Linux-shell exit code

source comes from:http://tldp.org/LDP/abs/html/exitcodes.html

Table E-1. Reserved Exit Codes

Exit Code NumberMeaningExampleComments1Catchall for general errorslet "var1 = 1/0"Miscellaneous errors, such as "divide by zero" and other impermissible operations2Misuse 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).126Command invoked cannot execute/dev/nullPermission problem or command is not an executable127"command not found"illegal_commandPossible problem with $PATH or a typo128Invalid argument to exitexit 3.14159exit takes only integer args in the range 0 - 255 (see first footnote)128+nFatal error signal "n"kill -9 $PPID of script$? returns 137 (128 + 9)130Script terminated by Control-CCtl-CControl-C is fatal error signal 2, (130 = 128 + 2, see above)255*Exit status out of rangeexit -1exit 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-shellor tcsh may give different values in some cases.