unset命令有什么作用 linux

Linux unset命令：
功能说明：unset是一个内建的Unix shell命令，在Bourne shell家族（sh、ksh、bash等）和C shell家族（csh、tcsh等）都有实现。它可以取消设置一个shell变量，从内存和shell的导出环境中删除它。它实现为一个壳层内建指令，因为它直接操纵shell的内部。
只读shell变量不能被unset。如果试图unset只读变量，unset命令将打印一个错误消息，并返回一个非零的退出代码。

This command removes one or more variables. Each name is a variable name, specified in any of the ways acceptable to the set command. If aname refers to an element of an array then that element is removed without affecting the rest of the array. If a name consists of an array name with no parenthesized index, then the entire array is deleted. The unset command returns an empty string as result. An error occurs if any of the variables doesn't exist, and any variables after the non-existent one are not deleted.
语　　法：
unset [-fv][变量或函数名称]

补充说明：unset为shell内建指令，可删除变量或函数。

参 　 数：
-f 　仅删除函数。
-v 　仅删除变量。

[3][root][test:/home/root/Desktop] set lx = "ls -lh"
[3][root][test:/home/root/Desktop] $lx
total 80K
drwxrwxr-x 2 root tt 4.0K 2014-02-19 09:32 bin
drwxrwxr-x 3 root tt 4.0K 2014-02-10 10:56 Data
drwxrwxr-x 2 root tt 4.0K 2014-02-19 09:32 lib
drwxrwxr-x 10 root tt 4.0K 2014-02-20 16:12 script
-r--r--r-- 1 root tt 312 2014-02-19 09:32 sut.cs
[3][root][test:/home/root/Desktop]
[3][root][test:/home/root/Desktop]
[3][root][test:/home/root/Desktop] set
......此处省略输出
[3][root][test:/home/root/Desktop] set |grep -i lx
lx ls -lh
[3][root][test:/home/root/Desktop]
[3][root][test:/home/root/Desktop] unset lx
[3][root][test:/home/root/Desktop] set | grep -i lx

set命令：
Linux and Unix set command

Quick links

About set
Syntax
Examples
Related commands
Linux and Unix main page

About set

set is a built-in function of the Bourne shell (sh), C shell (csh), and Korn shell (ksh), which is used to define and determine the values of the systemenvironment.

Syntax

Bourne shell (sh):

set [--aefhkntuvx[argument]]...

C shell (csh):

set [var[=value]]

set var [n] = word

Korn shell (ksh):

set [+-aefhkmnopstuvx] [+-o option]... [+-A name] [arg...]

Options: Bourne Shell (sh)

In sh, the set built-in command has the following options:

-- An option of a double-dash ("--") signifies the end of an option list. This is primarily useful when values listed after the options will start with a dash themselves.
-a Mark variables which are modified or created for "export"; environment variables set in this way will be passed on to the environments of any subsequent commands.
-e Exit immediately if a command exits with a non-zero exit status.
-f Disable file name generation (globbing).
-h Locate and remember function commands as functions are defined (function commands are normally located when the function is executed).
-k All keyword arguments are placed in the environment for a command, not just those that precede the command name.
-n Read commands but do not execute them.
-t Exit after reading and executing one command.
-u Treat unset variables as an error when substituting.
-v Print shell input lines as they are read.
-x Print commands and their arguments as they are executed.

Using + rather than - causes these flags to be turned off. These flags can also be used upon invocation of the shell itself. The current set of flags may be found in the variable $-. The remaining arguments are positional parameters and are assigned, in order, to $1, $2, .... If no arguments are given the values of all names are printed.

For each name, the unset command removes the corresponding variable or function value. The special variables PATH, PS1, PS2, MAILCHECK, and IF cannot be unset.

With the export built-in command, the given names are marked for automatic export to the environment of subsequently executed commands. If no arguments are given, variable names that have been marked for export during the current shell's execution are listed. Function names are not exported.

Options: C Shell (csh)

In csh, If no arguments are specified, set displays the values of all shell variables. Multiword values are displayed as a parenthesized list. With thevar argument alone, set assigns an empty (null) value to the variable var. With arguments of the form var = value, set assigns value to var, wherevalue is one of:

word A single word (or quoted string).
(wordlist) A space-separated list of words enclosed in parentheses.

Values are command and filename expanded before being assigned. The form set var[n]=word replaces the n'th word in a multiword value withword.

unset removes variables whose names match (using filename substitution) pattern. All variables are removed by "unset *"; this is a very bad idea if you don't know what you're doing, though.

Options: K Shell (ksh)

In ksh, the set command takes the following options:

-A Array assignment. Unset the variable name and assign values sequentially from the list arg. If +A is used, the variable name is not unset first.
-a All subsequent variables that are defined are automatically exported.
-e If a command has a non-zero exit status, execute the ERR trap (if set), and exit. This mode is disabled while reading profiles.
-f Disables file name generation (globbing).
-h Each command becomes a tracked alias when first encountered.
-k All variable assignment arguments are placed in the environment for a command, not just those that precede the command name.
-m Background jobs will run in a separate process group and a line will print upon completion. The exit status of background jobs is reported in a completion message. On systems with job control, this flag is turned on automatically for interactive shells.
-n Read commands and check them for syntax errors, but do not execute them. Ignored for interactive shells.
-ooption The option argument can be one of the following:

allexport Same as -a.
errexit Same as -e.
bgnice All background jobs are run at a lower priority. This is the default mode.
emacs Puts you in an emacs-style in-line editor for command entry.
emacs Puts you in a gmacs-style in-line editor for command entry.
ignoreeof The shell will not exit on end-of-file. The command exit must be used.
keyword Same as -k.
markdirs All directory names resulting from file name generation have a trailing "/" appended.
monitor Same as -m.
noclobber Prevents redirection ">" from truncating existing files. Require ">|" to truncate a file when turned on.
noexec Same as -n.
noglob Same as -f.
nolog Do not save function definitions in history file.
nounset Same as -u.
privileged Same as -p.
verbose Same as -v.
trackall Same as -h.
vi Puts you in insert mode of a vi-style in-line editor until you hit escape. This puts you in control mode. A return sends the line.
viraw Each character is processed as it is typed in vi mode.
xtrace Same as -x.

If no option name is supplied then the current option settings are printed.

-p Disables processing of the $HOME/.profile file and uses the file /etc/suid_profile instead of the ENV file. This mode is on whenever the effective uid (user ID) is not equal to the real uid, or when the effective gid (group ID) is not equal to the real gid. Turning this off causes the effective uid and gid to be set to the real uid and gid.
-s Sort the positional parameters lexicographically.
-t Exit after reading and executing one command.
-u Treat unset parameters as an error when substituting.
-v Print shell input lines as they are read.
-x Print commands and their arguments as they are executed.
- Turns off -x and -v flags and stops examining arguments for flags.
- Do not change any of the flags; useful in setting $1 to a value beginning with -. If no arguments follow this flag then the positional parameters are unset.

Using + rather than - causes these flags to be turned off. These flags can also be used upon invocation of the shell. The current set of flags may be found in $-. Unless -A is specified, the remaining arguments are positional parameters and are assigned, in order, to "$1 $2 ...". If no arguments are given, then the names and values of all variables are printed on the standard output.

The variables given by the list of names are automatically unassigned: their values and attributes are erased. Readonly variables cannot be unset. If the -f flag is set, then the names refer to function names. Unsetting ERRNO, LINENO, MAILCHECK, OPTARG, OPTIND, RANDOM, SECONDS,TMOUT, and _ removes their special meaning even if they are subsequently assigned.

When using unset, the variables given by the list of names are similarly unassigned: their values and attributes are erased. Readonly variables cannot be unset. If the -f, flag is set, then the names refer to function names. Unsetting ERRNO, LINENO, MAILCHECK, OPTARG, OPTIND,RANDOM, SECONDS, TMOUT, and _ removes their special meaning even if they are subsequently assigned.

With the export built-in command, the given names are marked for automatic export to the environment of subsequently-executed commands. kshcommands that are preceded by one or two "*" characters (asterisks) are treated specially in the following ways:

Variable assignment lists preceding the command remain in effect when the command completes.
I/O redirections are processed after variable assignments.
Errors cause a script that contains them to abort.
Words, following a command preceded by ** that are in the format of a variable assignment, are expanded with the same rules as a variable assignment. This means that tilde substitution is performed after the = sign and word splitting and file name generation are not performed.

Examples

set PATH="/bin:/usr/bin:/usr/sbin:usr/local/bin"

In csh, this command sets the environment variable PATH, such that the shell will search for files in the /bin, /usr/bin, /usr/sbin and /usr/local/bindirectories, in that order.

Related commands

csh
ksh
setenv
sh