📄 wait.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2004 IEEE and The Open Group, All Rights Reserved --><title>wait</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="wait"></a> <a name="tag_04_168"></a><!-- wait --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2004 Edition<br>Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_04_168_01"></a>NAME</h4><blockquote>wait - await process completion</blockquote><h4><a name="tag_04_168_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>wait</tt> <b>[</b><i>pid</i><tt>...</tt><b>]</b></code></p></blockquote><h4><a name="tag_04_168_03"></a>DESCRIPTION</h4><blockquote><p>When an asynchronous list (see <a href="xcu_chap02.html#tag_02_09_03_02"><i>Asynchronous Lists</i></a>) is started by theshell, the process ID of the last command in each element of the asynchronous list shall become known in the current shellexecution environment; see <a href="xcu_chap02.html#tag_02_12"><i>Shell Execution Environment</i></a>.</p><p>If the <i>wait</i> utility is invoked with no operands, it shall wait until all process IDs known to the invoking shell haveterminated and exit with a zero exit status.</p><p>If one or more <i>pid</i> operands are specified that represent known process IDs, the <i>wait</i> utility shall wait until allof them have terminated. If one or more <i>pid</i> operands are specified that represent unknown process IDs, <i>wait</i> shalltreat them as if they were known process IDs that exited with exit status 127. The exit status returned by the <i>wait</i> utilityshall be the exit status of the process requested by the last <i>pid</i> operand.</p><p>The known process IDs are applicable only for invocations of <i>wait</i> in the current shell execution environment.</p></blockquote><h4><a name="tag_04_168_04"></a>OPTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_168_05"></a>OPERANDS</h4><blockquote><p>The following operand shall be supported:</p><dl compact><dt><i>pid</i></dt><dd>One of the following: <ol><li><p>The unsigned decimal integer process ID of a command, for which the utility is to wait for the termination.</p></li><li><p>A job control job ID (see the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap03.html#tag_03_203">Section 3.203, Job Control Job ID</a>) that identifies a background process group to bewaited for. The job control job ID notation is applicable only for invocations of <i>wait</i> in the current shell executionenvironment; see <a href="xcu_chap02.html#tag_02_12"><i>Shell Execution Environment</i></a>. The exit status of <i>wait</i> shallbe determined by the last command in the pipeline. <basefont size="2"></p><dl><dt><b>Note:</b></dt><dd>The job control job ID type of <i>pid</i> is only available on systems supporting the User Portability Utilities option.</dd></dl><basefont size="3"></li></ol></dd></dl></blockquote><h4><a name="tag_04_168_06"></a>STDIN</h4><blockquote><p>Not used.</p></blockquote><h4><a name="tag_04_168_07"></a>INPUT FILES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_168_08"></a>ENVIRONMENT VARIABLES</h4><blockquote><p>The following environment variables shall affect the execution of <i>wait</i>:</p><dl compact><dt><i>LANG</i></dt><dd>Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume ofIEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap08.html#tag_08_02">Section 8.2, Internationalization Variables</a> forthe precedence of internationalization variables used to determine the values of locale categories.)</dd><dt><i>LC_ALL</i></dt><dd>If set to a non-empty string value, override the values of all the other internationalization variables.</dd><dt><i>LC_CTYPE</i></dt><dd>Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte asopposed to multi-byte characters in arguments).</dd><dt><i>LC_MESSAGES</i></dt><dd>Determine the locale that should be used to affect the format and contents of diagnostic messages written to standarderror.</dd><dt><i>NLSPATH</i></dt><dd><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">Determine the location of message catalogs for the processing of <i>LC_MESSAGES .</i> <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd></dl></blockquote><h4><a name="tag_04_168_09"></a>ASYNCHRONOUS EVENTS</h4><blockquote><p>Default.</p></blockquote><h4><a name="tag_04_168_10"></a>STDOUT</h4><blockquote><p>Not used.</p></blockquote><h4><a name="tag_04_168_11"></a>STDERR</h4><blockquote><p>The standard error shall be used only for diagnostic messages.</p></blockquote><h4><a name="tag_04_168_12"></a>OUTPUT FILES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_168_13"></a>EXTENDED DESCRIPTION</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_168_14"></a>EXIT STATUS</h4><blockquote><p>If one or more operands were specified, all of them have terminated or were not known by the invoking shell, and the status ofthe last operand specified is known, then the exit status of <i>wait</i> shall be the exit status information of the commandindicated by the last operand specified. If the process terminated abnormally due to the receipt of a signal, the exit status shallbe greater than 128 and shall be distinct from the exit status generated by other signals, but the exact value is unspecified. (Seethe <a href="../utilities/kill.html"><i>kill</i></a> <b>-l</b> option.) Otherwise, the <i>wait</i> utility shall exit with one ofthe following values:</p><dl compact><dt> 0</dt><dd>The <i>wait</i> utility was invoked with no operands and all process IDs known by the invoking shell have terminated.</dd><dt>1-126</dt><dd>The <i>wait</i> utility detected an error.</dd><dt> 127</dt><dd>The command identified by the last <i>pid</i> operand specified is unknown.</dd></dl></blockquote><h4><a name="tag_04_168_15"></a>CONSEQUENCES OF ERRORS</h4><blockquote><p>Default.</p></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_04_168_16"></a>APPLICATION USAGE</h4><blockquote><p>On most implementations, <i>wait</i> is a shell built-in. If it is called in a subshell or separate utility executionenvironment, such as one of the following:</p><pre><tt>(wait)nohup wait ...find . -exec wait ... \;</tt></pre><p>it returns immediately because there are no known process IDs to wait for in those environments.</p><p>Historical implementations of interactive shells have discarded the exit status of terminated background processes before eachshell prompt. Therefore, the status of background processes was usually lost unless it terminated while <i>wait</i> was waiting forit. This could be a serious problem when a job that was expected to run for a long time actually terminated quickly with a syntaxor initialization error because the exit status returned was usually zero if the requested process ID was not found. This volume ofIEEE Std 1003.1-2001 requires the implementation to keep the status of terminated jobs available until the status isrequested, so that scripts like:</p><pre><tt>j1&p1=$!j2&wait $p1echo Job 1 exited with status $?wait $!echo Job 2 exited with status $?</tt></pre><p>work without losing status on any of the jobs. The shell is allowed to discard the status of any process if it determines thatthe application cannot get the process ID for that process from the shell. It is also required to remember only {CHILD_MAX} numberof processes in this way. Since the only way to get the process ID from the shell is by using the <tt>'!'</tt> shell parameter, theshell is allowed to discard the status of an asynchronous list if <tt>"$!"</tt> was not referenced before another asynchronous listwas started. (This means that the shell only has to keep the status of the last asynchronous list started if the application didnot reference <tt>"$!"</tt>. If the implementation of the shell is smart enough to determine that a reference to <tt>"$!"</tt> wasnot saved anywhere that the application can retrieve it later, it can use this information to trim the list of saved information.Note also that a successful call to <i>wait</i> with no operands discards the exit status of all asynchronous lists.)</p><p>If the exit status of <i>wait</i> is greater than 128, there is no way for the application to know if the waited-for processexited with that value or was killed by a signal. Since most utilities exit with small values, there is seldom any ambiguity. Evenin the ambiguous cases, most applications just need to know that the asynchronous job failed; it does not matter whether itdetected an error and failed or was killed and did not complete its job normally.</p></blockquote><h4><a name="tag_04_168_17"></a>EXAMPLES</h4><blockquote><p>Although the exact value used when a process is terminated by a signal is unspecified, if it is known that a signal terminated aprocess, a script can still reliably determine which signal by using <a href="../utilities/kill.html"><i>kill</i></a> as shown bythe following script:</p><pre><tt>sleep 1000&pid=$!kill -kill $pidwait $pidecho $pid was terminated by a SIG$(kill -l $?) signal.</tt></pre><p>If the following sequence of commands is run in less than 31 seconds:</p><pre><tt>sleep 257 | sleep 31 &jobs -l %%</tt></pre><p>either of the following commands returns the exit status of the second <a href="../utilities/sleep.html"><i>sleep</i></a> in thepipeline:</p><pre><tt>wait</tt> <i><pid of sleep 31></i><tt>wait %%</tt></pre></blockquote><h4><a name="tag_04_168_18"></a>RATIONALE</h4><blockquote><p>The description of <i>wait</i> does not refer to the <a href="../functions/waitpid.html"><i>waitpid</i>()</a> function from theSystem Interfaces volume of IEEE Std 1003.1-2001 because that would needlessly overspecify this interface. However, thewording means that <i>wait</i> is required to wait for an explicit process when it is given an argument so that the statusinformation of other processes is not consumed. Historical implementations use the <a href="../functions/wait.html"><i>wait</i>()</a> function defined in the System Interfaces volume of IEEE Std 1003.1-2001 until<a href="../functions/wait.html"><i>wait</i>()</a> returns the requested process ID or finds that the requested process does notexist. Because this means that a shell script could not reliably get the status of all background children if a second backgroundjob was ever started before the first job finished, it is recommended that the <i>wait</i> utility use a method such as thefunctionality provided by the <a href="../functions/waitpid.html"><i>waitpid</i>()</a> function.</p><p>The ability to wait for multiple <i>pid</i> operands was adopted from the KornShell.</p><p>This new functionality was added because it is needed to determine the exit status of any asynchronous list accurately. The onlycompatibility problem that this change creates is for a script like</p><pre><tt>while sleep 60 do job& echo Job started $(date) as $! done</tt></pre><p>which causes the shell to monitor all of the jobs started until the script terminates or runs out of memory. This would not be aproblem if the loop did not reference <tt>"$!"</tt> or if the script would occasionally <i>wait</i> for jobs it started.</p></blockquote><h4><a name="tag_04_168_19"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_168_20"></a>SEE ALSO</h4><blockquote><p><a href="xcu_chap02.html#tag_02"><i>Shell Command Language</i></a>, <a href="kill.html"><i>kill</i>()</a>, <a href="sh.html"><i>sh</i></a>, the System Interfaces volume of IEEE Std 1003.1-2001, <a href="../functions/wait.html"><i>wait</i>()</a>, <a href="../functions/waitpid.html"><i>waitpid</i>()</a></p></blockquote><h4><a name="tag_04_168_21"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 2.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -