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141 lines
9.2 KiB
Markdown
141 lines
9.2 KiB
Markdown
# Exit Status
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- exit code
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- return status
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## Purpose
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The exit status is a numeric value that is returned by a program to the
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calling program or shell. In C programs, this is represented by the
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return value of the `main()` function or the value you give to
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`exit(3)`. The only part of the number that matters are the least
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significant 8 bits, which means there are only values from 0 to 255.
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In the shell, every operation generates an exit status (return status),
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even if no program is called. An example for such an operation is a
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redirection.
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The parameter to the
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- `exit` (exit the shell/script)
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- `return` (return from a function)
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builtin commands serve the purpose of giving the exit status to the
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calling component.
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This - and only this - makes it possible to determinate the success or
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failure of an operation. For scripting, always set exit codes.
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## Values
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The code is a number between 0 and 255, where the part from 126 to 255
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is reserved to be used by the Bash shell directly or for special
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purposes, like reporting a termination by a signal:
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Code Description
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--------- ------------------------------------------------------------------------------------------------------------------------------------------------------------
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0 success
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1-255 failure (in general)
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126 the requested command (file) can't be executed (but was found)
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127 command (file) not found
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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)
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128 + N the shell was terminated by the signal N (also used like this by various other programs)
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255 wrong argument to the exit builtin (see code 128)
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The lower codes 0 to 125 are not reserved and may be used for whatever
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the program likes to report. A value of **0 means successful**
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termination, a value **not 0 means unsuccessful** termination. This
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behavior (== 0, != 0) is also what Bash reacts on in some code flow
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control statements like `if` or `while`.
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## Portability
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Tables of shell behavior involving non-portable side-effects or common
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bugs with exit statuses. Note heirloom doesn't support pipeline
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negation (`! pipeline`).
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### Misc
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--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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test bash\ bash\ zsh 5.0.2\ ksh93\ mksh\ posh\ dash\ busybox\ heirloom\
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4.2.45 (POSIX) (emulate ksh) 93v- 2013-03-18 R44 2013/02/24 0.11 0.5.7.3 1.2.1 050706
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----------------------------------------------------------------------------------------------------- -------- --------- --------------- ----------------- ---------------- ------- --------- ---------- -----------
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`` :; : `false` `echo $? >&2` `` 1 1 1 1 0 0 0 0 1
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`false; eval; echo $?` 0 0 0 0 0 1 0 1 0
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`` x=`false` eval echo \$? `` 1 1 1 1 0 0 0 0 1
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`` eval echo \$? <&0`false` `` 1 1 1 1 0 0 0 0 1
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`while :; do ! break; done; echo $?` 1 1 1 1 0 0 1 1 -
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[discussion](https://lists.gnu.org/archive/html/bug-bash/2010-09/msg00009.html)`false; : | echo $?` 1 1 1 0 1 1 1 1 0
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`` (exit 2); for x in "`exit 3`"; do echo $?; done `` 3 3 3 3 2 2 0 0 3
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--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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### functions
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Measuring side-effects during the function call, during return, and
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transparency of the return builtin.
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-------------------------------------------------------------------------------------------------------------------------------------------
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test bash bash\ zsh\ ksh93 mksh posh dash busybox heirloom
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(POSIX) (emulate ksh)
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-------------------------------------------------------- ------ --------- --------------- ------- ------ ------ ------ --------- ----------
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`` f() { echo $?; }; :; f `false` `` 1 1 1 1 0 0 0 0 1
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`f() { return; }; false; f; echo $?` 1 1 1 0 1 1 1 1 1
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`f() { return $?; }; false; f; echo $?` 1 1 1 1 1 1 1 1 1
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`f() { ! return; }; f; echo $?` 0 0 1 0 0 0 1 1 -
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`f() { ! return; }; false; f; echo $?` 1 1 0 0 1 1 0 0 -
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`` f() { return; }; x=`false` f; echo $? `` 1 1 1 1 0 0 0 0 0
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`` f() { return; }; f <&0`false`; echo $? `` 1 1 1 1 0 0 0 0 1
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`` f() { x=`false` return; }; f; echo $? `` 1 1 1 0 0 0 0 0 1
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`` f() { return <&0`false`; }; f; echo $? `` 1 1 1 0 0 0 0 0 1
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`` f() { x=`false` return <&0`false`; }; f; echo $? `` 1 1 1 1 0 0 0 0 1
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-------------------------------------------------------------------------------------------------------------------------------------------
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### case..esac
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Statuses measured within the command and after, with matching and
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non-matching patterns.
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-------------------------------------------------------------------------------------------------------------------------------------------------
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test bash bash\ zsh\ ksh93 mksh posh dash busybox heirloom
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(POSIX) (emulate ksh)
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-------------------------------------------------------------- ------ --------- --------------- ------- ------ ------ ------ --------- ----------
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`(exit 2); case x in x) echo $?;; esac` 2 2 0 2 2 2 0 0 2
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`` (exit 2); case `exit 3`x in x) echo $?;; esac `` 3 3 0 3 2 2 0 0 3
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`` (exit 2); case x in `exit 4`x) echo $?;; esac `` 4 4 4 4 2 2 0 0 4
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`` (exit 2); case `exit 3`x in `exit 4`x) echo $?;; esac `` 4 4 4 4 2 2 0 0 4
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`(exit 2); case x in x);; esac; echo $?` 0 0 0 0 0 0 0 0 2
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`(exit 2); case x in "");; esac; echo $?` 0 0 0 0 0 0 0 0 2
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`` (exit 2); case `exit 3`x in x);; esac; echo $? `` 0 0 0 3 0 0 0 0 3
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`` (exit 2); case `exit 3`x in "");; esac; echo $? `` 0 0 0 3 0 0 0 0 3
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`` (exit 2); case x in `exit 4`x);; esac; echo $? `` 0 0 0 4 0 0 0 0 4
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`` (exit 2); case x in `exit 4`);; esac; echo $? `` 0 0 4 4 0 0 0 0 4
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`` (exit 2); case `exit 3`x in `exit 4`);; esac; echo $? `` 0 0 4 4 0 0 0 0 4
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`` (exit 2); case `exit 3`x in `exit 4`x);; esac; echo $? `` 0 0 0 4 0 0 0 0 4
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-------------------------------------------------------------------------------------------------------------------------------------------------
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