Appendix E. Exit Codes With Special Meanings
Table E-1. Reserved Exit Codes
| Exit Code Number | Meaning | Example | Comments |
|---|---|---|---|
| 1 | Catchall for general errors | let "var1 = 1/0" | Miscellaneous errors, such as "divide by zero" and other impermissible operations |
| 2 | Misuse of shell builtins (according to Bash documentation) | empty_function() {} | Missing keyword or command, or permission problem (and diff return code on a failed binary file comparison). |
| 126 | Command invoked cannot execute | /dev/null | Permission problem or command is not an executable |
| 127 | "command not found" | illegal_command | Possible problem with $PATH or a typo |
| 128 | Invalid argument to exit | exit 3.14159 | exit takes only integer args in the range 0 - 255 (see first footnote) |
| 128+n | Fatal error signal "n" | kill -9 $PPID of script | $? returns 137 (128 + 9) |
| 130 | Script terminated by Control-C | Ctl-C | Control-C is fatal error signal 2, (130 = 128 + 2, see above) |
| 255* | Exit status out of range | exit -1 | exit takes only integer args in the range 0 - 255 |
According to the above table, exit codes 1 - 2, 126 - 165, and 255 [1] have special meanings, and should therefore be avoided for user-specified exit parameters. Ending a script with exit 127 would certainly cause confusion when troubleshooting (is the error code a "command not found" or a user-defined one?). However, many scripts use an exit 1 as a general bailout-upon-error. Since exit code 1 signifies so many possible errors, it is not particularly useful in debugging.
There has been an attempt to systematize exit status numbers (see /usr/include/sysexits.h), but this is intended for C and C++ programmers. A similar standard for scripting might be appropriate. The author of this document proposes restricting user-defined exit codes to the range 64 - 113 (in addition to 0, for success), to conform with the C/C++ standard. This would allot 50 valid codes, and make troubleshooting scripts more straightforward. [2] All user-defined exit codes in the accompanying examples to this document conform to this standard, except where overriding circumstances exist, as in Example 9-2.
 | Issuing a $? from the command-line after a shell script exits gives results consistent with the table above only from the Bash or sh prompt. Running the C-shell or tcsh may give different values in some cases. |