Using Meld as external diff tool with Tortoise SVN

Since Git1.6.3, you can use the git difftool script: see my answer below.

May be this article will help you. Here are the best parts:

There are two different ways to specify an external diff tool.

The first is the method you used, by setting the GIT_EXTERNAL_DIFF variable. However, the variable is supposed to point to the full path of the executable. Moreover, the executable specified by GIT_EXTERNAL_DIFF will be called with a fixed set of 7 arguments:

path old-file old-hex old-mode new-file new-hex new-mode

As most diff tools will require a different order (and only some) of the arguments, you will most likely have to specify a wrapper script instead, which in turn calls the real diff tool.

The second method, which I prefer, is to configure the external diff tool via "git config". Here is what I did:

1) Create a wrapper script "git-diff-wrapper.sh" which contains something like

-->8-(snip)--
#!/bin/sh

# diff is called by git with 7 parameters:
# path old-file old-hex old-mode new-file new-hex new-mode

"<path_to_diff_executable>" "$2" "$5" | cat
--8<-(snap)--

As you can see, only the second ("old-file") and fifth ("new-file") arguments will be passed to the diff tool.

2) Type

$ git config --global diff.external <path_to_wrapper_script>

at the command prompt, replacing with the path to "git-diff-wrapper.sh", so your ~/.gitconfig contains

-->8-(snip)--
[diff]
    external = <path_to_wrapper_script>
--8<-(snap)--

Be sure to use the correct syntax to specify the paths to the wrapper script and diff tool, i.e. use forward slashed instead of backslashes. In my case, I have

[diff]
    external = \"c:/Documents and Settings/sschuber/git-diff-wrapper.sh\"

in .gitconfig and

"d:/Program Files/Beyond Compare 3/BCompare.exe" "$2" "$5" | cat

in the wrapper script. Mind the trailing "cat"!

(I suppose the '| cat' is needed only for some programs which may not return a proper or consistent return status. You might want to try without the trailing cat if your diff tool has explicit return status)

(Diomidis Spinellis adds in the comments:

The cat command is required, because diff(1), by default exits with an error code if the files differ.
Git expects the external diff program to exit with an error code only if an actual error occurred, e.g. if it run out of memory.
By piping the output of git to cat the non-zero error code is masked.
More efficiently, the program could just run exit with and argument of 0.)

That (the article quoted above) is the theory for external tool defined through config file (not through environment variable).
In practice (still for config file definition of external tool), you can refer to:

• How do I setup DiffMerge with msysgit / gitk? which illustrates the concrete settings of DiffMerge and WinMerge for MsysGit and gitk
• How can I set up an editor to work with Git on Windows? for the definition of Notepad++ as an external editor.

It should just be:

git diff --cached

--cached means show the changes in the cache/index (i.e. staged changes) against the current HEAD. --staged is a synonym for --cached.

--staged and --cached does not point to HEAD, just difference with respect to HEAD. If you cherry pick what to commit using git add --patch (or git add -p), --staged will return what is staged.