[QUESTION] mergetool environment variables

I am trying to integrate a custom mergetool with a shell wrapper. What I get from the online help is the following description referring to the command and environment variables.

mergetool.<tool>.cmd Specify the command to invoke the specified merge tool. The specified command is evaluated in shell with the following variables available: BASE is the name of a temporary file containing the common base of the files to be merged, if available; LOCAL is the name of a temporary file containing the contents of the file on the current branch; REMOTE is the name of a temporary file containing the contents of the file from the branch being merged; MERGED contains the name of the file to which the merge tool should write the results of a successful merge.

When I try to use this from a shell, simply with: #!/bin/sh env exit 1

the described environment variables: BASE, LOCAL, REMOTE, and MERGED, are not present.

The history is trivial, but has a 1 line conflict after a git merge. The file is called a, with two lines at the parent commit and the middle line changed in both branches being merged. There are correct temp files in the directory while the shell script is being run. The merged file looks as follows:

Hello <<<<<<< HEAD Ours ======= Theirs

World

This applies to git 2.45.1 in cygwin and 2.51.0 on NonStop. There are no upstreams. Is this the problem?

Any thoughts on this?

Thanks, Randall

-- Brief whoami: NonStop&UNIX developer since approximately UNIX(421664400) NonStop(211288444200000000) -- In real life, I talk too much.

Am 12.09.25 um 22:16 schrieb rsbecker@nexbridge.com:

Take note: this talks about "variables", not "environment variables".

Look at the scripts in the directory mergetools/ and note that they are only (large) shell code fragements without a shbang line. They are not even executable.

-- Hannes

On September 13, 2025 3:05 AM, Johannes Sixt wrote:

Let me try to infer what is happening and please correct me if my assumptions are wrong:

Git includes an existing shell fragment with the appropriate tool name from git-core/mergetools/tool-name instead of creating a dedicated shell in which to run the merge tool script.

It then uses ${merge-tool-path} to execute the tool based on whether diff_cmd() or merge_cmd() is invoked. Because BASE, LOCAL, REMOTE, and MERGED are not exported, they are not visible to sub-shells.

This does not apply if the tool is unknown to the mergetools directory, so must be contributed and packaged into git to work according to the documentation.

So a custom mergetool will not have access to these variables and has to hack through what is in the working index based on file_BASE_num, etc. instead of having direct information on what is being merged.

That about right?

Am 13.09.25 um 16:42 schrieb rsbecker@nexbridge.com:

I'm sorry to say that I can't help. I tried to disentangle what is going on, but this stuff is far too convoluted to be understood in a few minutes. I cannot tell if it is possible to write a mergetool that is not installed with Git.

I would just copy one of the existing tool scripts and run `make install` from the Git source directory.

-- Hannes

On September 13, 2025 5:04 PM, Johannes Sixt wrote:

Yes, it needs to be in the git install area. Adding export BASE export LOCAL, etc., works to resolve the situation. I wonder whether that should be documented.

<rsbecker@nexbridge.com> writes:

I am not a mergetool user, but I do not think that is how the thing was intended to be used in the first place.

The documentation that may talk about variables BASE etc. (not "environment variables") are meant for those who add their new tool to the existing mergetool/difftool infrastructure, which means that their tool's definition must be dot-sourceable just like any of these tools defined in mergetools/ directory. You'd add yours there, mimicking what they do, and add a handful of shell functions to be called. git-mergetool--lib would then use

    . "$MERGE_TOOLS_DIR/${tool%[0-9]}"

inside its setup_tool function to read your definition.

Hi Randall

On 14/09/2025 01:18, rsbecker@nexbridge.com wrote:

Looking at t7610-mergetool.sh I think you can use mergetool.<tool>.cmd to call a user defined merge tool. The value of the config variable is eval'd in the shell so if you run

     git config mergetool.my-tool.cmd 'my-tool "$BASE" "$LOCAL" "$REMOTE"'

then

     git mergetool --tool=my-tool

will run

     my-tool <base-file> <local-file> <remote-file>

on each unmerged file

Thanks

Phillip

The text of this reply sat in my drafts, so my apologies for getting it out later than I intended. I agree with Phillip's reply below, to which I've moved the text so it makes sense :)

On Sun, Sep 14, 2025 at 9:51 AM Phillip Wood <phillip.wood123@gmail.com> wrote:

I run a custom merge tool, and I am fairly certain it has access to these variables. But I pass them in the config defining the tool to the « main » command in the versions where I use them.

See history around https://github.com/benknoble/Dotfiles/blob/4a4fe9678bdc5b34dc826058fbe85d29cc4d7722/links/gitconfig#L130.

Otherwise this would seem an unfortunate interface.