Is it possible to do something along the lines of:
echo ${$(ls)/foo/bar}
I'm pretty sure i saw somewhere working example of something like that but this results in "bad substitution" error.
I know that there are other methods to do that but such a short oneliner would be useful. Am I missing something or is this impossible?
Syntax ${...} only allows referencing a variable (or positional parameter), optionally combined with parameter expansion.
Syntax $(...) (or, less preferably, its old-style equivalent, `...`), performs command substitution, which allows embedding arbitrary commands to whose stdout output the expression expands.
Thus, you could combine the two features as follows:
echo "$(lsOutput=$(ls); echo "${lsOutput//foo/bar}")"
Note the uncomplicated nested use of $(...), which is one of the main advantages over `...`, whose use would require escaping here.
That said, any variables you define inside the command substitution are confined to the subshell that the command runs in anyway, so you could make do with just a command that produces the desired output, given that it is only the stdout output that matters.
echo "$(ls | sed 's/foo/bar/')"