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Move UCB-licensed code from 4-clause to 3-clause licence.

Patches provided by Joel Baker in PR 22280, verified by myself.

.\"	$NetBSD: system.3,v 1.15 2003/08/07 16:43:45 agc Exp $
.\"
.\" Copyright (c) 1990, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
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.\" This code is derived from software contributed to Berkeley by
.\" the American National Standards Committee X3, on Information
.\" Processing Systems.
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.\"     from: @(#)system.3	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt SYSTEM 3
.Os
.Sh NAME
.Nm system
.Nd pass a command to the shell
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft int
.Fn system "const char *string"
.Sh DESCRIPTION
The
.Fn system
function
hands the argument
.Fa string
to the command interpreter
.Xr sh 1 .
The calling process waits for the shell to finish executing the command,
ignoring
.Dv SIGINT
and
.Dv SIGQUIT ,
and blocking
.Dv SIGCHLD .
.Pp
If
.Fa string
is a
.Dv NULL
pointer,
.Fn system
will return non-zero.
Otherwise,
.Fn system
returns the termination status of the shell in the format specified by
.Xr waitpid 2 .
.Sh RETURN VALUES
If a child process cannot be created, or the termination status of
the shell cannot be obtained,
.Fn system
returns -1 and sets
.Va errno
to indicate the error.
If execution of the shell fails,
.Fn system
returns the termination status for a program that terminates with a call of
.Fn exit 127 .
.Sh SEE ALSO
.Xr sh 1 ,
.Xr execve 2 ,
.Xr waitpid 2 ,
.Xr popen 3
.Sh STANDARDS
The
.Fn system
function
conforms to
.St -ansiC
and
.St -p1003.2-92 .
.Sh CAVEATS
Never supply the
.Fn system
function with a command containing any part of an unsanitized user-supplied
string.
Shell meta-characters present will be honored by the
.Xr sh 1
command interpreter.