;
              .profile) echo "You have a shell login file";;
            esac
         done