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

The last part of this page of the Advanced Bash Scripting Guide is "redirecting only stderr to a pipe".

# Redirecting only stderr to a pipe.

exec 3>&1                              # Save current "value" of stdout.
ls -l 2>&1 >&3 3>&- | grep bad 3>&-    # Close fd 3 for 'grep' (but not 'ls').
#              ^^^^   ^^^^
exec 3>&-                              # Now close it for the remainder of the script.

# Thanks, S.C.

This may be what you want. If not, some other part of the ABSG should be able to help you, it is excellent.