What does exit code 0 mean?

Exit code 0 means success — the command completed without error. By Unix convention any non-zero value indicates failure, which is why shell tests like if command; then treat 0 as true.

What does exit code 127 mean?

Exit code 127 means command not found. The shell could not locate the program in PATH, usually because of a typo, a missing binary, or PATH not including the directory. Exit code 126 is related: the file was found but is not executable.

What does exit code 137 mean?

Exit code 137 is 128 + 9, meaning the process was killed by SIGKILL (signal 9). This is frequently the Linux out-of-memory killer terminating a process, or a forced kill -9. Check memory limits and system logs.

Are exit codes the same as errno?

No. An exit code is the single byte a process returns when it finishes (0-255), reported to its parent. errno is an in-process integer set by system calls to describe the last error. A program may translate an errno into an exit code, but they are different mechanisms.

Is anything I search uploaded?

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What is the Unix Exit Codes Reference?

Reference for Unix process exit codes including POSIX conventions, bash special codes (126, 127, 128+N) and the sysexits.h values (64-78). Searchable and runs entirely in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Unix Exit Codes Reference

Name: Unix Exit Codes Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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This is a searchable reference for Unix and Linux process exit codes — the single byte a program returns when it finishes. It combines the POSIX convention, the special codes bash assigns (126, 127, and the 128+N “killed by signal” range), and the BSD sysexits.h registry (64–78) so you can decode an exit status from any of these worlds.

How it works

An exit status is one byte, so it ranges from 0 to 255. The shell exposes it as $? after each command. Zero means success and every other value means failure. Three layers define the meanings: POSIX reserves 0 for success and treats non-zero as error; bash assigns 126 (found but not executable), 127 (command not found), and 128 + N (terminated by signal N); and sysexits.h proposes a structured set from EX_USAGE (64) to EX_CONFIG (78) for programs that want consistent, meaningful codes. Search by number, source or keyword to find the matching entry and its explanation.

Decoding signal deaths (128 + N)

When a process is killed by a signal, the shell reports 128 + signal. The most common ones you will encounter:

Exit code	Signal	Typical cause
130	SIGINT (2)	User pressed Ctrl+C
137	SIGKILL (9)	Forced kill, often the Linux OOM killer
139	SIGSEGV (11)	Segmentation fault
143	SIGTERM (15)	Clean termination request (e.g. systemd stop)
129	SIGHUP (1)	Terminal disconnect

To find the signal from any 128+ code: subtract 128. Then look up that signal number to understand what sent it. Exit 137 in a container environment almost always means the container ran out of memory and the OOM killer terminated the process — check your memory limits and the host’s dmesg.

The sysexits.h codes (64–78)

BSD’s sysexits.h header defines structured exit codes for programs to use instead of ad-hoc numbers. Common ones used by sendmail, ssh, and many Unix utilities:

Code	Name	Meaning
64	EX_USAGE	Command line usage error
65	EX_DATAERR	Bad input data
66	EX_NOINPUT	Cannot open input
67	EX_NOUSER	Address unknown
68	EX_NOHOST	Host name unknown
69	EX_UNAVAILABLE	Service unavailable
70	EX_SOFTWARE	Internal software error
71	EX_OSERR	System error
73	EX_CANTCREAT	Can’t create (user) output file
75	EX_TEMPFAIL	Temp failure; user can retry
78	EX_CONFIG	Configuration error

Practical debugging example

Reading $? after a failed command:

$ ./missing-binary
bash: ./missing-binary: No such file or directory
$ echo $?
127

Exit code 127 confirms the shell could not find the command — fix the path or install the binary.

$ cat protected-file.txt
cat: protected-file.txt: Permission denied
$ echo $?
1

Exit code 1 is a generic failure. It tells you the command ran but failed — cat itself does not use sysexits, so the full error message is what carries meaning here.

For CI/CD pipelines and shell scripts, always check $? or use set -e to abort on non-zero exits. In Docker and Kubernetes, the exit code of PID 1 becomes the container exit code and appears in kubectl describe pod or docker inspect.

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