read

READ(1P)                    POSIX Programmer's Manual                   READ(1P)



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.

NAME
       read — read from standard input into shell variables

SYNOPSIS
       read [-r] var...

DESCRIPTION
       The read utility shall read a single logical line from standard input
       into one or more shell variables.

       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.

OPTIONS
       The read utility shall conform to the Base Definitions volume of
       POSIX.1‐2017, 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 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.1‐2017, 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.

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 POSIX.1‐2008.

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 POSIX.1‐2017.

       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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Chapter 2, Shell Command Language

       The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

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 .

       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                   2017                              READ(1P)