Can TortoiseMerge be used as a difftool with Windows Git Bash?

I'm just starting to work with Git. I would like to use TortoiseMerge as the difftool and mergetool.

In my $HOME/.gitconfig I have the following sections. I've removed the user and color sections for this question.

[merge]
	tool = tortoisemerge
[mergetool "tortoisemerge"]
	cmd = \"TortoiseMerge.exe\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\"
[diff]
	tool = tortoisemerge
[difftool "tortoisemerge"]
	cmd = \"TortoiseMerge.exe\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\"

If I type tortoisemerge at the Git Bash prompt it loads. It is known to be on the path. But if I type the command I get the following error.

Rich:mygittest (master *)
$ git difftool
error: 'tortoisemerge' can only be used to resolve merges
merge tool candidates: kompare emerge vimdiff
No known merge resolution program available.
external diff died, stopping at readme.txt.
Rich:mygittest (master *)
$ 

What am I not understanding to make this work? Tortoisemerge is installed with TortoiseSVN.

The following settings work fine for me. However, I am using TortoiseGit not TortoiseSVN. Notice the difference in the parameters for diff.

[diff]
  tool = tortoisediff
[difftool]
  prompt = false
[merge]
  tool = tortoisemerge
[mergetool]
  prompt = false
  keepBackup = false
[difftool "tortoisediff"]
  cmd = \""c:/Program Files/TortoiseGIT/bin/TortoiseGitMerge.exe"\" -mine "$REMOTE" -base "$LOCAL"
[mergetool "tortoisemerge"]
  cmd = \""c:/Program Files/TortoiseGIT/bin/TortoiseGitMerge.exe"\" -base "$BASE" -theirs "$REMOTE" -mine "$LOCAL" -merged "$MERGED"

So that filenames with spaces are handled correctly, you should change the last line of @melbourn's answer to

cmd = \""c:/Program Files/TortoiseGIT/bin/TortoiseGitMerge.exe"\" -base "$BASE" -theirs "$REMOTE" -mine "$LOCAL" -merged "$MERGED"

This is a simpler way, just run these commands in git bash:

git config --global merge.tool tortoisemerge

git config --global diff.tool tortoisediff
git config --global difftool.tortoisediff.cmd "TortoiseGitMerge \$LOCAL \$REMOTE"

It's done.

----for futher reading----

There are other settings you may need:

# you may want to disable prompt or merge backup
git config --global mergetool.keepBackup false
git config --global difftool.prompt false
git config --global mergetool.prompt false

# set the path only when it's not found in system path
git config --global mergetool.tortoisemerge.path "C:\Program Files\TortoiseGit\bin\TortoiseGitMerge.exe" 
git config --global difftool.tortoisdiff.path "C:\Program Files\TortoiseGit\bin\TortoiseGitMerge.exe"

To set the mergetool, you just need to set the right mergetool name. It's official supported.

Git for windows installer includes a shell file for running tortoisegitmerge as a merge tool. It locates at C:\Program Files\Git\mingw64\libexec\git-core\mergetools. (Git provides 4 variables for merging: $BASE $LOCAL $REMOTE $MERGED, you can check them in the shell file.)

You can check all supported merge/diff tools supported on your machine with the command:

git difftool --tool-help
git mergetool --tool-help

Run tortoisegitmerge as git diff tool is not official supported by git, maybe it's because this tool doesn't support directory compare (git difftool -d). So you need to config the difftool cmd to use it as a git difftool.

The tortoisegitmerge tool has a manual at:
<https://tortoisegit.org/docs/tortoisegitmerge/tme-automation.html>
You can find the simplest way to do file diff is:
TortoiseGitMerge BaseFilePath MyFilePath [ TheirFilePath ]

Great answer by Klas Mellbourn! Saved me a lotta time. One shortcoming is, newly Added or Removed files in the repository will not show up during the difftool command execution. There's nothing to compare them to! Here's what I did on top of this: (Inspired by my co-worker's answer).

1. Create a file named empty.empty in $Home directory (execute start ~ in your bash). And as the name suggests, keep it empty.
2. Create another file named tortoisediff.sh in $Home/bin directory with following content

:

#!/bin/sh
# $LOCAL $REMOTE seem to be swapped
# $1 is $LOCAL
# $2 is $REMOTE

difftool='/c/Program Files/TortoiseGit/bin/TortoiseGitMerge.exe'
NULL="/dev/null"
empty="C:/home/empty.empty"

if [ "$1" == "$NULL" ]; then
	echo "Added: " "$2"
	"$difftool" /base:"$empty" /mine:"$2" /readonly:"$empty"
elif [ "$2" == "$NULL" ]; then
	echo 'Removed: ' "$1"
	"$difftool" /base:"$1" /readonly:"$1" /mine:"$empty"
else
	echo 'Modified' "$2"
	"$difftool" /base:"$1" /basename:"$1" /readonly:"$1" /mine:"$2" /minename:"$2"
fi

# Checkout https://tortoisegit.org/docs/tortoisegitmerge/tme-automation.html for more

3. Modify your .gitconfig file (Line 11 of the answer)

`cmd = tortoisediff.sh "$LOCAL" "$REMOTE"`

This would now make difftool refer to tortoisediff.sh instead of opening the application directly.

4. Keep in mind: you'd have to run git add . followed by git difftool --staged instead of simply git difftool.

[diff]
  tool = tortoisediff
[difftool]
  prompt = false
[merge]
  tool = tortoisemerge
[mergetool]
  prompt = false
  keepBackup = false
[difftool "tortoisediff"]
  cmd = \""C:/Users/$USER/Desktop/TortoiseMerge.exe"\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\"
[mergetool "tortoisemerge"]
  cmd = \""C:/Users/$USER/Desktop/TortoiseMerge.exe"\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\"

This worked for me, using TortoiseMerge 1.6.7 (Portable)

When rebasing, I strongly recommend switch $theirs and $mine, because it is different when you merge and rebase-merge. Check here:

https://stackoverflow.com/questions/25576415/what-is-the-precise-meaning-of-ours-and-theirs-in-git

So, if you only use mergetool to rebase like me, do:

[mergetool "tortoisemerge"]
cmd = \""c:/Program Files/TortoiseGIT/bin/TortoiseGitMerge.exe"\" -base "$BASE" -theirs "$LOCAL" -mine "$REMOTE" -merged "$MERGED"

For using the TortoiseMerge with an existing TortoiseSVN installation in Windows:

[guitool "VisualDiff"]
	cmd = git difftool -y \"$FILENAME\"
	needsfile = yes
	noconsole = yes
[diff]
	tool = tortoisediff
[difftool]
	prompt = false
[merge]
	tool = tortoisemerge
[mergetool]
	prompt = false
	keepBackup = false
[difftool "tortoisediff"]
	cmd = \""W:/Programs/TortoiseSVN/bin/TortoiseMerge.exe"\" "/mine:$REMOTE" "/base:$LOCAL"
[mergetool "tortoisemerge"]
	cmd = \""W:/Programs/TortoiseSVN/bin/TortoiseMerge.exe"\" "/base:$BASE" "/theirs:$REMOTE" "/mine:$LOCAL" "/merged:$MERGED"