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107 lines
4.8 KiB
Markdown
107 lines
4.8 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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|test|bash 4.2.45|bash (POSIX)|zsh 5.0.2 (emulate ksh)|ksh93 93v- 2013-03-18|mksh R44 2013/02/24|posh 0.11|dash 0.5.7.3|busybox 1.2.1|heirloom 050706|
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|--|--|--|--|--|--|--|--|--|--|
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|<pre>:; : \`false\` \`echo $? >&2\`</pre>|1|1|1|1|0|0|0|0|1|
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|<pre>false; eval; echo $?</pre>|0|0|0|0|0|1|0|1|0|
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|<pre>x=\`false\` eval echo $?</pre>|1|1|1|1|0|0|0|0|1|
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|<pre>eval echo \$? <&0\`false\`</pre>|1|1|1|1|0|0|0|0|1|
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|<pre>while :; do ! break; done; echo $?</pre>|1|1|1|1|0|0|1|1|-|
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|<pre>false; :| echo $?</pre><br>[discussion](https://lists.gnu.org/archive/html/bug-bash/2010-09/msg00009.html)|1|1|1|0|1|1|1|1|0|
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|<pre>(exit 2); for x in "\`exit 3\`"; do echo $?; done</pre>|3|3|3|3|2|2|0|0|3|
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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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|test|bash|bash<br>(POSIX)|zsh<br>(emulate ksh)|ksh93|mksh|posh|dash|busybox|heirloom
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|--|--|--|--|--|--|--|--|--|--|
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|<pre>f() { echo $?; }; :; f \`false\`</pre>|1|1|1|1|0|0|0|0|1|
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|<pre>f() { return; }; false; f; echo $?</pre>|1|1|1|0|1|1|1|1|1|
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|<pre>f() { return $?; }; false; f; echo $?</pre>|1|1|1|1|1|1|1|1|1|
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|<pre>f() { ! return; }; f; echo $?</pre>|0|0|1|0|0|0|1|1|-|
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|<pre>f() { ! return; }; false; f; echo $?</pre>|1|1|0|0|1|1|0|0|-|
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|<pre>f() { return; }; x=\`false\` f; echo $?</pre>|1|1|1|1|0|0|0|0|0|
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|<pre>f() { return; }; f <&0\`false\`; echo $?</pre>|1|1|1|1|0|0|0|0|1|
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|<pre>f() { x=\`false\` return; }; f; echo $?</pre>|1|1|1|0|0|0|0|0|1|
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|<pre>f() { return <&0\`false\`; }; f; echo $?</pre>|1|1|1|0|0|0|0|0|1|
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|<pre>f() { x=\`false\` return <&0\`false\`; }; f; echo $?</pre>|1|1|1|1|0|0|0|0|1|
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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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|test|bash|bash<br>(POSIX)|zsh<br>(emulate ksh)|ksh93|mksh|posh|dash|busybox|heirloom|
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|--|--|--|--|--|--|--|--|--|--|
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|<pre>(exit 2); case x in x) echo $?;; esac</pre>|2|2|0|2|2|2|0|0|2|
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|<pre> (exit 2); case \`exit 3\`x in x) echo $?;; esac</pre>|3|3|0|3|2|2|0|0|3|
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|<pre> (exit 2); case x in \`exit 4\`x) echo $?;; esac</pre>|4|4|4|4|2|2|0|0|4|
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|<pre> (exit 2); case \`exit 3\`x in \`exit 4\`x) echo $?;; esac</pre>|4|4|4|4|2|2|0|0|4|
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|<pre>(exit 2); case x in x);; esac; echo $?</pre>|0|0|0|0|0|0|0|0|2|
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|<pre>(exit 2); case x in "");; esac; echo $?</pre>|0|0|0|0|0|0|0|0|2|
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|<pre>(exit 2); case \`exit 3\`x in x);; esac; echo $?</pre>|0|0|0|3|0|0|0|0|3|
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|<pre>(exit 2); case \`exit 3\`x in "");; esac; echo $?</pre>|0|0|0|3|0|0|0|0|3|
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|<pre>(exit 2); case x in \`exit 4\`x);; esac; echo $?</pre>|0|0|0|4|0|0|0|0|4|
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|<pre>(exit 2); case x in \`exit 4\`);; esac; echo $?</pre>|0|0|4|4|0|0|0|0|4|
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|<pre>(exit 2); case \`exit 3\`x in \`exit 4\`);; esac; echo $?</pre>|0|0|4|4|0|0|0|0|4|
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|<pre> (exit 2); case \`exit 3\`x in \`exit 4\`x);; esac; echo $?</pre>|0|0|0|4|0|0|0|0|4|
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