UNSET(1POSIX)

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
unset — unset values and attributes of variables and functions

SYNOPSIS

unset [-fv] name...

DESCRIPTION
Each variable or function specified by name shall be unset.

If -v is specified, name refers to a variable name and the shell shall unset it and remove it from the environment. Read-only variables cannot be unset.

If -f is specified, name refers to a function and the shell shall unset the function definition.

If neither -f nor -v is specified, name refers to a variable; if a variable by that name does not exist, it is unspecified whether a function by that name, if any, shall be unset.

Unsetting a variable or function that was not previously set shall not be considered an error and does not cause the shell to abort.

The unset special built-in shall support the Base Definitions volume of POSIX.1-2017, Section 12.2, Utility Syntax Guidelines.

Note that:

    VARIABLE=

is not equivalent to an unset of VARIABLE; in the example, VARIABLE is set to "". Also, the variables that can be unset should not be misinterpreted to include the special parameters (see Section 2.5.2, Special Parameters).

OPTIONS
See the DESCRIPTION.

OPERANDS
See the DESCRIPTION.

STDIN
Not used.

INPUT FILES
None.

ENVIRONMENT VARIABLES
None.

ASYNCHRONOUS EVENTS
Default.

STDOUT
Not used.

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 unset.

>0  At least one name could not be unset.

CONSEQUENCES OF ERRORS
Default.

The following sections are informative.

APPLICATION USAGE
None.

EXAMPLES
Unset VISUAL variable:

    unset -v VISUAL

Unset the functions foo and bar:

    unset -f foo bar

RATIONALE
Consideration was given to omitting the -f option in favor of an unfunction utility, but the standard developers decided to retain historical practice.

The -v option was introduced because System V historically used one name space for both variables and functions. When unset is used without options, System V historically unset either a function or a variable, and there was no confusion about which one was intended. A portable POSIX application can use unset without an option to unset a variable, but not a function; the -f option must be used.

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 .