readonly man page


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.

readonly — set the readonly attribute for variables


readonly name[=word]...

readonly -p


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:

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


See the Description.


See the Description.


Not used.

Asynchronous Events



See the Description.


The standard error shall be used only for diagnostic messages.

Exit Status


Consequences of Errors


The following sections are informative.


readonly HOME PWD


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.

See Also

Section 2.14, Special Built-In Utilities

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