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External Tools: configuring a 3-way diff

You can set up custom external 3-way diffs which can be invoked from a File Log:

1. In the Preferences, be sure to have Low-level property tools.useFileLogPathInsteadOfRepositoryRoot set to true. This is necessary to have the externals tools variable ${repositoryRootPath} providing the path to the logged file instead of just the path to the repository root.

2. Create a bash script wrapper, similar to the one below:

   #!/bin/bash
   includeWt=0
   windows=0
   POSITIONAL=()
   while [[ $# -gt 0 ]]
   do
     key="$1"
     case $key in
       --include-wt)
         includeWt=1
         shift
         ;;
       --windows)
         windows=1
         shift
         ;;
       *)
         POSITIONAL+=("$1")
         shift
       ;;
     esac
   done
   set -- "${POSITIONAL[@]}"
   
   abspath=$1
   if [ $windows == 1 ]; then
       abspath=$(echo "$abspath" | sed 's/\\/\//g')
       abspath=$(echo "$abspath" | tr '[:upper:]' '[:lower:]')
   fi
   
   commit=$2
   difftool=$3
   
   repo=$(cd "$(dirname "$abspath")" && git rev-parse --show-toplevel)
   if [ $windows == 1 ]; then
       repo=$(echo "$repo" | tr '[:upper:]' '[:lower:]')
   fi
   
   relpath=$(realpath --relative-to="$repo" "$abspath")
   
   cd $repo
   theirs=$(mktemp "/tmp/diff.XXXXXX")
   git show $commit:$relpath > $theirs
   
   if [ $includeWt == 0 ]; then
     ours=$(mktemp "/tmp/diff.XXXXXX")
     git show HEAD:$relpath > $ours
   else    
     ours=$abspath
   fi
   
   mergebase=$(git merge-base HEAD $commit)
   
   base=$(mktemp "/tmp/diff.XXXXXX")
   git show $mergebase:$relpath > $base
   
   "$difftool" $ours $theirs $base
   

3. In the Preferences, configure this tool for your custom diff program, for example:
   

   Note the optional usage of --include-wt which will use the current working tree file, instead of the clean version of the file from
   HEAD. --windows signals the wrapper that it will be called on a Windows platform (which requires some special handling).

4. Now, in the File Log, you can select a target commit to diff your current working tree file (or HEAD) with and invoke the tool from the Commits graph context menu.