Terminally Incoherent

A relatively unknown feature of the windows shell (cmd.exe) is that it has a built in substring function. You can use the :~ operator in conjunction with any variable to obtain a substring of that variable. For example:

set foo=foobar
echo %foo:~3%

The above will output bar. You can also use two comma separated parameters if you want. The first one is the start index, and the second one is length:

echo %foo:~1,2%

This will display oo. You can also have a negative offset, but not the length:

echo %foo:~-3,3%

This will also produce bar. What would be a practical application for this? You tell me - there are hundreds of ways you could exploit this feature to your advantage. I personally use it in my scripts to create dated directories. Sometimes you want to create a folder labeled with the current date. The easy way to do this of course is:

mkdir "%date%"

The only problem is that this method doesn’t work. Because by default dates contain slashes (”/”) which are interpreted as path delimiters by the mkdir command. What you end up with is a directory tree:

Fri 10\27\2006\

This might be useful, but not exactly what we have wanted. What I really meant to do was this:

mkdir "%date:~0,3% %date:~4,2%-%date:~7,2%-%date:~-4,4%"

Yes, the line is much more verbose, but the end effect is well worth it - you end up with a neatly named folder Fri 10-27-2006. This is really useful trick for quick backup scripts and etc…

This entry was posted on Friday, October 27th, 2006 at 1:16 am and is filed under technology, windows. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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