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
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
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
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 Special Parameters ).
Consideration was given to omitting the -f option in favor of an
unfunction utility, but the standard developers decided to retain
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.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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