While this code works
#!/bin/bash
d="test_files/*"
for f in $d.{mp3,txt} ;do
do something
done
putting the {mp3,txt} in to a variable does not, see code below.
#!/bin/bash
a={mp3,txt}
d="test_files/*"
for f in $d."$a" ;do
do smoething
done
the output here is /*.{mp3,txt}
Putting {mp3,txt} in to an array
a=({mp3,txt})
outputs only files with the *.mp3 extension.
It doesn't work because brace expansion happens before all other expansions.
From man bash:
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. To avoid conflicts with parameter expansion, the string ‘${’ is not considered eligible for brace expansion
You can use eval to do brace expansion stored in variables, but it is not recommended. For example:
eval echo "$d.$a"
ais an array of two elements, and$aexpands to only the first (it's equivalent to${a[0]}).