BASH Expression to replace beginning and ending of a string in one operation?

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 directly in bash. You can use sed, of course:

b="$(sed 's|^source/(.*).c$|obj/$1.obj|' <<< "$f")"

Answer 2

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

Answer 3

you shouldn't have to worry about your code being "wordier" or not. In fact, being a bit verbose is no harm, consider how much it will improve your(or someone else) understanding of the script. Besides, for performance, using bash's internal string manipulation is much faster than calling external commands. Lastly, you are not going to retype your commands every time you use it right? So why worry that its "wordier" since these commands are already in your script?

Answer 4

Why not simply using cd to remove the "source/" part?

This way we can avoid the temporary variables a and b:

for f in $(cd source; printf "%s\n" *.c); do
   echo process "source/${f}" "obj/${f%.*}.obj"
done