Batch files, $ : and ~

Question

Hello all :)

I've been writing an application which will need to expand environment strings in a file.

To that effect, I could use the standard windows API function, ExpandEnvironmentStrings: http://msdn.microsoft.com/en-us/library/ms724265(VS.85).aspx

I do have a few problems with that function though. First: The size of the lpSrc and lpDst buffers is limited to 32K.

Next: Note that this function does not support all the features that Cmd.exe supports. For example, it does not support %variableName:str1=str2% or %variableName:~offset,length%.

I would like to implement these extras cmd.exe allows, but I'm not sure exactly what they are. :~offset,length is a bit odvious ... substring. But not sure what the first one is.

Any ideas?

Billy3

Answer 1

It's string substitution.

Basically, if variableName is set to "I am three", then "%variableName:three=four%" generates "I am four" (double quotes put in for nicer formatting, they do not form part of the strings).

C:\Documents and Settings\Administrator>set x=I am three

C:\Documents and Settings\Administrator>echo %x%
I am three

C:\Documents and Settings\Administrator>echo %x:three=four%
I am four

You can also replace with an empty string (obvious) and replace from the start of the string (not so obvious):

C:\Documents and Settings\Administrator>echo %x:three=%
I am 

C:\Documents and Settings\Administrator>echo %x:*am=I am not%
I am not three

In addition, the substring variant is Pythonesque in that negative numbers work from the end of the string:

C:\Documents and Settings\Administrator>echo %x:~,4%
I am

C:\Documents and Settings\Administrator>echo %x:~-5%
three