Bash script – read stdin line by line

Posted by Phil Lavin on Feb 7, 2012 in Blog | 1 comment

This is a really useful bit of code which I use almost daily to speed up routine  tasks. You can pipe text, split into multiple lines, into a while loop and read through them line by line. This is how you do it:

Read a file line by line and echo (reversed) the line:

Source code

cat file.txt | while read line
do
echo $line | rev
done

Read the output of an ls and cat each file through sed:

Source code

ls -1 | while read line
do
cat $line | sed 's/foo/bar/g'
done

You’ll see that you ‘while read XXXX’. $XXXX then becomes the variable that references that line. So in the above example ‘while read line’ produces $line. Naturally, there are better ways to do the above tasks but it hopefully gives you an idea of the power of a while loop in bash.

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