.BAT File Format – Variables

set

• Names are not case sensitive but set shows them as typed. A name can hold most characters except =, and spaces count: set a = 1 sets a variable named a (with a space) to 1.
• Because < > | & ^ are special, they need a caret or quotes in a value. Microsoft: set v=New^&Name gives New&Name, and set v="New&Name" keeps the quotes in the value.
• Limits (Microsoft’s cmd page): one variable up to 8,192 bytes, and the whole environment of a process up to 65,536 characters; the longest line is 8191 characters.
• A variable set in a batch file lasts for the cmd.exe process, and is not passed up to the parent that started it.

Expansion

These forms need command extensions (on by default). A variable that is not set expands to nothing, which is why a missing variable in if %x%==1 makes a syntax error: write if "%x%"=="1".

The command line of a batch file argument is %1, not a variable; see Parameters and labels.

Delayed expansion

Normal expansion happens when a line, or a whole parenthesised block, is read, before any command in it runs. So a variable changed inside a block or a for body still has its old value in the same block:

set n=1
if 1==1 (
    set n=2
    echo %n%
    setlocal enabledelayedexpansion
    echo !n!
)

• Turn it on with setlocal enabledelayedexpansion (until endlocal), with cmd /v:on, or with a registry setting; disabledelayedexpansion and /v:off turn it off. It is off by default.
• When it is on, !NAME! is replaced when the command runs. !NAME:~n,m! and !NAME:old=new! work as with percent signs.
• Every ! in a line becomes special while it is on. To keep a literal one, write ^^! (or quote the text and write ^!). Values that hold a !, such as file names, can be damaged by a later expansion.
• Loop variables such as %%i are expanded in an earlier pass than !var!, so !array[%%i]! works.

set /a

The expression is evaluated with whole numbers; a name in it is taken as a variable and its value is read as a number, and a name that is not set counts as 0, so % signs are not needed. The operators are, from the strongest binding to the weakest (Microsoft):

• Numbers are decimal, hexadecimal with 0x, or octal with a leading 0: 0x12, 18 and 022 are equal (Microsoft). So 08 and 09 are errors.
• Arithmetic is reported to use 32-bit signed integers, so results wrap and division truncates toward zero. Microsoft does not state the width.
• The characters &, |, ^, <, > and in a batch file the remainder % (written %%) need quotes or carets: set /a "x=a|b". Microsoft says to quote an expression that uses &&, || or %.
• Typed at the prompt, set /a also prints the result; in a batch file it does not.
• There is no floating point, no string arithmetic and no exponent operator.

Special variables

These are computed each time they are expanded, are not stored in the environment, and are missing when command extensions are disabled (SS64).

Microsoft’s if page adds that if defined makes %errorlevel%, %cmdcmdline% and %cmdextversion% available as environment variables, and that a real variable of the same name takes precedence over the dynamic one.

Windows fills the environment with ordinary variables, which a script may rely on but Microsoft’s command pages do not list: COMSPEC (the path of cmd.exe), PATH, PATHEXT, PROMPT, DIRCMD (default dir options), TEMP and TMP, USERNAME, USERPROFILE, APPDATA, LOCALAPPDATA, COMPUTERNAME, SystemRoot and windir, ProgramFiles and ProgramFiles(x86), NUMBER_OF_PROCESSORS, PROCESSOR_ARCHITECTURE and OS (Windows_NT). The order of precedence given by Microsoft’s cmd page: built-in system variables, then the system and user registry variables, then Autoexec.bat, then a logon script, then variables set in the script.

setlocal and endlocal

setlocal [enableextensions | disableextensions] [enabledelayedexpansion | disabledelayedexpansion] starts a local environment in a batch file: variable changes, and changes to the current directory, made after it are undone by the matching endlocal or at the end of the file (Microsoft).

• setlocal and endlocal can nest, and each called subroutine file has its own.
• The four options set command extensions and delayed expansion until the matching endlocal, whatever they were before.
• Used with an option, setlocal sets ERRORLEVEL to 0, otherwise to 1: a test for whether extensions are available (Microsoft’s example uses verify other 2>nul first).
• At the prompt, outside a script, both do nothing.
• To carry a value out of a local environment, expand it on the same line as endlocal, which is read before it runs: endlocal & set result=%result%.

An example

@echo off
setlocal enabledelayedexpansion
set "count=0"
for %%f in (*.txt) do (
    set /a count+=1
    set "name[!count!]=%%~nxf"
)
for /l %%i in (1,1,%count%) do echo %%i: !name[%%i]!
set "text=Hello, World"
echo %text:~0,5% %text:World=there% %text:~-5%
endlocal & set "files=%count%"

Not described