Things you learn the Hard Way

2007-08-13 by Thomas Fischer [en]

There are things in life that you learn the hard way. The same holds true for Linux and, in the following example, for bash scripts.

Lets have a look on the following two code examples:

counter=0
while read line ; do
    echo $line | grep a >/dev/null || continue
    counter=$(( $counter + 1 ))
done < test.txt
echo $counter

counter=0
grep a < test.txt | while read line ; do
    counter=$(( $counter + 1 ))
done
echo $counter

Apple
Banana
Orange
Cheese

As it turns out, in the second case the variable counter is always reset to 0, regardless of the hits in test.txt, whereas the first example has a final result of 2 as expected. It seems, that changes to counter do not propagate to the world outside the while-do-loop, if the output of grep is piped into the loop.
I'm not sure why this happens, but it can be reproduced both on a Gentoo and a Fedora box.

Although the examples above are artificial, the problem occurred within a larger script and it took me three days to both notice that the problem exists and to find the reason...

Keywords: Linux, University
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