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, as described
in Escape Character (Backslash) . If standard input is a terminal
device and the
invoking shell is interactive, read shall prompt for a continuation
line when:
The shell reads an input line ending with a backslash, unless the
-r option is specified.
A here-document is not terminated after a <newline> is entered.
The line shall be split into fields as in the shell (see 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 var operands specified than there are fields,
the leftover fields and their intervening separators shall
be assigned to the last var. If there are fewer fields than
vars, the remaining vars shall be set to empty
strings.
The setting of variables specified by the var operands shall
affect the current shell execution environment; see Shell Execution
Environment . If it is called in a subshell or separate utility
execution
environment, such as one of the following:
it shall not affect the shell variables in the caller's environment.
OPTIONS
The read utility shall conform to the Base Definitions volume
of IEEE Std 1003.1-2001, 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.
OPERANDS
The following operand shall be supported:
var
The name of an existing or nonexisting shell variable.
STDIN
The standard input shall be a text file.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of
read:
IFS
Determine the internal field separators used to delimit fields; see
Shell
Variables .
LANG
Provide a default value for the internationalization variables that
are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, 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 is read
and the -r option was not specified, or if a here-document is
not terminated after a <newline> is entered.
ASYNCHRONOUS EVENTS
Default.
STDOUT
Not used.
STDERR
The standard error shall be used for diagnostic messages and prompts
for continued input.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0
Successful completion.
>0
End-of-file was detected or an error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
The -r option is included to enable read to subsume the
purpose of the line utility, which is not included
in IEEE Std 1003.1-2001.
The results are undefined if an end-of-file is detected following
a backslash at the end of a line when -r is not
specified.
EXAMPLES
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.
RATIONALE
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 IEEE Std 1003.1-2001.
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:
it does not affect the shell variables in the environment of the caller.
FUTURE DIRECTIONS
None.
SEE ALSO
Shell Command Language
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
