Appendix D. Exit Codes With Special Meanings
Table D-1. "Reserved"Exit Codes
| Exit Code Number | Meaning | Example | Comments |
|---|---|---|---|
| 1 | Catchall for general errors | let "var1 = 1/0" | Miscellaneous errors, such as "divide byzero" |
| 2 | Misuse of shell builtins (according to Bash documentation) | Seldom seen, usually defaults to exit code |
1126Command invoked cannot executePermission problem or command is not an executable127"command not found"Possible problem with $PATHor a typo128Invalid argument to exitexit 3.14159exittakes only integer args in therange0 - 255(seefootnote)128+nFatal error signal "n"kill -9$PPIDof script$?returns
137(128 + 9)130Script terminated by Control-CControl-C is fatal error signal
2, (130 = 128 + 2, see above)255*Exit status out of rangeexit-1exittakes only integer args in therange0 - 255
According to the above table, exit codes1 - 2,126 - 165, and 255
have special meanings, and should therefore be avoided foruser-specified exit parameters. Ending a script with exit127would certainly cause confusion when troubleshooting(is the error code a "command not found"or auser-defined one?). However, many scripts use an exit1as a general bailout upon error. Since exit code
1signifies so many possible errors,this probably would not be helpful 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 similarstandard for scripting might be appropriate. The author ofthis document proposes restricting user-defined exit codes tothe range64 - 113(in addition to
0, for success), to conform withthe C/C++ standard. This would allot 50 valid codes, and maketroubleshooting scripts more straightforward.
All user-defined exit codes in the accompanying examplesto this document now conform to this standard, exceptwhere overriding circumstances exist, as in Example 9-2.
Issuing a $?fromthe command line after a shell script exits gives resultsconsistent with the table above only from the Bash orshprompt. Running the C-shell ortcshmay give different values in somecases.
Notes
Out of range exit values can result inunexpected exit codes. An exit value greaterthan255returns an exitcode modulo
256. For example, exit3809gives an exit code of225(3809 % 256 = 225).