EnableDelayedExpansion

EnableDelayedExpansion is a feature of the Windows command interpreter (cmd.exe) that controls how environment variable values are expanded in batch scripts. When enabled, variables inside a code block can be expanded at execution time using !VAR! instead of being expanded at parse time with %VAR%. This allows a script to reflect changes to variables made inside loops or other blocks.

EnableDelayedExpansion is activated with the setlocal command: setlocal EnableDelayedExpansion. The effect lasts until endlocal or the end of the script. Alternatively, delaying can be enabled for an entire session by running cmd /V:ON, which configures the command interpreter globally for that session.

In practice, use !VAR! inside blocks to see updated values. Example:

setlocal EnableDelayedExpansion

set i=0

for %%F in (*) do (

set /a i+=1

echo File %%F is number !i!

)

endlocal

Notes and caveats:

- Percent expansions (%VAR%) are still evaluated when the block is parsed, so they may not reflect changes made inside the block. Delayed expansion with !VAR! provides the current value during execution.

- Literal exclamation marks can be problematic inside a block with delayed expansion. Printing a literal ! often requires turning off delayed expansion or careful handling, as escaping options are limited in this context.

EnableDelayedExpansion is commonly used to compute counters, accumulate values, or otherwise react to changes within loops and conditional blocks in batch scripts. See also CMD.EXE, setlocal, endlocal, and command-line options for enabling delayed expansion.