path: root/man-pages-posix-2017/man1p/read.1p

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.TH READ "1P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
read
\(em read from standard input into shell variables
.SH SYNOPSIS
.LP
.nf
read \fB[\fR-r\fB] \fIvar\fR...
.fi
.SH DESCRIPTION
The
.IR read
utility shall read a single logical line from standard input into one
or more shell variables.
.P
By default, unless the
.BR \-r
option is specified,
<backslash>
shall act as an escape character. An unescaped
<backslash>
shall preserve the literal value of the following character, with the
exception of a
<newline>.
If a
<newline>
follows the
<backslash>,
the
.IR read
utility shall interpret this as line continuation. The
<backslash>
and
<newline>
shall be removed before splitting the input into fields. All other
unescaped
<backslash>
characters shall be removed after splitting the input into fields.
.P
If standard input is a terminal device and the invoking shell is
interactive,
.IR read
shall prompt for a continuation line when it reads an input line ending
with a
<backslash>
<newline>,
unless the
.BR \-r
option is specified.
.P
The terminating
<newline>
(if any) shall be removed from the input and the results shall be split
into fields as in the shell for the results of parameter expansion (see
.IR "Section 2.6.5" ", " "Field Splitting");
the first field shall be assigned to the first variable
.IR var ,
the second field to the second variable
.IR var ,
and so on. If there are fewer fields than there are
.IR var
operands, the remaining
.IR var s
shall be set to empty strings. If there are fewer
.IR var s
than fields, the last
.IR var
shall be set to a value comprising the following elements:
.IP " *" 4
The field that corresponds to the last
.IR var
in the normal assignment sequence described above
.IP " *" 4
The delimiter(s) that follow the field corresponding to the last
.IR var
.IP " *" 4
The remaining fields and their delimiters, with trailing
.IR IFS
white space ignored
.P
The setting of variables specified by the
.IR var
operands shall affect the current shell execution environment; see
.IR "Section 2.12" ", " "Shell Execution Environment".
If it is called in a subshell or separate utility execution
environment, such as one of the following:
.sp
.RS 4
.nf

(read foo)
nohup read ...
find . -exec read ... \e;
.fi
.P
.RE
.P
it shall not affect the shell variables in the caller's environment.
.SH OPTIONS
The
.IR read
utility shall conform to the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 12.2" ", " "Utility Syntax Guidelines".
.P
The following option is supported:
.IP "\fB\-r\fP" 10
Do not treat a
<backslash>
character in any special way. Consider each
<backslash>
to be part of the input line.
.SH OPERANDS
The following operand shall be supported:
.IP "\fIvar\fR" 10
The name of an existing or nonexisting shell variable.
.SH STDIN
The standard input shall be a text file.
.SH "INPUT FILES"
None.
.SH "ENVIRONMENT VARIABLES"
The following environment variables shall affect the execution of
.IR read :
.IP "\fIIFS\fP" 10
Determine the internal field separators used to delimit fields; see
.IR "Section 2.5.3" ", " "Shell Variables".
.IP "\fILANG\fP" 10
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 8.2" ", " "Internationalization Variables"
for the precedence of internationalization variables used to determine
the values of locale categories.)
.IP "\fILC_ALL\fP" 10
If set to a non-empty string value, override the values of all the
other internationalization variables.
.IP "\fILC_CTYPE\fP" 10
Determine the locale for the interpretation of sequences of bytes of
text data as characters (for example, single-byte as opposed to
multi-byte characters in arguments).
.IP "\fILC_MESSAGES\fP" 10
.br
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard error.
.IP "\fINLSPATH\fP" 10
Determine the location of message catalogs for the processing of
.IR LC_MESSAGES .
.IP "\fIPS2\fP" 10
Provide the prompt string that an interactive shell shall write to
standard error when a line ending with a
<backslash>
<newline>
is read and the
.BR \-r
option was not specified.
.SH "ASYNCHRONOUS EVENTS"
Default.
.SH STDOUT
Not used.
.SH STDERR
The standard error shall be used for diagnostic messages and
prompts for continued input.
.SH "OUTPUT FILES"
None.
.SH "EXTENDED DESCRIPTION"
None.
.SH "EXIT STATUS"
The following exit values shall be returned:
.IP "\00" 6
Successful completion.
.IP >0 6
End-of-file was detected or an error occurred.
.SH "CONSEQUENCES OF ERRORS"
Default.
.LP
.IR "The following sections are informative."
.SH "APPLICATION USAGE"
The
.BR \-r
option is included to enable
.IR read
to subsume the purpose of the
.IR line
utility, which is not included in POSIX.1\(hy2008.
.SH EXAMPLES
The following command:
.sp
.RS 4
.nf

while read -r xx yy
do
    printf "%s %s\en$yy$xx"
done < \fIinput_file\fR
.fi
.P
.RE
.P
prints a file with the first field of each line moved to the end of the
line.
.SH RATIONALE
The
.IR read
utility historically has been a shell built-in. It was separated off
into its own utility to take advantage of the richer description of
functionality introduced by this volume of POSIX.1\(hy2017.
.P
Since
.IR read
affects the current shell execution environment,
it is generally provided as a shell regular built-in. If it is called
in a subshell or separate utility execution environment, such as one of
the following:
.sp
.RS 4
.nf

(read foo)
nohup read ...
find . -exec read ... \e;
.fi
.P
.RE
.P
it does not affect the shell variables in the environment of the
caller.
.P
Although the standard input is required to be a text file, and
therefore will always end with a
<newline>
(unless it is an empty file), the processing of continuation lines
when the
.BR \-r
option is not used can result in the input not ending with a
<newline>.
This occurs if the last line of the input file ends with a
<backslash>
<newline>.
It is for this reason that ``if any'' is used in ``The terminating
<newline>
(if any) shall be removed from the input'' in the description.
It is not a relaxation of the requirement for standard input to
be a text file.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Chapter 2" ", " "Shell Command Language"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 8" ", " "Environment Variables",
.IR "Section 12.2" ", " "Utility Syntax Guidelines"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .