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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, 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: (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 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.
The following operand shall be supported: var The name of an existing or nonexisting shell variable.
The standard input shall be a text file.
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.
The standard error shall be used for diagnostic messages and prompts for continued input.
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.
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.
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 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: (read foo) nohup read ... find . -exec read ... \; it does not affect the shell variables in the environment of the caller.
Shell Command Language
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 .