next up previous contents index
Next: C-Shell summary Up: Shell Scripts Previous: Shell Scripts

C Shell Scripts

A shell script is a plain ASCII text file and can hence be created with any text editor (even vi!). It is a list of commands which you would normally have issued yourself on the commandline (quite similar in concept to command files under VMS or batch files under MSDOS). In addition, most UNIX shells have the capability of command flow logic ( goto, if/then/else/endif etc.), retrieving the command line parameters, defining and using (shell) variables etc.

Under the UNIX environment one can also choose which shell to use, although we shall only give examples in the most commonly used shell, the C-shell (csh). As an example, we will show a shell script which copies all files from one directory to a new one, also creating that new directory. The new directory must not exist yet, otherwise the script will fail with an error message. It's usage would look like:

    % csh -f scriptname dir1 dir2 
or shorter:
    % scriptname dir1 dir2
The second form, in which the script is not told through which shell it should process its commands, is the recommended practice. The first line of this script file must then contain the magic line
    #! /bin/csh -f
to denote the script is to be run via the C-shell, which on standard UNIX systems is located in /bin/csh. In this case the script also needs to be made executable, i.e.
    % chmod +x scriptname

In effect your operating system will then issue the first form of the command, The second form has the advantage that you don't have to remember which shell to use, and in the end saves a few keystrokes, always considered a big issue in UNIX!

Now lets look at the full text of the script first:

    #! /bin/csh -f
    #
    # Example of a shell script to copy all files from one directory 
    # to another. The input directory must not contain any subdirectories, 
    # and it will not copy any so-called (hidden) dot-files.
    #
    ##                 	 check if called properly
    if ($#argv != 2) then
        echo "Usage: $0 dir1 dir2"
        echo "copies all files from one directory to another"
        goto done
    endif
    ##                  save command line args in variables
    set dir1=$1
    set dir2=$2
    ##                  check if dir1 indeed is an existing dir
    if (! -d $dir1) then
        echo "$dir1 is not a directory" ; exit 1
    endif
    ##                  check if dir2 does not exist
    if (-e $dir2) then
        echo "$dir2 already exists" ; exit 1
    endif
    ##                  create new dir2
    mkdir $dir2
    if ($status != 0) goto error
    ##                  loop through all files in dir1
    foreach file ($dir1/*)
        if (-d $file) then
            echo "Skipping $file (is a directory)"
        else
            echo "Copying $file"
            cp $file $dir2
        endif
    end
    ##           Labels to jump to exit OK (done) or not OK (error)
    done:
     exit 0
    error:
     exit 1

A few things can be noted:


next up previous contents index
Next: C-Shell summary Up: Shell Scripts Previous: Shell Scripts
Peter Teuben
9/29/2000