Appendix E. Exit Codes With Special Meanings
Table E-1. Reserved Exit Codes
Exit Code
Number
Meaning Example Comments
1
2
126
127
128
128+n
130
Catchall for general errors
Misuse of shell builtins
(according to Bash documentation)
Command invoked cannot execute
"command not found"
Invalid argument to exit
Fatal error signal "n"
Script terminated by
Control-C let "var1 = 1/0" empty_function()
{}
/dev/null
Miscellaneous errors, such as "divide by zero" and other impermissible operations
Missing keyword or command, or
permission problem (and diff return code on a failed binary file comparison).
Permission problem or command is not an executable illegal_command Possible problem with
$PATH
or a typo exit 3.14159
exit
takes only integer args in the range 0 -
255 (see first footnote)
$?
returns 137 (128 + 9)
kill -9
$PPID
of script
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 [144] 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. [145] 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.
Appendix E. Exit Codes With Special Meanings 802
