Apply filter to STDERR in Linux

STDOUT ────────────────┐
                       ├─────> terminal/file/whatever
STDERR ── [ filter ] ──┘

Method

If ./a.out outputs as below

In STDERR:

stderr output

In STDOUT:

more regular

Then the following command will output as below.

# ./a.out 3>&1 1>&2 2>&3 3>&- | sed 's/e/E/g'
more regular
stdErr output

Explanation

    First save stdout as &3 (&1 is duped into 3)
    Next send stdout to stderr (&2 is duped into 1)
    Send stderr to &3 (stdout) (&3 is duped into 2)
    close &3 (&- is duped into 3)

References

Pipe only STDERR through a filter