How do SETLOCAL and ENABLEDELAYEDEXPANSION work?

Question

I notice in most scripts, the two are usually in the same line as so:

SETLOCAL ENABLEDELAYEDEXPANSION 

Are the two in fact separate commands and can be written on separate lines?

Will setting ENABLEDELAYEDEXPANSION have an adverse effect on a script if it is set on the first lines of the script and not disabled until the end of the script?

Answer 1

I think you should understand what delayed expansion is. The existing answers don't explain it (sufficiently) IMHO.

Typing SET /? explains the thing reasonably well:

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% 

Another example is this batch file:

@echo off setlocal enabledelayedexpansion set b=z1 for %%a in (x1 y1) do (  set b=%%a  echo !b:1=2! ) 

This prints x2 and y2: every 1 gets replaced by a 2.

Without setlocal enabledelayedexpansion, exclamation marks are just that, so it will echo !b:1=2! twice.

Because normal environment variables are expanded when a (block) statement is read, expanding %b:1=2% uses the value b has before the loop: z2 (but y2 when not set).

Answer 2

ENABLEDELAYEDEXPANSION is a parameter passed to the SETLOCAL command (look at setlocal /?)

Its effect lives for the duration of the script, or an ENDLOCAL:

When the end of a batch script is reached, an implied ENDLOCAL is executed for any outstanding SETLOCAL commands issued by that batch script.

In particular, this means that if you use SETLOCAL ENABLEDELAYEDEXPANSION in a script, any environment variable changes are lost at the end of it unless you take special measures.