9.2 KiB
Exit Status
- exit code
- return status
Purpose
The exit status is a numeric value that is returned by a program to the
calling program or shell. In C programs, this is represented by the
return value of the main() function or the value you give to
exit(3). The only part of the number that matters are the least
significant 8 bits, which means there are only values from 0 to 255.
In the shell, every operation generates an exit status (return status), even if no program is called. An example for such an operation is a redirection.
The parameter to the
exit(exit the shell/script)return(return from a function)
builtin commands serve the purpose of giving the exit status to the calling component.
This - and only this - makes it possible to determinate the success or failure of an operation. For scripting, always set exit codes.
Values
The code is a number between 0 and 255, where the part from 126 to 255 is reserved to be used by the Bash shell directly or for special purposes, like reporting a termination by a signal:
Code Description
0 success 1-255 failure (in general) 126 the requested command (file) can't be executed (but was found) 127 command (file) not found 128 according to ABS it's used to report an invalid argument to the exit builtin, but I wasn't able to verify that in the source code of Bash (see code 255) 128 + N the shell was terminated by the signal N (also used like this by various other programs) 255 wrong argument to the exit builtin (see code 128)
The lower codes 0 to 125 are not reserved and may be used for whatever
the program likes to report. A value of 0 means successful
termination, a value not 0 means unsuccessful termination. This
behavior (== 0, != 0) is also what Bash reacts on in some code flow
control statements like if or while.
Portability
Tables of shell behavior involving non-portable side-effects or common
bugs with exit statuses. Note heirloom doesn't support pipeline
negation (! pipeline).
Misc
test bash\ bash\ zsh 5.0.2\ ksh93\ mksh\ posh\ dash\ busybox\ heirloom
4.2.45 (POSIX) (emulate ksh) 93v- 2013-03-18 R44 2013/02/24 0.11 0.5.7.3 1.2.1 050706
:; : `false` `echo $? >&2` 1 1 1 1 0 0 0 0 1
false; eval; echo $? 0 0 0 0 0 1 0 1 0
x=`false` eval echo \$? 1 1 1 1 0 0 0 0 1
eval echo \$? <&0`false` 1 1 1 1 0 0 0 0 1
while :; do ! break; done; echo $? 1 1 1 1 0 0 1 1 -
discussionfalse; : | echo $? 1 1 1 0 1 1 1 1 0
(exit 2); for x in "`exit 3`"; do echo $?; done 3 3 3 3 2 2 0 0 3
functions
Measuring side-effects during the function call, during return, and transparency of the return builtin.
test bash bash\ zsh\ ksh93 mksh posh dash busybox heirloom (POSIX) (emulate ksh)
f() { echo $?; }; :; f `false` 1 1 1 1 0 0 0 0 1
f() { return; }; false; f; echo $? 1 1 1 0 1 1 1 1 1
f() { return $?; }; false; f; echo $? 1 1 1 1 1 1 1 1 1
f() { ! return; }; f; echo $? 0 0 1 0 0 0 1 1 -
f() { ! return; }; false; f; echo $? 1 1 0 0 1 1 0 0 -
f() { return; }; x=`false` f; echo $? 1 1 1 1 0 0 0 0 0
f() { return; }; f <&0`false`; echo $? 1 1 1 1 0 0 0 0 1
f() { x=`false` return; }; f; echo $? 1 1 1 0 0 0 0 0 1
f() { return <&0`false`; }; f; echo $? 1 1 1 0 0 0 0 0 1
f() { x=`false` return <&0`false`; }; f; echo $? 1 1 1 1 0 0 0 0 1
case..esac
Statuses measured within the command and after, with matching and non-matching patterns.
test bash bash\ zsh\ ksh93 mksh posh dash busybox heirloom (POSIX) (emulate ksh)
(exit 2); case x in x) echo $?;; esac 2 2 0 2 2 2 0 0 2
(exit 2); case `exit 3`x in x) echo $?;; esac 3 3 0 3 2 2 0 0 3
(exit 2); case x in `exit 4`x) echo $?;; esac 4 4 4 4 2 2 0 0 4
(exit 2); case `exit 3`x in `exit 4`x) echo $?;; esac 4 4 4 4 2 2 0 0 4
(exit 2); case x in x);; esac; echo $? 0 0 0 0 0 0 0 0 2
(exit 2); case x in "");; esac; echo $? 0 0 0 0 0 0 0 0 2
(exit 2); case `exit 3`x in x);; esac; echo $? 0 0 0 3 0 0 0 0 3
(exit 2); case `exit 3`x in "");; esac; echo $? 0 0 0 3 0 0 0 0 3
(exit 2); case x in `exit 4`x);; esac; echo $? 0 0 0 4 0 0 0 0 4
(exit 2); case x in `exit 4`);; esac; echo $? 0 0 4 4 0 0 0 0 4
(exit 2); case `exit 3`x in `exit 4`);; esac; echo $? 0 0 4 4 0 0 0 0 4