Amazon.com Widgets

Safer in-place edit?

Sometimes I need to edit a file in batch mode. Say you have /etc/default/foo and you want to change BAR="-c5" to BAR="-c4". One of the popular methods is using sed in-place edit, like this:

sed -i -e 's/BAR="-c5"/BAR="-c4"/' foo

If your version of sed is antiquated and doesn’t support in-place editing you can do the same using perl:

perl -i -p -e 's/BAR="-c5"/BAR="-c4"/' foo

The problem with this approach is that if the needle doesn’t exist, sed and perl will silently skip doing the replacement. For example if upstream changes BAR="-c5" to BAR="-c3" then your sed/perl will happily and silently leave it at -c3 instead of changing to -c4 and most probably you will not notice until much later.

One can use patch instead, but it is somewhat clumsy to save a diff file with at least 5 lines in it, just to change one character in the input file…

There’s always the possibility to add some more perl code and make it:

perl -i -pe 's/BAR="-c5"/BAR="-c4"/ and $e|=1; END{exit(!$e)}' foo

I feel this is hackish and somewhat error prone though.

What’s your solution?

Leave a Reply

Spam Protection by WP-SpamFree