gawk: Exit Status
2.6 'gawk''s Exit Status
========================
If the 'exit' statement is used with a value (Exit Statement),
then 'gawk' exits with the numeric value given to it.
Otherwise, if there were no problems during execution, 'gawk' exits
with the value of the C constant 'EXIT_SUCCESS'. This is usually zero.
If an error occurs, 'gawk' exits with the value of the C constant
'EXIT_FAILURE'. This is usually one.
If 'gawk' exits because of a fatal error, the exit status is two. On
non-POSIX systems, this value may be mapped to 'EXIT_FAILURE'.