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       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

       readonly — set the readonly attribute for variables

SYNOPSIS

       readonly name[=word]...

       readonly −p

DESCRIPTION

       The  variables whose names are specified shall be given the readonly attribute. The values
       of variables with the readonly attribute cannot be changed by subsequent  assignment,  nor
       can  those  variables be unset by the unset utility. If the name of a variable is followed
       by =word, then the value of that variable shall be set to word.

       The readonly special built-in shall support the Base Definitions volume  of  POSIX.1‐2008,
       Section 12.2, Utility Syntax Guidelines.

       When  −p  is specified, readonly writes to the standard output the names and values of all
       read-only variables, in the following format:

           "readonly %s=%s\n", <name>, <value>

       if name is set, and

           "readonly %s\n", <name>

       if name is unset.

       The shell shall format the output, including the proper use of  quoting,  so  that  it  is
       suitable  for  reinput  to  the shell as commands that achieve the same value and readonly
       attribute-setting results in a shell execution environment in which:

        1. Variables with values at the time they were output do not have the readonly  attribute
           set.

        2. Variables that were unset at the time they were output do not have a value at the time
           at which the saved output is reinput to the shell.

       When no arguments are given, the results are unspecified.

OPTIONS

       See the DESCRIPTION.

OPERANDS

       See the DESCRIPTION.

STDIN

       Not used.

INPUT FILES

       None.

ENVIRONMENT VARIABLES

       None.

ASYNCHRONOUS EVENTS

       Default.

STDOUT

       See the DESCRIPTION.

STDERR

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES

       None.

EXTENDED DESCRIPTION

       None.

EXIT STATUS

       Zero.

CONSEQUENCES OF ERRORS

       Default.

       The following sections are informative.

APPLICATION USAGE

       None.

EXAMPLES

       readonly HOME PWD

RATIONALE

       Some historical shells preserve the readonly attribute across separate  invocations.  This
       volume of POSIX.1‐2008 allows this behavior, but does not require it.

       The  −p  option  allows  portable  access  to  the values that can be saved and then later
       restored using, for example, a dot script.  Also  see  the  RATIONALE  for  export  for  a
       description of the no-argument and −p output cases and a related example.

       Read-only  functions  were  considered,  but  they  were  omitted  as not being historical
       practice or particularly useful. Furthermore,  functions  must  not  be  read-only  across
       invocations  to preclude ``spoofing'' (spoofing is the term for the practice of creating a
       program that acts like a well-known utility with the intent of subverting the real  intent
       of the user) of administrative or security-relevant (or security-conscious) shell scripts.

FUTURE DIRECTIONS

       None.

SEE ALSO

       Section 2.14, Special Built-In Utilities

       The Base Definitions volume of POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines

COPYRIGHT

       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 .