gawk versus mawk: BEGIN block handling of empty piped input

Answer

Both gawk and mawk execute the BEGIN block before any input is read and the END block after all input has been processed, even when the input stream is empty. Consequently, no pattern‑action (rule) block is run for lack of records.

Confirmed facts

• BEGIN runs first in both implementations.
• END runs last in both implementations.
• With empty input (e.g., echo -n | … or piping /dev/null) there are zero records, so the pattern‑action block is never entered.
• Redefining FS or RS inside BEGIN has no observable effect when no records are read.

Steps to verify

1. Run with gawk:

echo -n | gawk 'BEGIN{print "BEGIN"} {print $0} END{print "END"}'

Expected output:

BEGIN
END

2. Repeat with mawk:

echo -n | mawk 'BEGIN{print "BEGIN"} {print $0} END{print "END"}'

Expected output is identical.

3. Test FS redefinition:

echo -n | gawk 'BEGIN{FS=":"} {print $1} END{print "done"}'

Only done appears after END; no middle line.

Version note

Older mawk releases (pre‑1.3.4) were reported to skip the END block on empty input. If you must support legacy systems, check your mawk version:

mawk -W version 2>/dev/null || mawk --version

If the version is older than 1.3.4, consider upgrading or adding an explicit END guard.

Missing diagnostic detail

If you are targeting a system with mawk < 1.3.4, please confirm the exact mawk version in use, as this changes the recommendation (you may need to rely on an explicit END block or upgrade).