read(1p) — Linux manual page

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)

PROLOG         top

       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         top

       read — read from standard input into shell variables

SYNOPSIS         top

       read [-r] var...

DESCRIPTION         top

       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         top

       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         top

       The following operand shall be supported:

       var       The name of an existing or nonexisting shell variable.

STDIN         top

       The standard input shall be a text file.

INPUT FILES         top

       None.

ENVIRONMENT VARIABLES         top

       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         top

       Default.

STDOUT         top

       Not used.

STDERR         top

       The standard error shall be used for diagnostic messages and
       prompts for continued input.

OUTPUT FILES         top

       None.

EXTENDED DESCRIPTION         top

       None.

EXIT STATUS         top

       The following exit values shall be returned:

        0    Successful completion.

       >0    End-of-file was detected or an error occurred.

CONSEQUENCES OF ERRORS         top

       Default.

       The following sections are informative.

APPLICATION USAGE         top

       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         top

       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         top

       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         top

       None.

SEE ALSO         top

       Chapter 2, Shell Command Language

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

COPYRIGHT         top

       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)

Pages that refer to this page: line(1)