unset(1) [linux man page]
UNSET(P) POSIX Programmer's Manual UNSET(P) 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 func- tion 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. 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 mis- interpreted to include the special parameters (see 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 histori- cal 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
Special Built-In Utilities COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol- ogy -- 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 . IEEE
/The Open Group 2003 UNSET(P)
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READONLY(P) POSIX Programmer's Manual READONLY(P) 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 IEEE Std 1003.1-2001, 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 ", <name>, <value> if name is set, and "readonly %s ", <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 IEEE Std 1003.1-2001 allows this behav- ior, 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
Special Built-In Utilities COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol- ogy -- 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 . IEEE
/The Open Group 2003 READONLY(P)