Example of delayed expansion in batch file

Can someone give me an example of where a batch script would act differently with or without delayed expansion? Are there any situations where you would NOT want to use delayed expansion? Thanks.

Look at the following examples...

Example 1: The following code DOESN'T use delayed expansion, so the variables in the for loop are expanded only one time. This means that %Count% will always expand to 0 in each iteration of the loop, no matter what we do to it with the set command:

@echo off
set COUNT=0

for %%v in (1 2 3 4) do (
  set /A COUNT=%COUNT% + 1
  echo Count = %COUNT%
)
pause

So this script will output:

Count = 0
Count = 0
Count = 0
Count = 0

This is not how this loop is supposed to work.

Example 2: On the other hand, if we use delayed expansion, we have the following script, which will run as expected.

setlocal ENABLEDELAYEDEXPANSION
set COUNT=0

for %%v in (1 2 3 4) do (
  set /A COUNT=!COUNT! + 1
  echo Count = !COUNT!
)

pause

and, as expected, it will output:

Count = 1
Count = 2
Count = 3
Count = 4

When you use the ENABLEDELAYEDEXPANSION, and expand a variable using ! instead of %, the variable is re-expanded each time, and everything works as it's supposed to.

I wanted to add a great example on how "EnableDelayedExpansion" (EDE) can be useful outside of the ubiquitous FOR loop examples.

Here is a line of earthquake data that I wish to parse (I call it it 1line.txt)

ak_11574812 2015.04.29.193822 62.9525 -148.8849 1.0 9.5 1 49km S of Cantwell, Alaska

The problem I ran into was that last segment of this line does not always start at the same column number. So I needed to create a flexible SET command that will accurately pluck out the last segment of this line.

ECHO OFF
setlocal enableDelayedExpansion
set where=72
set /p line=<1line.txt
set locate=!line:~%where%,28!
echo %locate%

EDE allows me to place a variable (where) inside another variable (line). EDE will translate the variable bracketed by % first, then process the variable bracketed by ! and (in this case) push out the results into the "locate" variable.

Max's answer gives an example of where a batch script would act differently with or without delayed expansion.

For the sake of completeness, let's answer another part of the question and show a situation where you would NOT want to use delayed expansion when your data contain an exclamation mark ! (and show two ways of processing such data):

@ECHO OFF
SETLOCAL EnableExtensions DisableDelayedExpansion
  
  set "_auxFile=%temp%\%~n0.txt"
  rem create multiline sample file
  >"%_auxFile%" ( for /L %%G in (1,1,3) do echo line %%G is 100%% valid! Sure! Hurrah!)
  rem create one-line sample file
  >"%_auxFile%" echo this line is 100%% valid! Sure! Hurrah!

  echo(
  echo --- file content 
  type "%_auxFile%"
  
  echo(
  SETLOCAL EnableDelayedExpansion
    echo --- enabled delayed expansion chokes down unescaped exclamation marks^^^! "^!"
    for /F "usebackq delims=" %%G in ("%_auxFile%") do (
      set "_auxLine=%%~G"
      echo loop var=%%~G
      echo _auxLine=!_auxLine!
    )
  ENDLOCAL
  echo(
  SETLOCAL DisableDelayedExpansion
    echo --- toggled delayed expansion works although might be laborious!
    for /F "usebackq delims=" %%G in ("%_auxFile%") do (
      set "_auxLine=%%G"
      echo loop var=%%G
      SETLOCAL EnableDelayedExpansion
        echo _auxLine=!_auxLine!
      ENDLOCAL
    )
  ENDLOCAL
  echo(
  SETLOCAL DisableDelayedExpansion
    echo --- keep delayed expansion DISABLED: use CALL command!
    for /F "usebackq delims=" %%G in ("%_auxFile%") do (
      set "_auxLine=%%G"
      echo loop var=%%G
      call :ProcessVar
    )
  ENDLOCAL

  rem delete the sample file
  del "%_auxFile%"
ENDLOCAL
goto :eof

:ProcessVar
  echo _auxLine=%_auxLine%
  echo WARNING: neither !_auxLine! nor %%G loop variable is available here!  
goto :eof

Note that above script shows proper ways of escaping

• % percent sign by %% doubling it (delayed expansion does not matter), and
• ! exclamation mark if delayed expansion is enabled:
• "^!" if enclosed in a pair of double quotes, then use the cmd and batch-script general escape character ^ caret;
• ^^^! otherwise, use three ^ carets.

Output:

==> D:\bat\SO\10558316.bat

--- file content
this line is 100% valid! Sure! Hurrah!

--- enabled delayed expansion chokes down unescaped exclamation marks! "!"
loop var=this line is 100% valid Hurrah
_auxLine=this line is 100% valid Hurrah

--- toggled delayed expansion works although might be laborious!
loop var=this line is 100% valid! Sure! Hurrah!
_auxLine=this line is 100% valid! Sure! Hurrah!

--- keep delayed expansion DISABLED: use CALL command!
loop var=this line is 100% valid! Sure! Hurrah!
_auxLine=this line is 100% valid! Sure! Hurrah!
WARNING: !_auxLine! as well as %G loop variables are not available here!

==>

As pointed in the answer the main usage of the delayed expansion is the setting and accessing variables in brackets context.

Though it can be useful in another situations too.

Parametrizing substring and string substitution:

@echo off
setlocal enableDelayedExpansion

set "string=test string value"
set start=5
set get_next=6

echo #!string:~%start%,%get_next%!#

set "search_for=string"
set "replace_with=text"

echo #!string:%search_for%=%replace_with%!#

the output will be:

#string#
#test text value#

though this can be achieved with additional call this way is more performant

Using shift command within brackets parameterized argument access

@echo off

echo first attempt:
(
	echo "%~1"
	shift
	echo "%~1"
)
::now the shift command will take effect

setlocal enableDelayedExpansion
echo second attempt:

(	
	set /a argument=1
	call echo %%!argument! 
	shift
	call echo %%!argument! 
)

the output will be:

first attempt:
"first argument"
"first argument"
second attempt:
"second argument"
"third argument"

As you can see parameterized argument access can be done only with delayed expansion.

Using for tokens (or function arguments) for parameterization

One more approach for mixing !s and %s this could be useful for nested loops:

@echo off
setlocal enabledelayedexpansion
set begin=2
set end=2
set string=12345

for /f "tokens=1,2" %%A in ("!begin! !end!") do set "string2=!string:~%%A,%%B!"
echo !string2!

endlocal

as you can see now the for command tokens are used as parameters.

Several answers here answer the "How to use delayed expansion?" question or what happen if you don't use delayed expansion. However, the second question is "Are there any situations where you would NOT want to use delayed expansion?" and a couple answers take this question as "how to avoid the problems caused by using delayed expansion?"

My answer answers the question as I understand it: "In which situations is better to NOT use delayed expansion (instead of use it)?"

If you want to exchange the contents of two variables, the simplest way to perform this is using the %standard% variable expansion:

set "var1=%var2%"  &  set "var2=%var1%"

The way that the %standard% expansion works makes possible to achieve this replacemenmt without using any auxiliary variable. As far as I know, the Batch-file "programming language" is the only one that allows to perform this exchange in this way, that is, making good use of a language "feature" (not via a specialized "exchange" instruction/statement).