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 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
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. |
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