Unix Exit Codes

"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

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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.

Source: http://www.faqs.org/docs/abs/HTML/exitcodes.html