use Winmerge inside of Git to file diff

Since the thread is getting confusing and bifurcated, here are consolidated instructions for the Directory Listing "--dir-diff" WinMerge method for msysgit Git Windows.

Step 1 - Create a file named winmerge.sh at a location accessible to your path (such as /home/bin/winmerge.sh) with following contents.

#!/bin/sh
echo Launching WinMergeU.exe: $1 $2
"$PROGRAMFILES/WinMerge/WinMergeU.exe" -r -ub -dl "Remote" -dr "Local" "$1" "$2"

Step 2 - Type the following commands in Git Bash to instruct git to use winmerge.sh as difftool (these options get stored in /home/.gitconfig):

git config --replace --global diff.tool winmerge
git config --replace --global difftool.winmerge.cmd "winmerge.sh \"$LOCAL\" \"$REMOTE\""
git config --replace --global difftool.prompt false

Step 3 - Now you can test by typing the following command in Git Bash to start your WinMerge diff:

git difftool --dir-diff

Step 4 - For quicker access, create an alias for this command by adding this line to .bashrc in your home folder (or create .bashrc file with this line if file does not already exist):

alias diffdir='git difftool --dir-diff'

Step 5 - Now you can quickly see a diff in WinMerge just by typing the following command into Git Bash

diffdir

I have a script that will set the Diff and Merge tools in the Git config with the proper parameters that doesn't require a separate .sh file to exist. It seems to be working fine for me.

git config --global diff.tool winmerge
git config --global difftool.prompt false
git config --global difftool.winmerge.cmd "\"\$PROGRAMFILES\"/WinMerge/WinMergeU.exe -r -u -e -dl \"Local\" -dr \"Remote\" \"\$LOCAL\" \"\$REMOTE\""

git config --global merge.tool winmerge
git config --global mergetool.prompt false
git config --global mergetool.winmerge.trustExitCode true
git config --global mergetool.winmerge.cmd "\"\$PROGRAMFILES\"/WinMerge/WinMergeU.exe -r -u -e -dl \"Local\" -dr \"Remote\" \"\$LOCAL\" \"\$REMOTE\" \"\$BASE\" \"\$MERGED\""

Note - the entire .cmd portion is quoted so that the parameters will be listed in the .gitconfig properly

I faced trouble using the first part in 2 places and fixed it as follows

1. The second command for setting up winmerge.cmd required an extra slash on cmdline (before $LOCAL and $REMOTE), else cygwin was substituting the variable in cmdline

   C:\myGitRepo>git config --replace --global difftool.winmerge.cmd "winmerge.sh \"\$LOCAL\" \"\$REMOTE\""
   

2. changed the winmerge.sh file to (without this, was getting right-path-invalid error)

   #!/bin/sh
   echo Launching WinMergeU.exe: "$(cygpath -aw "$1")" "$(cygpath -aw "$2")"
   "$PROGRAMFILES/WinMerge/WinMergeU.exe" -e -ub -dl "Base" -dr "Mine" "$(cygpath -aw "$1")" "$(cygpath -aw "$2")"
   

Update June 2015, 6 years later:

As detailed in "git mergetool winmerge", a simple git config diff.tool winmerge will be enough.

Git 2.5+ (Q2, 2015) is now aware of Winmerge as a diff or merge tool!

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Original answer (2009-2012)

(msysgit, 1.6.5, DOS session)

The first part (using winmerge) is described in "How do I view ‘git diff’ output with visual diff program?"

C:\myGitRepo>git config --replace --global diff.tool winmerge
C:\myGitRepo>git config --replace --global difftool.winmerge.cmd "winmerge.sh \"$LOCAL\" \"$REMOTE\""
C:\myGitRepo>git config --replace --global difftool.prompt false

With winmerge.sh stored in a directory part of your PATH:

#!/bin/sh
echo Launching WinMergeU.exe: $1 $2
"$PROGRAMFILES/WinMerge/WinMergeU.exe" -e -u -dl "Local" -dr "Remote" "$1" "$2"

(see WinMerge Command-line options)

git difftool

will now launch WinMerge.
If you want git diff to launch WinMerge, just set:

set GIT_EXTERNAL_DIFF=winmerge.sh

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But the real added value comes from the ability to use that same diff tool to present all differences in one batch instead of presenting them sequentially, forcing you to close the diff tool windows one file at a time.

Update June 2012 (2-and-a-half years later):

Comparing directories instead of file-by-file will be available soon:
See [ANNOUNCE] Git 1.7.11.rc1:

"git difftool" learned the "--dir-diff" option to spawn external diff tools that can compare two directory hierarchies at a time after populating two temporary directories, instead of running an instance of the external tool once per a file pair.

See "Patch difftool: teach difftool to handle directory diffs", and the answer "Directory comparison of Git branches" for more details.

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Original difftool by directories script (December 2009)

As Seba Illingworth mentions in his answer, a script git-diffall.sh (also put in the path) can do just that:

#!/bin/sh
git diff --name-only "$@" | while read filename; do
    git difftool "$@" --no-prompt "$filename" &
done

But that only works by opening n windows for n files (if you try to use the -s option of WinMerge, it will not work because of the temp files being deleted by the difftool too early)

---

That is why I like the approach of GitDiff.bat - power-diffing with GI, which allows you to review the list of files with a difference, before selecting one to examine its internal differences.
I have tweaked it to use only DOS commands

@echo off

setlocal

if "%1" == "-?" (
    echo GitDiff - enables diffing of file lists, instead of having to serially
    echo diff files without being able to go back to a previous file.
    echo Command-line options are passed through to git diff.
    echo If GIT_FOLDER_DIFF is set, it is used to diff the file lists. Default is windff.
    goto END
)

if "%GIT_DIFF_COPY_FILES%"=="" (
    rd /s /q %TEMP%\GitDiff
    mkdir %TEMP%\GitDiff
    mkdir %TEMP%\GitDiff\old
    mkdir %TEMP%\GitDiff\new

    REM This batch file will be called by git diff. This env var indicates whether it is
    REM being called directly, or inside git diff
    set GIT_DIFF_COPY_FILES=1

    set GIT_DIFF_OLD_FILES=%TEMP%\GitDiff\old
    set GIT_DIFF_NEW_FILES=%TEMP%\GitDiff\new

    set GIT_EXTERNAL_DIFF=%~dp0\GitDiff.bat
    echo Please wait and press q when you see "(END)" printed in reverse color...
    call git diff %*

    if defined GIT_FOLDER_DIFF (
        REM This command using GIT_FOLDER_DIFF just does not work for some reason.
        %GIT_FOLDER_DIFF% %TEMP%\GitDiff\old %TEMP%\GitDiff\new
        goto END
    )

    if exist "%ProgramFiles%\Beyond Compare 2\BC2.exe" (
        set GIT_FOLDER_DIFF="%ProgramFiles%\Beyond Compare 2\BC2.exe"
        "%ProgramFiles%\Beyond Compare 2\BC2.exe" %TEMP%\GitDiff\old %TEMP%\GitDiff\new
        goto END
    )

    "%ProgramFiles(x86)%\WinMerge\WinMergeU.exe" -r -e -dl "Local" -dr "Remote"  %TEMP%\GitDiff\old %TEMP%\GitDiff\new
    goto END
)

REM diff is called by git with 7 parameters:
REM     path old-file old-hex old-mode new-file new-hex new-mode
copy %TEMP%\%~nx2 %GIT_DIFF_OLD_FILES%\%1
copy %5 %GIT_DIFF_NEW_FILES%

:END

It is not robust enough to handle files with same names in different directories, but it gives you a general idea of what is possible:
Here only one WinMerge will open up, with the list of files having internal differences. You can click on the ones you want to examines, then a simple ESC will close the all WinMerge-diff session.