Pipe only STDERR through a filter

Question

Is there any way, in bash, to pipe STDERR through a filter before unifying it with STDOUT? That is, I want

STDOUT ────────────────┐
                       ├         


        

Answer 1

A naive use of process substitution seems to allow filtering of stderr separately from stdout:

:; ( echo out ; echo err >&2 ) 2> >( sed s/^/e:/ >&2 )
out
e:err

Note that stderr comes out on stderr and stdout on stdout, which we can see by wrapping the whole thing in another subshell and redirecting to files o and e

( ( echo out ; echo err >&2 ) 2> >( sed s/^/e:/ >&2 ) ) 1>o 2>e