Exit Codes With Special Meanings

1

Catchall for general errors

Example: let "var1 = 1/0"

Miscellaneous errors, such as "divide by zero" and other impermissible operations

2

Misuse of shell builtins (according to Bash documentation)

Example: empty_function() {}

Missing keyword or command, or permission problem (and diff return code on a failed binary file comparison).

126

Command invoked cannot execute

Example: /dev/null

Permission problem or command is not an executable

127

"command not found"

Possible problem with $PATH or a typo

128

Invalid argument to exit

Example: exit 3.14159

exit takes only integer args in the range 0 - 255 (see first footnote)

128+n

Fatal error signal "n"

Example: kill -9 $PPID

$? returns 137 (128 + 9)

130

Script terminated by Control-C

Example: Ctl-C

Control-C is fatal error signal 2, (130 = 128 + 2, see above)

255*

Exit status out of range

Example: 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 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. [1]

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. All user-defined exit codes in the accompanying examples to this document conform to this standard, except where overriding circumstances exist, as in TODO Example 9-2. [2]