Question 1

What does exit code 137 mean?

Accepted Answer

Exit code 137 = 128 + 9 = SIGKILL — the process was forcibly killed. In Docker, 137 almost always means the container hit its memory limit and the kernel OOM-killer sent SIGKILL. Outside Docker, you may have run `kill -9 <pid>`. SIGKILL cannot be caught, blocked, or ignored, so the process is terminated immediately by the kernel.

Question 2

How does the 128+N signal rule work?

Accepted Answer

When a Unix process is terminated by a signal `N`, the parent shell reports the exit status as `128 + N`. So 130 = 128 + 2 = SIGINT (Ctrl+C), 139 = 128 + 11 = SIGSEGV (segfault), 143 = 128 + 15 = SIGTERM. Note: this rule only applies to signal-terminated processes — your own `exit 137` does NOT mean SIGKILL, it just exits with the integer 137.

Question 3

What are the reserved bash exit codes I should not use?

Accepted Answer

Avoid 0 (success), 1 (general error), 2 (misuse of builtins), 126 (not executable), 127 (command not found), 128 (invalid exit argument), 128+N (signal termination, N=1..31), and 255 (out-of-range). The sysexits.h range 64–78 is also reserved by convention (EX_USAGE … EX_CONFIG). For your own scripts, pick codes in 3–63 or 79–125 to avoid collisions.

Question 4

Why does exit 256 still return 0?

Accepted Answer

Unix exit codes are a single unsigned byte (0–255). Any integer passed to `exit` is taken modulo 256: `exit 256` becomes 0, `exit 257` becomes 1, `exit -1` becomes 255. This is why 255 is often shown as 'out-of-range' — it is what you get from any negative or >255 input. The tool explains this wrap-around for every code above 128.

Question 5

What extra features does this tool have compared to scattered blog posts?

Accepted Answer

(1) Decode any code 0–255 with the 128+N signal rule auto-applied. (2) Complete signal table — number, name, default action, catchable/ignorable flags. (3) Reverse lookup by signal name (SIGKILL → 9) or by number (9 → SIGKILL). (4) sysexits.h reference (EX_USAGE 64 … EX_CONFIG 78) with usage notes. (5) Reserved-range guide + safe custom-code suggestion. (6) Bash scripting snippets ($?, trap, set -e, set -o pipefail, command || exit). (7) Docker OOM 137 callout. (8) Day-of-week vs day-of-month-style confusions (SIGINT 130 vs SIGTERM 143 vs SIGKILL 137). (9) Code-wrap explanation (mod 256). (10) Searchable both ways (code → meaning, signal name ↔ number). (11) Shareable deep-link to any code or signal. (12) Recent history (localStorage, last 20). 100% client-side — no ads, no tracking.

Exit Code & Signal Reference

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Choosing your own exit codes

How to use

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Choosing your own exit codes