ea9572e1d0
If 'e' is used, the kernel must support the recently added pipe2() system call. The use of pipe2() with O_CLOEXEC also fixes race conditions between concurrent popen() calls from different threads, even if the close-on-exec flag on the fd of the returned FILE is later cleared (because popen() closes all file descriptors from earlier popen() calls in the child process). Therefore, this approach should be used in all cases when pipe2() can be assumed present. The old version of popen() rejects "re" and "we" but treats "r+e" like "r+".
203 lines
4.9 KiB
Groff
203 lines
4.9 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)popen.3 8.2 (Berkeley) 5/3/95
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.\" $FreeBSD$
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.\"
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.Dd May 20, 2013
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.Dt POPEN 3
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.Os
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.Sh NAME
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.Nm popen ,
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.Nm pclose
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.Nd process
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.Tn I/O
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdio.h
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.Ft FILE *
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.Fn popen "const char *command" "const char *type"
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.Ft int
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.Fn pclose "FILE *stream"
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.Sh DESCRIPTION
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The
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.Fn popen
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function
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.Dq opens
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a process by creating a bidirectional pipe
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forking,
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and invoking the shell.
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Any streams opened by previous
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.Fn popen
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calls in the parent process are closed in the new child process.
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Historically,
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.Fn popen
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was implemented with a unidirectional pipe;
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hence many implementations of
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.Fn popen
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only allow the
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.Fa type
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argument to specify reading or writing, not both.
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Since
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.Fn popen
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is now implemented using a bidirectional pipe, the
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.Fa type
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argument may request a bidirectional data flow.
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The
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.Fa type
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argument is a pointer to a null-terminated string
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which must be
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.Ql r
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for reading,
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.Ql w
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for writing, or
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.Ql r+
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for reading and writing.
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.Pp
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A letter
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.Ql e
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may be appended to that to request that the underlying file descriptor
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be set close-on-exec.
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.Pp
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The
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.Fa command
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argument is a pointer to a null-terminated string
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containing a shell command line.
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This command is passed to
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.Pa /bin/sh
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using the
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.Fl c
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flag; interpretation, if any, is performed by the shell.
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.Pp
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The return value from
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.Fn popen
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is a normal standard
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.Tn I/O
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stream in all respects
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save that it must be closed with
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.Fn pclose
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rather than
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.Fn fclose .
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Writing to such a stream
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writes to the standard input of the command;
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the command's standard output is the same as that of the process that called
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.Fn popen ,
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unless this is altered by the command itself.
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Conversely, reading from a
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.Dq popened
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stream reads the command's standard output, and
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the command's standard input is the same as that of the process that called
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.Fn popen .
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.Pp
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Note that output
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.Fn popen
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streams are fully buffered by default.
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.Pp
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The
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.Fn pclose
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function waits for the associated process to terminate
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and returns the exit status of the command
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as returned by
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.Xr wait4 2 .
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.Sh RETURN VALUES
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The
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.Fn popen
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function returns
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.Dv NULL
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if the
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.Xr fork 2
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or
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.Xr pipe 2
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calls fail,
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or if it cannot allocate memory.
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.Pp
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The
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.Fn pclose
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function
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returns \-1 if
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.Fa stream
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is not associated with a
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.Dq popened
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command, if
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.Fa stream
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already
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.Dq pclosed ,
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or if
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.Xr wait4 2
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returns an error.
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.Sh ERRORS
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The
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.Fn popen
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function does not reliably set
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.Va errno .
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.Sh SEE ALSO
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.Xr sh 1 ,
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.Xr fork 2 ,
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.Xr pipe 2 ,
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.Xr wait4 2 ,
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.Xr fclose 3 ,
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.Xr fflush 3 ,
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.Xr fopen 3 ,
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.Xr stdio 3 ,
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.Xr system 3
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.Sh HISTORY
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A
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.Fn popen
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and a
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.Fn pclose
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function appeared in
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.At v7 .
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.Pp
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Bidirectional functionality was added in
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.Fx 2.2.6 .
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.Sh BUGS
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Since the standard input of a command opened for reading
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shares its seek offset with the process that called
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.Fn popen ,
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if the original process has done a buffered read,
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the command's input position may not be as expected.
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Similarly, the output from a command opened for writing
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may become intermingled with that of the original process.
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The latter can be avoided by calling
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.Xr fflush 3
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before
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.Fn popen .
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.Pp
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Failure to execute the shell
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is indistinguishable from the shell's failure to execute command,
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or an immediate exit of the command.
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The only hint is an exit status of 127.
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.Pp
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The
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.Fn popen
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function
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always calls
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.Xr sh 1 ,
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never calls
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.Xr csh 1 .
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