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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>system</title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_009_360"> </a>NAME</h4><blockquote>system - issue a command</blockquote><h4><a name = "tag_000_009_361"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="stdlib.h.html">stdlib.h</a>>int system(const char *<i>command</i>);</code></pre></blockquote><h4><a name = "tag_000_009_362"> </a>DESCRIPTION</h4><blockquote>The<i>system()</i>function passes the string pointed to by<i>command</i>to the host environment to be executed by a command processor in animplementation-dependent manner. If the implementation supportsthe <b>XCU</b> specification commands, the environment of the executedcommand will be as if a child process were created using<i><a href="fork.html">fork()</a></i>,and the child process invoked the<i><a href="../xcu/sh.html">sh</a></i>utility (see<i><a href="../xcu/sh.html">sh</a></i>in the <b>XCU</b> specification) using<i><a href="exec.html">execl()</a></i>as follows:<pre><code>execl(<<i>shell path</i>>, "sh", "-c", <i>command</i>, (char *)0);</code></pre>where<i><shell path></i>is an unspecified pathname for the<i><a href="../xcu/sh.html">sh</a></i>utility.<p>The<i>system()</i>function ignores the SIGINT and SIGQUIT signals, and blocks the SIGCHLDsignal, while waiting for the command to terminate.If this might cause the application to miss a signal that would havekilled it, then the application should examine the return value from<i>system()</i>and take whatever action is appropriate to the application ifthe command terminated due to receipt of a signal.<p>The<i>system()</i>function will not affect the termination status ofany child of the calling processes other than the process orprocesses it itself creates.<p>The<i>system()</i>function will not return until the child process has terminated.</blockquote><h4><a name = "tag_000_009_363"> </a>RETURN VALUE</h4><blockquote>If<i>command</i>is a null pointer,<i>system()</i>returns non-zero only if a command processor is available.<p>If<i>command</i>is not a null pointer,<i>system()</i>returns the termination status of thecommand language interpreter in the format specified by<i><a href="waitpid.html">waitpid()</a></i>.The termination status of the command language interpreter is asspecified for the<i><a href="../xcu/sh.html">sh</a></i>utility, except that if some error prevents the command language interpreterfrom executing after the child process is created, the return value from<i>system()</i>will be as if the command language interpreterhad terminated using<i>exit</i>(127)or<i>_exit</i>(127).If a child process cannot be created, or if the termination status for thecommand language interpreter cannot be obtained,<i>system()</i>returns -1 and sets<i>errno</i>to indicate the error.</blockquote><h4><a name = "tag_000_009_364"> </a>ERRORS</h4><blockquote>The<i>system()</i>function may set<i>errno</i>values as described by<i><a href="fork.html">fork()</a></i>.<p>In addition,<i>system()</i>may fail if:<dl compact><dt>[ECHILD]<dd>The status of the child process created by<i>system()</i>is no longer available.</dl></blockquote><h4><a name = "tag_000_009_365"> </a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_009_366"> </a>APPLICATION USAGE</h4><blockquote>If the return value of<i>system()</i>is not -1, its value can be decoded through the use of themacros described in<i><a href="syswait.h.html"><sys/wait.h></a></i>.For convenience, these macros are also provided in<i><a href="stdlib.h.html"><stdlib.h></a></i>.<p>To determine whether or not the <b>XCU</b> specification's environment is present, use:<pre><code>sysconf(_SC_2_VERSION)</code></pre><p>Note that, while<i>system()</i>must ignore SIGINT and SIGQUIT and block SIGCHLD while waiting for thechild to terminate, the handling of signals in the executedcommand is as specified by<i><a href="fork.html">fork()</a></i>and<i>exec</i>.For example, ifSIGINT is being caught or is set to SIG_DFL when<i>system()</i>is called, then the child will be started withSIGINT handling set to SIG_DFL.<p>Ignoring SIGINT and SIGQUITin the parent process prevents coordination problems (two processesreading from the same terminal, for example) when the executed commandignores or catches one of the signals.It is also usually the correct action when the user has given acommand to the application to be executed synchronously (as in the "!"command in many interactive applications).In either case, the signal should be delivered only to the childprocess, not to the application itself.There is one situation where ignoring the signals might have less thanthe desired effect. This is when the application uses<i>system()</i>to perform some task invisible to the user. If the user typed the interruptcharacter (^C, for example) while<i>system()</i>is being used in this way, one would expect the application to be killed, butonly the executed command will be killed. Applications that use<i>system()</i>in this way should carefully check the return status from<i>system()</i>to see if the executed command was successful, and should take appropriateaction when the command fails.<p>Blocking SIGCHLDwhile waiting for the child to terminate prevents the application fromcatching the signal and obtaining status from<i>system()</i>'schild process before<i>system()</i>can get the status itself.<p>The context in which the utility is ultimately executed maydiffer from that in which<i>system()</i>was called.For example, file descriptors that have the FD_CLOEXECflag set will be closed, and the process ID and parent process IDwill be different.Also,if the executed utility changes its environmentvariables or its current workingdirectory, that change will not be reflected in the caller's context.<p>There is no defined way for an application to find the specific path for theshell. However,<i><a href="confstr.html">confstr()</a></i>can provide a value forthat is guaranteed to find the<i><a href="../xcu/sh.html">sh</a></i>utility.</blockquote><h4><a name = "tag_000_009_367"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_009_368"> </a>SEE ALSO</h4><blockquote><i><a href="exec.html">exec</a></i>,<i><a href="pipe.html">pipe()</a></i>,<i><a href="waitpid.html">waitpid()</a></i>,<i><a href="limits.h.html"><limits.h></a></i>,<i><a href="signal.h.html"><signal.h></a></i>,<i><a href="stdlib.h.html"><stdlib.h></a></i>,the <b><a href="../xcuix.html">XCU</a></b> specification.</blockquote><h4>DERIVATION</h4><blockquote>Derived from Issue 1 of the SVID.</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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