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<!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) 2001-2003 The Open Group, All Rights Reserved --><title>bg</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="bg"></a> <a name="tag_04_10"></a><!-- bg --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_04_10_01"></a>NAME</h4><blockquote>bg - run jobs in the background</blockquote><h4><a name="tag_04_10_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><div class="box"><code><tt><sup>[<a href="javascript:open_code('UP')">UP</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> bg</tt> <b>[</b><i>job_id</i> <tt>...</tt><b>]</b><tt><img src="../images/opt-end.gif" alt="[Option End]" border="0"></tt></code></div></blockquote><h4><a name="tag_04_10_03"></a>DESCRIPTION</h4><blockquote><p>If job control is enabled (see the description of <a href="../utilities/set.html"><i>set</i></a> <b>-m</b>), the<i>bg</i> utility shall resume suspended jobs from the current environment (see <a href="xcu_chap02.html#tag_02_12"><i>ShellExecution Environment</i></a> ) by running them as background jobs. If the job specified by <i>job_id</i> is already a runningbackground job, the <i>bg</i> utility shall have no effect and shall exit successfully.</p><p>Using <i>bg</i> to place a job into the background shall cause its process ID to become "known in the current shell executionenvironment", as if it had been started as an asynchronous list; see <a href="xcu_chap02.html#tag_02_09_03_02"><i>AsynchronousLists</i></a> .</p></blockquote><h4><a name="tag_04_10_04"></a>OPTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_10_05"></a>OPERANDS</h4><blockquote><p>The following operand shall be supported:</p><dl compact><dt><i>job_id</i></dt><dd>Specify the job to be resumed as a background job. If no <i>job_id</i> operand is given, the most recently suspended job shallbe used. The format of <i>job_id</i> is described in 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>.</dd></dl></blockquote><h4><a name="tag_04_10_06"></a>STDIN</h4><blockquote><p>Not used.</p></blockquote><h4><a name="tag_04_10_07"></a>INPUT FILES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_10_08"></a>ENVIRONMENT VARIABLES</h4><blockquote><p>The following environment variables shall affect the execution of <i>bg</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_10_09"></a>ASYNCHRONOUS EVENTS</h4><blockquote><p>Default.</p></blockquote><h4><a name="tag_04_10_10"></a>STDOUT</h4><blockquote><p>The output of <i>bg</i> shall consist of a line in the format:</p><pre><tt>"[%d] %s\n", <</tt><i>job-number</i><tt>>, <</tt><i>command</i><tt>></tt></pre><p>where the fields are as follows:</p><dl compact><dt><<i>job-number</i>></dt><dd>A number that can be used to identify the job to the <a href="../utilities/wait.html"><i>wait</i></a>, <a href="../utilities/fg.html"><i>fg</i></a>, and <a href="../utilities/kill.html"><i>kill</i></a> utilities. Using these utilities, thejob can be identified by prefixing the job number with <tt>'%'</tt> .</dd><dt><<i>command</i>></dt><dd>The associated command that was given to the shell.</dd></dl></blockquote><h4><a name="tag_04_10_11"></a>STDERR</h4><blockquote><p>The standard error shall be used only for diagnostic messages.</p></blockquote><h4><a name="tag_04_10_12"></a>OUTPUT FILES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_10_13"></a>EXTENDED DESCRIPTION</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_10_14"></a>EXIT STATUS</h4><blockquote><p>The following exit values shall be returned:</p><dl compact><dt> 0</dt><dd>Successful completion.</dd><dt>>0</dt><dd>An error occurred.</dd></dl></blockquote><h4><a name="tag_04_10_15"></a>CONSEQUENCES OF ERRORS</h4><blockquote><p>If job control is disabled, the <i>bg</i> utility shall exit with an error and no job shall be placed in the background.</p></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_04_10_16"></a>APPLICATION USAGE</h4><blockquote><p>A job is generally suspended by typing the SUSP character (<control>-Z on most systems); see the Base Definitions volumeof IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap11.html#tag_11">Chapter 11, General Terminal Interface</a>. At thatpoint, <i>bg</i> can put the job into the background. This is most effective when the job is expecting no terminal input and itsoutput has been redirected to non-terminal files. A background job can be forced to stop when it has terminal output by issuing thecommand:</p><pre><tt>stty tostop</tt></pre><p>A background job can be stopped with the command:</p><pre><tt>kill -s stop</tt> <i>job ID</i></pre><p>The <i>bg</i> utility does not work as expected when it is operating in its own utility execution environment because thatenvironment has no suspended jobs. In the following examples:</p><pre><tt>... | xargs bg(bg)</tt></pre><p>each <i>bg</i> operates in a different environment and does not share its parent shell's understanding of jobs. For this reason,<i>bg</i> is generally implemented as a shell regular built-in.</p></blockquote><h4><a name="tag_04_10_17"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_10_18"></a>RATIONALE</h4><blockquote><p>The extensions to the shell specified in this volume of IEEE Std 1003.1-2001 have mostly been based on featuresprovided by the KornShell. The job control features provided by <i>bg</i>, <a href="../utilities/fg.html"><i>fg</i></a>, and <ahref="../utilities/jobs.html"><i>jobs</i></a> are also based on the KornShell. The standard developers examined the characteristicsof the C shell versions of these utilities and found that differences exist. Despite widespread use of the C shell, the KornShellversions were selected for this volume of IEEE Std 1003.1-2001 to maintain a degree of uniformity with the rest of theKornShell features selected (such as the very popular command line editing features).</p><p>The <i>bg</i> utility is expected to wrap its output if the output exceeds the number of display columns.</p></blockquote><h4><a name="tag_04_10_19"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_10_20"></a>SEE ALSO</h4><blockquote><p><a href="xcu_chap02.html#tag_02_09_03_02"><i>Asynchronous Lists</i></a> , <a href="fg.html"><i>fg</i></a> , <a href="kill.html"><i>kill</i>()</a> , <a href="jobs.html"><i>jobs</i></a> , <a href="wait.html"><i>wait</i>()</a></p></blockquote><h4><a name="tag_04_10_21"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 4.</p></blockquote><h4><a name="tag_04_10_22"></a>Issue 6</h4><blockquote><p>This utility is marked as part of the User Portability Utilities option.</p><p>The JC margin marker on the SYNOPSIS is removed since support for Job Control is mandatory in this issue. This is a FIPSrequirement.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr><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
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