bash Cookbook

Looping with a read

Problem

What can you do with a while loop? One common technique is to read the output of previous commands. Let’s say you’re using the Subversion revision control system, which is executable as svn. (This example is very similar to what you would do for cvs as well.) When you check the status of a directory subtree to see what files have been changed, you might see something like this:

$ svn status bcb
M      bcb/amin.c
?      bcb/dmin.c
?      bcb/mdiv.tmp
A      bcb/optrn.c
M      bcb/optson.c
?      bcb/prtbout.4161
?      bcb/rideaslist.odt
?      bcb/x.maxc
$

The lines that begin with question marks are files about which Subversion has not been told; in this case they’re scratch files and temporary copies of files. The lines that begin with an A are newly added files, and those that begin with M have been modified since the last changes were committed.

To clean up this directory it would be nice to get rid of all the scratch files, which are those files named in lines that begin with a question mark.

Solution

Try:

svn status mysrc | grep '^?' | cut -c8- | \
    while read FN; do echo "$FN"; rm -rf "$FN"; done

or:

svn status mysrc | \
while read TAG FN
do
    if [[ $TAG == \? ]]
    then
        echo $FN
        rm -rf "$FN"
    fi
done

Discussion

Both scripts will do the same thing—remove files that svn reports with a question mark.

The first approach uses several subprograms to do its work (not a big deal in these days of gigahertz processors), and would fit on a single line in a typical terminal window. It uses grep to select only the ...