Возвращаемое значение функции тайм-аута

Возвращаемое значение timeoutзависит от сигнала, который вы используете для завершения дочернего процесса.

SIGTERMвозвращает 124:

timeout -s SIGTERM 1 sleep 2; echo $?

Но SIGKILLвозвращает 137:

timeout -s SIGKILL 1 sleep 2; echo $?

См. страницу руководства:

If the command times out, and --preserve-status is not set, then exit with status 124. Otherwise, exit with the status of COMMAND. If no signal is specified, send the TERM signal upon timeout. The TERM signal kills any process that does not block or catch that signal. It may be necessary to use the KILL (9) signal, since this signal cannot be caught, in which case the exit status is 128+9 rather than 124.