|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
READ(1P) POSIX Programmer's Manual READ(1P)
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.
read — read a line from standard input
read [−r] var...
The read utility shall read a single line from standard input.
By default, unless the −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 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.
If standard input is a terminal device and the invoking shell is
interactive, read shall prompt for a continuation line when it reads
an input line ending with a <backslash> <newline>, unless the −r
option is specified.
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 Section 2.6.5, Field Splitting);
the first field shall be assigned to the first variable var, the
second field to the second variable var, and so on. If there are
fewer fields than there are var operands, the remaining vars shall be
set to empty strings. If there are fewer vars than fields, the last
var shall be set to a value comprising the following elements:
* The field that corresponds to the last var in the normal
assignment sequence described above
* The delimiter(s) that follow the field corresponding to the last
var
* The remaining fields and their delimiters, with trailing IFS
white space ignored
The setting of variables specified by the var operands shall affect
the current shell execution environment; see Section 2.12, Shell
Execution Environment. If it is called in a subshell or separate
utility execution environment, such as one of the following:
(read foo)
nohup read ...
find . −exec read ... \;
it shall not affect the shell variables in the caller's environment.
The read utility shall conform to the Base Definitions volume of
POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.
The following option is supported:
−r Do not treat a <backslash> character in any special way.
Consider each <backslash> to be part of the input line.
The following operand shall be supported:
var The name of an existing or nonexisting shell variable.
The standard input shall be a text file.
None.
The following environment variables shall affect the execution of
read:
IFS Determine the internal field separators used to delimit
fields; see Section 2.5.3, Shell Variables.
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions
volume of POSIX.1‐2008, Section 8.2, Internationalization
Variables for the precedence of internationalization
variables used to determine the values of locale
categories.)
LC_ALL If set to a non-empty string value, override the values of
all the other internationalization variables.
LC_CTYPE 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).
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
PS2 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 −r option was not
specified.
Default.
Not used.
The standard error shall be used for diagnostic messages and prompts
for continued input.
None.
None.
The following exit values shall be returned:
0 Successful completion.
>0 End-of-file was detected or an error occurred.
Default.
The following sections are informative.
The −r option is included to enable read to subsume the purpose of
the line utility, which is not included in POSIX.1‐2008.
The following command:
while read −r xx yy
do
printf "%s %s\n$yy$xx"
done < input_file
prints a file with the first field of each line moved to the end of
the line.
The 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.1‐2008.
Since 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:
(read foo)
nohup read ...
find . −exec read ... \;
it does not affect the shell variables in the environment of the
caller.
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 −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.
None.
Chapter 2, Shell Command Language
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
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 .
IEEE/The Open Group 2013 READ(1P)