I’m a fairly recent convert to git, and have been moving a bunch of my coding & doc projects to it. It’s been mostly seamless, but I had one kinda tricky piece left: the daily commit (automated, of course) of any notable changes to my Cisco router & switch configs. The tricky part is handling certain lines in the configs that change each time, but aren’t necessarily of interest (for example, ntp clock-period), and I don’t want to kick off a commit if that’s all that’s changed.

cvs has this nifty -I cli option, similar to the -I option to gnu diff – it allows you to specify a regexp and the cvs diff will ignore all lines that match that regexp.

Here’s a sampling of what I had:
cvs diff \
-I 'clock-period' \
-I '#time' \
-I 'set.spantree.port.*cost' \
[filename]


(Note that you can’t have spaces in the regexp you pass to cvs.)

git doesn’t have a cli switch for this; I was having a tough time figuring out how to make it use gnu diff. This gave me the tip I needed.

So, here we go!

0. If you don’t have gnu diff on your machine, install it. (I got mine from sunfreeware.com.) You can just run diff without any args to see the options – if you’re missing “I”, you don’t have the right diff.

1. Set up a wrapper script that uses gnu diff:
:::-->cat /home/gabrielle/bin/ciscodiff.sh
#!/usr/bin/bash
#make git use gnu diff and ignore certain lines
/path/to/gnu/diff \
-I 'clock-period' \
-I '#time' \
-I 'set spantree port.*cost' \
$2 $5 | cat

Notes:
- I don’t need the . instead of the space, like I did in the cvs regexp – so this is a more restrictive match. (Which I like.)
- $2 and $5 specify which of the parameters for git diff actually are passed through to this diff. See the “git Diffs” section of the manual.

Make sure to make this executable.

2. Then, back in my git repo, I added the following to .git/config:
[diff "ciscoconf"]
command = /home/gabrielle/bin/ciscodiff.sh

3. Then I created .gitattributes, like so:
*-confg diff=ciscoconf

git will now use my special diff wrapper *only* on files with names that match the *-confg glob pattern.

Voila.