bash Cookbook

Branching Many Ways

Problem

You have a series of comparisons to make, and the if/then/else is getting pretty long and repetitive. Isn’t there an easier way?

Solution

Use the case statement for a multiway branch:

case $FN in
    *.gif) gif2png $FN
        ;;
    *.png) pngOK $FN
        ;;
    *.jpg) jpg2gif $FN
        ;;
    *.tif | *.TIFF) tif2jpg $FN
        ;;
    *) printf "File not supported: %s" $FN
        ;;
esac

The equivalent to this using if/then/else statements is:

if [[ $FN == *.gif ]]
then
    gif2png $FN
elif [[ $FN == *.png ]]
then
    pngOK $FN
elif [[ $FN == *.jpg ]]
then
    jpg2gif $FN
elif [[ $FN == *.tif || $FN == *.TIFF ]]
then
    tif2jpg $FN
else
    printf "File not supported: %s" $FN
fi

Discussion

The case statement will expand the word (including parameter substitution) between the case and the in keywords. It will then try to match the word with the patterns listed in order. This is a very powerful feature of the shell. It is not just doing simple value comparisons, but string pattern matches. We have simple patterns in our example: *.gif matches any character sequence (signified by the *) that ends with the literal characters .gif.

Use |, a vertical bar meaning logical OR, to separate different patterns for which you want to take the same action. In the example above, if $FN ends either with .tif or .TIFF then the pattern will match and the (fictional) tif2jpg command will be executed.

Use the double semicolon to end the set of statements or else bash will continue executing into the next set of statements.

There is no else or default keyword ...