[QUESTION] mergetool environment variables

[QUESTION] mergetool environment variables

[rsbecker]

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
>>>>>>> Incoming
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.

Re: [QUESTION] mergetool environment variables

[Johannes Sixt]

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

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

> 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.

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

RE: [QUESTION] mergetool environment variables

[rsbecker]

On September 13, 2025 3:05 AM, Johannes Sixt wrote:
>Am 12.09.25 um 22:16 schrieb rsbecker@nexbridge.com:
>> 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
>
>Take note: this talks about "variables", not "environment variables".
>
>> 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.
>
>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.

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?

Re: [QUESTION] mergetool environment variables

[Johannes Sixt]

Am 13.09.25 um 16:42 schrieb rsbecker@nexbridge.com:
> Let me try to infer what is happening and please correct me if my assumptions
> are wrong:
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

RE: [QUESTION] mergetool environment variables

[rsbecker]

On September 13, 2025 5:04 PM, Johannes Sixt wrote:
>To: rsbecker@nexbridge.com
>Cc: git@vger.kernel.org
>Subject: Re: [QUESTION] mergetool environment variables
>
>Am 13.09.25 um 16:42 schrieb rsbecker@nexbridge.com:
>> Let me try to infer what is happening and please correct me if my
>> assumptions are wrong:
>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.

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.

Re: [QUESTION] mergetool environment variables

[Junio C Hamano]

<rsbecker@nexbridge.com> writes:

> 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.

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.

Re: [QUESTION] mergetool environment variables

[Phillip Wood]

Hi Randall

On 14/09/2025 01:18, rsbecker@nexbridge.com wrote:
> On September 13, 2025 5:04 PM, Johannes Sixt wrote:
>> To: rsbecker@nexbridge.com
>> Cc: git@vger.kernel.org
>> Subject: Re: [QUESTION] mergetool environment variables
>>
>> Am 13.09.25 um 16:42 schrieb rsbecker@nexbridge.com:
>>> Let me try to infer what is happening and please correct me if my
>>> assumptions are wrong:
>> 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.
> 
> 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.

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

Re: [QUESTION] mergetool environment variables

[D. Ben Knoble]

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:
>
> Hi Randall
>
> On 14/09/2025 01:18, rsbecker@nexbridge.com wrote:
> > On September 13, 2025 5:04 PM, Johannes Sixt wrote:
> >> To: rsbecker@nexbridge.com
> >> Cc: git@vger.kernel.org
> >> Subject: Re: [QUESTION] mergetool environment variables
> >>
> >> Am 13.09.25 um 16:42 schrieb rsbecker@nexbridge.com:
> >>> Let me try to infer what is happening and please correct me if my
> >>> assumptions are wrong:
> >> 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.
> >
> > 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.
>
> 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

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.

-- 
D. Ben Knoble