Using sed and grep/egrep to search and replace

Use this command:

egrep -lRZ "\.jpg|\.png|\.gif" . \
    | xargs -0 -l sed -i -e 's/\.jpg\|\.gif\|\.png/.bmp/g'

• egrep: find matching lines using extended regular expressions

  • -l: only list matching filenames

  • -R: search recursively through all given directories

  • -Z: use \0 as record separator

  • "\.jpg|\.png|\.gif": match one of the strings ".jpg", ".gif" or ".png"

  • .: start the search in the current directory

• xargs: execute a command with the stdin as argument

  • -0: use \0 as record separator. This is important to match the -Z of egrep and to avoid being fooled by spaces and newlines in input filenames.

  • -l: use one line per command as parameter

• sed: the stream editor

  • -i: replace the input file with the output without making a backup

  • -e: use the following argument as expression

  • 's/\.jpg\|\.gif\|\.png/.bmp/g': replace all occurrences of the strings ".jpg", ".gif" or ".png" with ".bmp"

Honestly, much as I love sed for appropriate tasks, this is definitely a task for perl -- it's truly more powerful for this kind of one-liners, especially to "write it back to where it comes from" (perl's -i switch does it for you, and optionally also lets you keep the old version around e.g. with a .bak appended, just use -i.bak instead).

perl -i.bak -pe 's/\.jpg|\.png|\.gif/.jpg/

rather than intricate work in sed (if even possible there) or awk...