Set&Setlocal命令注释

Displays, sets, or removes cmd.exe environment variables.

SET [variable=[string]]

variable Specifies the environment-variable name.

string Specifies a series of characters to assign to the variable.

Type SET without parameters to display the current environment variables.

If Command Extensions are enabled SET changes as follows:

SET command invoked with just a variable name, no equal sign or value

will display the value of all variables whose prefix matches the name

given to the SET command. For example:

SET P

would display all variables that begin with the letter 'P'

SET command will set the ERRORLEVEL to 1 if the variable name is not

found in the current environment.

SET command will not allow an equal sign to be part of the name of

a variable.

Two new switches have been added to the SET command:

SET /A expression

SET /P variable=[promptString]

The /A switch specifies that the string to the right of the equal sign

is a numerical expression that is evaluated. The expression evaluator

is pretty simple and supports the following operations, in decreasing

order of precedence:

() - grouping

! ~ - - unary operators

* / % - arithmetic operators

+ - - arithmetic operators

<< >> - logical shift

& - bitwise and

^ - bitwise exclusive or

| - bitwise or

= *= /= %= += -= - assignment

&= ^= |= <<= >>=

, - expression separator

If you use any of the logical or modulus operators, you will need to

enclose the expression string in quotes. Any non-numeric strings in the

expression are treated as environment variable names whose values are

converted to numbers before using them. If an environment variable name

is specified but is not defined in the current environment, then a value

of zero is used. This allows you to do arithmetic with environment

variable values without having to type all those % signs to get their

values. If SET /A is executed from the command line outside of a

command script, then it displays the final value of the expression. The

assignment operator requires an environment variable name to the left of

the assignment operator. Numeric values are decimal numbers, unless

prefixed by 0x for hexadecimal numbers, and 0 for octal numbers.

So 0x12 is the same as 18 is the same as 022. Please note that the octal

notation can be confusing: 08 and 09 are not valid numbers because 8 and

9 are not valid octal digits.

The /P switch allows you to set the value of a variable to a line of inpu

entered by the user. Displays the specified promptString before reading

the line of input. The promptString can be empty.

Environment variable substitution has been enhanced as follows:

%PATH:str1=str2%

would expand the PATH environment variable, substituting each occurrence

of \"str1\" in the expanded result with \"str2\". \"str2\" can be the empty

string to effectively delete all occurrences of \"str1\" from the expanded

output. \"str1\" can begin with an asterisk, in which case it will match

everything from the beginning of the expanded output to the first

occurrence of the remaining portion of str1.

May also specify substrings for an expansion.

%PATH:~10,5%

would expand the PATH environment variable, and then use only the 5

characters that begin at the 11th (offset 10) character of the expanded

result. If the length is not specified, then it defaults to the

remainder of the variable value. If either number (offset or length) is

negative, then the number used is the length of the environment variable

value added to the offset or length specified.

%PATH:~-10% would extract the last 10 characters of the PATH variable.

%PATH:~0,-2% would extract all but the last 2 characters of the PATH variable.

Finally, support for delayed environment variable expansion has been

added. This support is always disabled by default, but may be

enabled/disabled via the /V command line switch to CMD.EXE. See CMD /?

Delayed environment variable expansion is useful for getting around

the limitations of the current expansion which happens when a line

of text is read, not when it is executed. The following example

demonstrates the problem with immediate variable expansion:

set VAR=before

if \"%VAR%\" == \"before\" (

set VAR=after

if \"%VAR%\" == \"after\" @echo If you see this, it worked

)

would never display the message, since the %VAR% in BOTH IF statements

is substituted when the first IF statement is read, since it logically

includes the body of the IF, which is a compound statement. So the

IF inside the compound statement is really comparing \"before\" with

\"after\" which will never be equal. Similarly, the following example

will not work as expected:

set LIST=

for %i in (*) do set LIST=%LIST% %i

echo %LIST%

in that it will NOT build up a list of files in the current directory,

but instead will just set the LIST variable to the last file found.

Again, this is because the %LIST% is expanded just once when the

FOR statement is read, and at that time the LIST variable is empty.

So the actual FOR loop we are executing is:

for %i in (*) do set LIST= %i

which just keeps setting LIST to the last file found.

Delayed environment variable expansion allows you to use a different

character (the exclamation mark) to expand environment variables at

execution time. If delayed variable expansion is enabled, the above

examples could be written as follows to work as intended:

set VAR=before

if \"%VAR%\" == \"before\" (

set VAR=after

if \"!VAR!\" == \"after\" @echo If you see this, it worked

)

set LIST=

for %i in (*) do set LIST=!LIST! %i

echo %LIST%

If Command Extensions are enabled, then there are several dynamic

environment variables that can be expanded but which don't show up in

the list of variables displayed by SET. These variable values are

computed dynamically each time the value of the variable is expanded.

If the user explicitly defines a variable with one of these names, then

that definition will override the dynamic one described below:

%CD% - expands to the current directory string.

%DATE% - expands to current date using same format as DATE command.

%TIME% - expands to current time using same format as TIME command.

%RANDOM% - expands to a random decimal number between 0 and 32767.

%ERRORLEVEL% - expands to the current ERRORLEVEL value

%CMDEXTVERSION% - expands to the current Command Processor Extensions

version number.

%CMDCMDLINE% - expands to the original command line that invoked the

Command Processor.

Begins localization of environment changes in a batch file. Environment

changes made after SETLOCAL has been issued are local to the batch file.

ENDLOCAL must be issued to restore the previous settings. When the end

of a batch script is reached, an implied ENDLOCAL is executed for any

outstanding SETLOCAL commands issued by that batch script.

SETLOCAL

If Command Extensions are enabled SETLOCAL changes as follows:

SETLOCAL batch command now accepts optional arguments:

ENABLEEXTENSIONS / DISABLEEXTENSIONS

enable or disable command processor extensions. See

CMD /? for details.

ENABLEDELAYEDEXPANSION / DISABLEDELAYEDEXPANSION

enable or disable delayed environment variable

expansion. See SET /? for details.

These modifications last until the matching ENDLOCAL command,

regardless of their setting prior to the SETLOCAL command.

The SETLOCAL command will set the ERRORLEVEL value if given

an argument. It will be zero if one of the two valid arguments

is given and one otherwise. You can use this in batch scripts

to determine if the extensions are available, using the following

technique:

VERIFY OTHER 2>nul

SETLOCAL ENABLEEXTENSIONS

IF ERRORLEVEL 1 echo Unable to enable extensions

This works because on old versions of CMD.EXE, SETLOCAL does NOT

set the ERRORLEVEL value. The VERIFY command with a bad argument

initializes the ERRORLEVEL value to a non-zero value.