mirror of https://github.com/mkerrisk/man-pages
152 lines
4.2 KiB
Groff
152 lines
4.2 KiB
Groff
.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" License.
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.\" Modified Sat Jul 24 17:51:15 1993 by Rik Faith (faith@cs.unc.edu)
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.\" Modified 11 May 1998 by Joseph S. Myers (jsm28@cam.ac.uk)
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.\" Modified 14 May 2001, 23 Sep 2001 by aeb
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.\" 2004-12-20, mtk
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.\"
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.TH SYSTEM 3 2004-12-20 "" "Linux Programmer's Manual"
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.SH NAME
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system \- execute a shell command
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.SH SYNOPSIS
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.nf
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.B #include <stdlib.h>
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.sp
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.BI "int system(const char *" "command" );
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.fi
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.SH DESCRIPTION
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.BR system ()
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executes a command specified in
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.I command
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by calling
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.BR "/bin/sh \-c"
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.IR command ,
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and returns after the command has been completed.
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During execution of the command,
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.B SIGCHLD
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will be blocked, and
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.B SIGINT
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and
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.B SIGQUIT
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will be ignored.
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.SH "RETURN VALUE"
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The value returned is \-1 on error (e.g.
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.BR fork (2)
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failed),
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and the return status of the command otherwise.
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This latter return status is in the format
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specified in
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.BR wait (2).
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Thus, the exit code of the command will be
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.IR WEXITSTATUS(status) .
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In case
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.I "/bin/sh"
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could not be executed, the exit status will be that of
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a command that does
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.IR exit(127) .
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.PP
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If the value of
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.I command
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is NULL,
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.BR system ()
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returns non-zero if the shell is available, and zero if not.
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.PP
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.BR system ()
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does not affect the wait status of any other children.
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.SH "CONFORMING TO"
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C89, C99, POSIX.1-2001.
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.SH NOTES
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.PP
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If the
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.B _XOPEN_SOURCE
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feature test macro is defined, then the macros described in
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.BR wait (2)
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.RB ( WEXITSTATUS (),
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etc.) are made available when including
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.IR <stdlib.h> .
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.PP
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As mentioned,
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.BR system ()
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ignores
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.B SIGINT
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and
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.BR SIGQUIT .
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This may make programs that call it
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from a loop uninterruptible, unless they take care themselves
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to check the exit status of the child.
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E.g.
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.br
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.nf
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while (something) {
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int ret = system("foo");
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if (WIFSIGNALED(ret) &&
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(WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
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break;
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}
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.fi
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.PP
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Do not use
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.BR system ()
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from a program with set-user-ID or set-group-ID privileges,
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because strange values for some environment variables
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might be used to subvert system integrity.
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Use the
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.BR exec (3)
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family of functions instead, but not
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.BR execlp (3)
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or
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.BR execvp (3).
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.BR system ()
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will not, in fact, work properly from programs with set-user-ID or
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set-group-ID privileges on systems on which
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.I /bin/sh
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is bash version 2, since bash 2 drops privileges on startup.
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(Debian uses a modified bash which does not do this when invoked as
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.BR sh .)
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.PP
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In versions of glibc before 2.1.3, the check for the availability of
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.I /bin/sh
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was not actually performed if
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.I command
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was NULL; instead it was always assumed to be available, and
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.BR system ()
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always returned 1 in this case.
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Since glibc 2.1.3, this check is performed because, even though
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POSIX.1-2001 requires a conforming implementation to provide
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a shell, that shell may not be available or executable if
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the calling program has previously called
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.BR chroot (2)
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(which is not specified by POSIX.1-2001).
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.PP
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It is possible for the shell command to return 127, so that code is not
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a sure indication that the
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.BR execve (2)
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call failed.
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.SH "SEE ALSO"
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.BR sh (1),
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.BR signal (2),
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.BR wait (2),
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.BR exec (3)
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