How to use both a variable and a changed variable in the same command

Question

in simple I want to execute one command over many files and am trying to do that with a for do done loop.

dwebp foo.webp -o ./PNGs/foo.png is the command that I want to execute over all files that correspond to *.webp in my current directory.

I am able to get the /PNGs/foo.png working by doing

for f in *.webp; do echo "$f" "${f%.webp}.png"; done. I'm now however not able to use $f as the original file name (which is still needed).

The fact that I am using the ${f%.webp}.png without really understanding why or how it works doesn't help... So, does anyone know how I'd be able to run the dwebp foo.webp -o ./PNGs/foo.png command with a for do done loop, or a better way to achieve the same thing?

"${f%.webp}.png" doesn't change f's value. You can still use $f as the original filename. — oguz ismail
– oguz ismail, Commented Jan 12, 2021 at 15:48

markp-fuso · Accepted Answer · 2021-01-12 15:53:34Z

1

For an explanation on how ${f%.webp} works, see Parameter Expansion.

${f%.webp} does not modify the value of ${f}, consider:

$ f=foo.webp
$ echo "${f%.webp}.png"
foo.png
$ echo "${f}"
foo.webp

Net result is that you could try something like:

for f in *.webp
do
    echo "${f}"
    dwebp "${f}" -o "./PNGs/${f%.webp}.png"
done

Or if you'll need to perform the same parameter expansion a few times, you can do it once and store in a new variable, eg:

for f in *.webp
do
    newf="${f%.webp}.png"
    echo "original file : ${f} / new file : ${newf}"

    dwebp "${f}" -o "./PNGs/${newf}"
done

answered Jan 12, 2021 at 15:53

markp-fuso

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