Advanced Bash-Scripting Guide:
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Appendix C. Exit Codes With Special Meanings

Table C-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"
`2`	misuse of shell builtins, according to Bash documentation		Seldom seen, usually defaults to exit code 1
`126`	command invoked cannot execute		permission problem or command is not an executable
`127`	"command not found"		possible problem with `$PATH` or a typo
`128`	invalid argument to exit	exit 3.14159	exit takes only integer args in the range 0 - 255
`128+n`	fatal error signal "n"	kill -9 `$PPID` of script	`$?` returns 137 (128 + 9)
`130`	script terminated by Control-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 table, exit codes 1 - 2, 126 - 165, and 255 [1] have special meanings, and should therefore be avoided as user-specified exit parameters. Ending a script with exit 127 would certainly cause confusion when troubleshooting (is the error 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, this might not add any additional ambiguity, but, on the other hand, it probably would not be very informative either.

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 now 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.

Notes

[1]	Out of range exit values can result in unpredictable exit codes. For example, exit 3809 gives an exit code of 225.

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