Git on Windows: How do you set up a mergetool?

To follow-up on Charles Bailey's answer, here's my git setup that's using p4merge (free cross-platform 3way merge tool); tested on msys Git (Windows) install:

git config --global merge.tool p4merge
git config --global mergetool.p4merge.cmd 'p4merge.exe \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"'

or, from a windows cmd.exe shell, the second line becomes :

git config --global mergetool.p4merge.cmd "p4merge.exe \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\""

The changes (relative to Charles Bailey):

• added to global git config, i.e. valid for all git projects not just the current one
• the custom tool config value resides in "mergetool.[tool].cmd", not "merge.[tool].cmd" (silly me, spent an hour troubleshooting why git kept complaining about non-existing tool)
• added double quotes for all file names so that files with spaces can still be found by the merge tool (I tested this in msys Git from Powershell)
• note that by default Perforce will add its installation dir to PATH, thus no need to specify full path to p4merge in the command

Download: http://www.perforce.com/product/components/perforce-visual-merge-and-diff-tools

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EDIT (Feb 2014)

As pointed out by @Gregory Pakosz, latest msys git now "natively" supports p4merge (tested on 1.8.5.2.msysgit.0).

You can display list of supported tools by running:

git mergetool --tool-help

You should see p4merge in either available or valid list. If not, please update your git.

If p4merge was listed as available, it is in your PATH and you only have to set merge.tool:

git config --global merge.tool p4merge

If it was listed as valid, you have to define mergetool.p4merge.path in addition to merge.tool:

git config --global mergetool.p4merge.path c:/Users/my-login/AppData/Local/Perforce/p4merge.exe

• The above is an example path when p4merge was installed for the current user, not system-wide (does not need admin rights or UAC elevation)
• Although ~ should expand to current user's home directory (so in theory the path should be ~/AppData/Local/Perforce/p4merge.exe), this did not work for me
• Even better would have been to take advantage of an environment variable (e.g. $LOCALAPPDATA/Perforce/p4merge.exe), git does not seem to be expanding environment variables for paths (if you know how to get this working, please let me know or update this answer)

setting mergetool.p4merge.cmd will not work anymore since Git has started trying to support p4merge, see libexec/git-core/git-mergetool--lib.so we just need to specify the mergetool path for git,for example the p4merge:

git config --global mergetool.p4merge.path 'C:\Program Files\Perforce\p4merge.exe'
git config --global merge.tool p4merge

Then it will work.

I'm using Portable Git on WinXP (works a treat!), and needed to resolve a conflict that came up in branching. Of all the gui's I checked, KDiff3 proved to be the most transparent to use.

But I found the instructions I needed to get it working in Windows in this blog post, instructions which differ slightly from the other approaches listed here. It basically amounted to adding these lines to my .gitconfig file:

[merge]
    tool = kdiff3

[mergetool "kdiff3"]
    path = C:/YourPathToBinaryHere/KDiff3/kdiff3.exe
    keepBackup = false
    trustExitCode = false

Working nicely now!