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

       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.1‐2017,
       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

        0    All name operands were successfully marked readonly.

       >0    At least one name could not be marked readonly, or the -p option was  specified  and
             an error occurred.

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.1‐2017 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.1‐2017, 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 .