Variables are not behaving as expected

Question

I've been wrestling trying to get the syntax right on this batch file and I cannot figure out why some things aren't working.

1. The variable i is not getting incremented each time I do it.
2. Concatenation on strc doesn't seem to concatenate.

Here is my code:

set i=0 set "strc=concat:"  for %%f in (*.mp4) do (     set /a i+=1     set "str=intermediate%i%.ts"      set strc="%strc% %str%|"      ffmpeg -i "%%f" -c copy -bsf:v h264_mp4toannexb -f mpegts "%str%" )  set strc="%strc:-1%" ffmpeg -i "%strc%" -c copy -bsf:a aac_adtstoasc Output.mp4 

Answer 1

You are not the first, who fell into the famous "delayed expansion trap" (and you won't be the last).

You need delayed expansion if you want to use a variable, that you changed in the same block (a block is a series of commands within parentheses (and )).

Delayed variables are referenced with !var! instead of %var%.

Reason is the way, cmd parses the code. A complete line or block is parsed at once, replacing normal variables with their value at parse time. Delayed variables are evaluated at runtime.

Two simple batch files to demonstrate:

setlocal EnableDelayedExpansion set "var=hello" if 1==1 (     set "var=world"     echo %var% !var! ) 

setlocal EnableDelayedExpansion for /L %%i in (1,1,5) do (     echo %random% !random! ) 

Note: A line is also treated as a block:

set "var=old" set "var=new" & echo %var%  

With delayed expansion:

setlocal EnableDelayedExpansion set "var=old" set "var=new" & echo !var!  

Delayed expansion is per default turned off at the command prompt. If you really need it, you can do:

cmd /V:ON /C "set "var=hello" & echo !var!" 

Also there is a way to do the same without delayed expansion (but call costs some time, so it's slower, but if for some reason you can't / don't want to use delayed expansion, it's an alternative):

setlocal DISabledelayedexpansion for /L %%i in (1 1 5) do (     call echo %random% %%random%%  ) 

Both methods can also be used to display array-like variables:

(This is often asked like "variable which contains another variable" or "nested variables")

Here is a collection for using such array-like variables in different situations:

With delayed expansion:

setlocal ENableDelayedExpansion set "num=4" set "var[%num%]=HELLO" echo plain delayed: !var[%num%]! for /L %%i in (4 1 4) do (     echo for delayed: !var[%%i]!     set a=%%i     call echo for delayed with variable: %%var[!a!]%% ) 

without delayed expansion:

setlocal DISableDelayedExpansion set "num=4" set "var[%num%]=HELLO" call echo plain called: %%var[%num%]%% for /L %%i in (4 1 4) do (     call echo FOR called: %%var[%%i]%%     set a=%%i     call echo FOR called with variable: %%var[%a%]%% ) 

Note: setlocal has no effect outside of batchfiles, so delayedexpansion works only:

• In batch files
• When the cmd was started with delayed expansion enabled (cmd /V:ON) (by default, the cmd runs with delayed expansion disabled)

(Follow the links, when you are interested in the technical background or even the advanced technical stuff)