Using custom diff tool with `git show`

Looks like you're looking for the --ext-diff option.

Here's what git show docs say about it:

--ext-diff

Allow an external diff helper to be executed. If you set an external diff driver with gitattributes, you need to use this option with git-log and friends.

Like @LucasTrzesniewski said, you can use --ext-diff from the command line to set a diff for the current session.

You can also use the .gitattributes to set the git-diff perfile.

A git diff implementation exists of 2 parts:

• A definition in $GIT_DIR/config or $HOME/.gitconfig
• A bound between a file and a definition in gitattributes

Git has choosing for this design to separate the executable code from the source code, this makes it impossible for a git clone or other git command to run any harmful code.

Writing a definition

To write a definition, we start with a header consisting of [diff "namehere"], then followed by a line-break.

The next line consists of the command definition, this line is as follows: command = commandlinehere. This command is then called with 7 arguments if it is ran, these are documented for the GIT_EXTERNAL_DIFF enviroment vriable in the docs.

GIT_EXTERNAL_DIFF When the environment variable GIT_EXTERNAL_DIFF is set, the program named by it is called, instead of the diff invocation described above. For a path that is added, removed, or modified, GIT_EXTERNAL_DIFF is called with 7 parameters:

path old-file old-hex old-mode new-file new-hex new-mode where:

<old|new>-file are files GIT_EXTERNAL_DIFF can use to read the contents of <old|new>,

<old|new>-hex are the 40-hexdigit SHA-1 hashes,

<old|new>-mode are the octal representation of the file modes.

The file parameters can point at the user’s working file (e.g. new-file in "git-diff-files"), /dev/null (e.g. old-file when a new file is added), or a temporary file (e.g. old-file in the index). GIT_EXTERNAL_DIFF should not worry about unlinking the temporary file, it is removed when GIT_EXTERNAL_DIFF exits.

For a path that is unmerged, GIT_EXTERNAL_DIFF is called with 1 parameter, <path>.

For each path GIT_EXTERNAL_DIFF is called, two environment variables, GIT_DIFF_PATH_COUNTER and GIT_DIFF_PATH_TOTAL are set.

The total example looks like this:

[diff "jcdiff"]
command = j-c-diff

Writing the gitattributes

We need to modify our gitattributes to use our custom driver. The gitattributes file consist of a syntax similar to filename value [value2 [value3 [value4 [...]]]].

Examples:

*           diff=jcdiff

^{We to use our custom git diff for every file.}

*.java      diff=javadiff
*.python    diff=pythondiff

^{Use javadiff for java files, pythondiff for python files.}

*           diff=globaldiff
*.java      diff=javadiff

^{Use javadiff for java files, globaldiff for remaining files.}

Configuring git to automatically set --ext-diff

You can add an alias using git config alias.showobject 'show --ext-diff' to define a new command called git showobject that automatically uses our filter.

As an alternate answer for others in search of using a diff tool like meld with git show

git difftool --tool=meld HEAD~..HEAD

Most of the time I like to see my small diffs on the command line because it's quick, but every once in a while I do want to boot up a diff tool to inspect things a little deeper - especially with the work of others. But this excludes the ability to use .gitattributes because this would then be the default behavior.

In order to show a particular commit or branch, then I've created this alias in my ~/.gitconfig

[alias]
    showm = "!f(){ if [ -z $1 ]; then c='HEAD'; else c=$1; fi; git difftool --tool=meld -y $c~..$c; }; f"

This will launch meld to show the diffs introduced by that commit:

git showm <branch or hash>

Without an argument, it will use HEAD as a default:

git showm

As a bonus, you can follow that up with a bash alias to get tab completion for branch names by adding this to the ~/.bashrc

alias showm='git showm'
__git_complete showm _git_show