Classic Shell Scripting

Looping

Besides the if and case statements, the shell's looping constructs are the workhorse facilities for getting things done.

for Loops

The for loop iterates over a list of objects, executing the loop body for each individual object in turn. The objects may be command-line arguments, filenames, or anything else that can be created in list format. In Section 3.2.7.1, we showed this two-line script to update an XML brochure file:

mv atlga.xml atlga.xml.old
sed 's/Atlanta/&, the capital of the South/' < atlga.xml.old > atlga.xml

Now suppose, as is much more likely, that we have a number of XML files that make up our brochure. In this case, we want to make the change in all the XML files. The for loop is perfect for this:

for i in atlbrochure*.xml
do
    echo $i
    mv $i $i.old
    sed 's/Atlanta/&, the capital of the South/' < $i.old > $i
done

This loop moves each original file to a backup copy by appending a .old suffix, and then processing the file with sed to create the new file. It also prints the filename as a sort of running progress indicator, which is helpful when there are many files to process.

The in list part of the for loop is optional. When omitted, the shell loops over the command-line arguments. Specifically, it's as if you had typed for i in "$@":

for i       # loop over command-line args
do
    case $i in
    -f)  ...
         ;;
    ...
    esac
done

while and until Loops

The shell's while and until loops are similar to loops in conventional programming languages. The syntax is:

while condition                until condition