mirror of https://github.com/mkerrisk/man-pages
206 lines
5.8 KiB
Groff
206 lines
5.8 KiB
Groff
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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "BG" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" bg
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.SH NAME
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bg \- run jobs in the background
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.SH SYNOPSIS
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.LP
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\fBbg\fP \fB[\fP\fIjob_id\fP \fB...\fP\fB]\fP\fB\fP
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.SH DESCRIPTION
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.LP
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If job control is enabled (see the description of \fIset\fP \fB-m\fP),
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the
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\fIbg\fP utility shall resume suspended jobs from the current environment
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(see \fIShell
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Execution Environment\fP ) by running them as background jobs. If
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the job specified by \fIjob_id\fP is already a running
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background job, the \fIbg\fP utility shall have no effect and shall
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exit successfully.
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.LP
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Using \fIbg\fP to place a job into the background shall cause its
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process ID to become "known in the current shell execution
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environment", as if it had been started as an asynchronous list; see
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\fIAsynchronous
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Lists\fP .
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.SH OPTIONS
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.LP
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None.
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.SH OPERANDS
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.LP
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The following operand shall be supported:
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.TP 7
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\fIjob_id\fP
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Specify the job to be resumed as a background job. If no \fIjob_id\fP
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operand is given, the most recently suspended job shall
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be used. The format of \fIjob_id\fP is described in the Base Definitions
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volume of IEEE\ Std\ 1003.1-2001, Section 3.203, Job Control Job ID.
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.sp
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.SH STDIN
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.LP
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Not used.
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.SH INPUT FILES
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.LP
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None.
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.SH ENVIRONMENT VARIABLES
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.LP
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The following environment variables shall affect the execution of
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\fIbg\fP:
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.TP 7
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\fILANG\fP
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Provide a default value for the internationalization variables that
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are unset or null. (See the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, Section 8.2, Internationalization Variables
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for
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the precedence of internationalization variables used to determine
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the values of locale categories.)
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.TP 7
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\fILC_ALL\fP
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If set to a non-empty string value, override the values of all the
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other internationalization variables.
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.TP 7
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\fILC_CTYPE\fP
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Determine the locale for the interpretation of sequences of bytes
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of text data as characters (for example, single-byte as
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opposed to multi-byte characters in arguments).
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.TP 7
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\fILC_MESSAGES\fP
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Determine the locale that should be used to affect the format and
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contents of diagnostic messages written to standard
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error.
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.TP 7
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\fINLSPATH\fP
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Determine the location of message catalogs for the processing of \fILC_MESSAGES
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\&.\fP
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.sp
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.SH ASYNCHRONOUS EVENTS
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.LP
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Default.
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.SH STDOUT
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.LP
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The output of \fIbg\fP shall consist of a line in the format:
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.sp
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.RS
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.nf
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\fB"[%d] %s\\n", <\fP\fIjob-number\fP\fB>, <\fP\fIcommand\fP\fB>
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\fP
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.fi
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.RE
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.LP
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where the fields are as follows:
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.TP 7
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<\fIjob-number\fP>
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A number that can be used to identify the job to the \fIwait\fP, \fIfg\fP,
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and \fIkill\fP utilities. Using these utilities, the
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job can be identified by prefixing the job number with \fB'%'\fP .
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.TP 7
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<\fIcommand\fP>
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The associated command that was given to the shell.
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.sp
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.SH STDERR
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.LP
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The standard error shall be used only for diagnostic messages.
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.SH OUTPUT FILES
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.LP
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None.
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.SH EXTENDED DESCRIPTION
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.LP
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None.
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.SH EXIT STATUS
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.LP
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The following exit values shall be returned:
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.TP 7
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\ 0
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Successful completion.
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.TP 7
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>0
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An error occurred.
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.sp
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.SH CONSEQUENCES OF ERRORS
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.LP
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If job control is disabled, the \fIbg\fP utility shall exit with an
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error and no job shall be placed in the background.
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.LP
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\fIThe following sections are informative.\fP
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.SH APPLICATION USAGE
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.LP
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A job is generally suspended by typing the SUSP character (<control>-Z
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on most systems); see the Base Definitions volume
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of IEEE\ Std\ 1003.1-2001, Chapter 11, General Terminal Interface.
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At that
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point, \fIbg\fP can put the job into the background. This is most
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effective when the job is expecting no terminal input and its
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output has been redirected to non-terminal files. A background job
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can be forced to stop when it has terminal output by issuing the
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command:
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.sp
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.RS
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.nf
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\fBstty tostop
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\fP
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.fi
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.RE
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.LP
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A background job can be stopped with the command:
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.sp
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.RS
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.nf
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\fBkill -s stop\fP \fIjob ID\fP
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.fi
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.RE
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.LP
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The \fIbg\fP utility does not work as expected when it is operating
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in its own utility execution environment because that
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environment has no suspended jobs. In the following examples:
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.sp
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.RS
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.nf
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\fB\&... | xargs bg
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(bg)
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\fP
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.fi
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.RE
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.LP
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each \fIbg\fP operates in a different environment and does not share
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its parent shell's understanding of jobs. For this reason,
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\fIbg\fP is generally implemented as a shell regular built-in.
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.SH EXAMPLES
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.LP
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None.
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.SH RATIONALE
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.LP
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The extensions to the shell specified in this volume of IEEE\ Std\ 1003.1-2001
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have mostly been based on features
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provided by the KornShell. The job control features provided by \fIbg\fP,
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\fIfg\fP, and \fIjobs\fP are also based on the KornShell. The standard
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developers examined the characteristics
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of the C shell versions of these utilities and found that differences
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exist. Despite widespread use of the C shell, the KornShell
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versions were selected for this volume of IEEE\ Std\ 1003.1-2001 to
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maintain a degree of uniformity with the rest of the
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KornShell features selected (such as the very popular command line
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editing features).
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.LP
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The \fIbg\fP utility is expected to wrap its output if the output
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exceeds the number of display columns.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIAsynchronous Lists\fP , \fIfg\fP , \fIkill\fP() , \fIjobs\fP ,
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\fIwait\fP()
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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