Question

Here’s a simple problem that’s been bugging me for some time. I often find I have a number of input files in some directory, and I want to construct output file names by replacing beginning and ending portions. For example, given this:

source/foo.c
source/bar.c
source/foo_bar.c

I often end up writing BASH expressions like:

for f in source/*.c; do
  a="obj/${f##*/}"
  b="${a%.*}.obj"
  process "$f" "$b"
done

to generate the commands

process "source/foo.c"    "obj/foo.obj"
process "source/bar.c     "obj/bar.obj"
process "source/foo_bar.c "obj/foo_bar.obj"

The above works, but its a lot wordier than I like, and I would prefer to avoid the temporary variables. Ideally there would be some command that could replace the beginning and ends of a string in one shot, so that I could just write something like:

for f in source/*.c; do process "$f" "obj/${f##*/%.*}.obj"; done

Of course, the above doesn’t work. Does anyone know something that will? I’m just trying to save myself some typing here.

Answer 1

Not the prettiest thing in the world, but you can use a regular expression to group the content you want to pick out, and then refer to the BASH_REMATCH array:

if [[ $f =~ ^source/(.*).c$ ]] ; then f="obj/${BASH_REMATCH[1]}.o"; fi