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78 lines
6.2 KiB
Plaintext
78 lines
6.2 KiB
Plaintext
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====== 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 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.
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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.
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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 calling component.
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This - and only this - makes it possible to determinate the success or 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 is reserved to be used by the Bash shell directly or for special purposes, like reporting a termination by a signal:
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^ Code ^ Description ^
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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 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''.
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===== Portability =====
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Tables of shell behavior involving non-portable side-effects or common bugs with exit statuses. Note heirloom doesn't support pipeline 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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| <code>:; : `false` `echo $? >&2`</code> | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 1 |
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| <code>false; eval; echo $?</code> | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 |
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| <code>x=`false` eval echo \$?</code> | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 1 |
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| <code>eval echo \$? <&0`false`</code> | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 1 |
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| <code>while :; do ! break; done; echo $?</code> | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 1 | - |
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| [[https://lists.gnu.org/archive/html/bug-bash/2010-09/msg00009.html | discussion]]<code>false; : | echo $?</code>| 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 0 |
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| <code>(exit 2); for x in "`exit 3`"; do echo $?; done</code> | 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 transparency of the return builtin.
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^ test ^ bash ^ bash\\ (POSIX) ^ zsh\\ (emulate ksh) ^ ksh93 ^ mksh ^ posh ^ dash ^ busybox ^ heirloom ^
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| <code>f() { echo $?; }; :; f `false`</code> | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 1 |
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| <code>f() { return; }; false; f; echo $?</code> | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
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| <code>f() { return $?; }; false; f; echo $?</code> | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
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| <code>f() { ! return; }; f; echo $?</code> | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 1 | - |
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| <code>f() { ! return; }; false; f; echo $?</code> | 1 | 1 | 0 | 0 | 1 | 1 | 0 | 0 | - |
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| <code>f() { return; }; x=`false` f; echo $?</code> | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 |
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| <code>f() { return; }; f <&0`false`; echo $?</code> | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 1 |
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| <code>f() { x=`false` return; }; f; echo $?</code> | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 1 |
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| <code>f() { return <&0`false`; }; f; echo $?</code> | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 1 |
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| <code>f() { x=`false` return <&0`false`; }; f; echo $?</code> | 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 non-matching patterns.
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^ test ^ bash ^ bash\\ (POSIX) ^ zsh\\ (emulate ksh) ^ ksh93 ^ mksh ^ posh ^ dash ^ busybox ^ heirloom ^
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| <code>(exit 2); case x in x) echo $?;; esac</code> | 2 | 2 | 0 | 2 | 2 | 2 | 0 | 0 | 2 |
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| <code>(exit 2); case `exit 3`x in x) echo $?;; esac</code> | 3 | 3 | 0 | 3 | 2 | 2 | 0 | 0 | 3 |
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| <code>(exit 2); case x in `exit 4`x) echo $?;; esac</code> | 4 | 4 | 4 | 4 | 2 | 2 | 0 | 0 | 4 |
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| <code>(exit 2); case `exit 3`x in `exit 4`x) echo $?;; esac</code> | 4 | 4 | 4 | 4 | 2 | 2 | 0 | 0 | 4 |
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| <code>(exit 2); case x in x);; esac; echo $?</code> | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 2 |
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| <code>(exit 2); case x in "");; esac; echo $?</code> | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 2 |
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| <code>(exit 2); case `exit 3`x in x);; esac; echo $?</code> | 0 | 0 | 0 | 3 | 0 | 0 | 0 | 0 | 3 |
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| <code>(exit 2); case `exit 3`x in "");; esac; echo $?</code> | 0 | 0 | 0 | 3 | 0 | 0 | 0 | 0 | 3 |
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| <code>(exit 2); case x in `exit 4`x);; esac; echo $?</code> | 0 | 0 | 0 | 4 | 0 | 0 | 0 | 0 | 4 |
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| <code>(exit 2); case x in `exit 4`);; esac; echo $?</code> | 0 | 0 | 4 | 4 | 0 | 0 | 0 | 0 | 4 |
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| <code>(exit 2); case `exit 3`x in `exit 4`);; esac; echo $?</code> | 0 | 0 | 4 | 4 | 0 | 0 | 0 | 0 | 4 |
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| <code>(exit 2); case `exit 3`x in `exit 4`x);; esac; echo $?</code> | 0 | 0 | 0 | 4 | 0 | 0 | 0 | 0 | 4 |
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