behaviour of git diff, GIT_DIR & checked out tree

behaviour of git diff, GIT_DIR & checked out tree

[David Tweed]

I'm doing something "sensible people don't do"
-- I know this -- but I'm getting results I don't
understand, so I'd appreciate any insight.

In a git tracked tree rooted at $HOME/V with
git-dir of $HOME/V/.git, if I'm in $HOME/V then

git diff master@{midnight}

tells me the difference between the current modified files
in the tree being tracked in V and the specified commit. In
a different directory, OUTSIDE of $HOME/V, I tried

env GIT_DIR=$HOME/V/.git git diff master@{midnight}

to get the same effect but, whilst I do get a diff output, it
looks like a diff of the commit against an empty tree. (Using

env GIT_DIR=$HOME/V/.git git diff HEAD master@{midnight}

works ok.) I suppose I could alias it to

cd $HOME/V && command && cd -

but that seems a bit clunky; is there another way to explicitly
say "working tree for repo with this GIT_DIR" to git diff?

[Why I'm doing this: I actually work on files in the $HOME/V
via various other directories containing symlinks to the basic
files (with tracked files being pointed to by multiple symlinks).
So I'm almost never actually "in" the tracked tree. This
usage makes more sense for my tasks than being within
the work tree.]

-- 
cheers, dave tweed__________________________
david.tweed@gmail.com
Rm 124, School of Systems Engineering, University of Reading.
"we had no idea that when we added templates we were adding a Turing-
complete compile-time language." -- C++ standardisation committee

Re: behaviour of git diff, GIT_DIR & checked out tree

[Johannes Schindelin]

Hi,

On Mon, 24 Sep 2007, David Tweed wrote:

> In a git tracked tree rooted at $HOME/V with
> git-dir of $HOME/V/.git, if I'm in $HOME/V then
> 
> git diff master@{midnight}
> 
> tells me the difference between the current modified files
> in the tree being tracked in V and the specified commit. In
> a different directory, OUTSIDE of $HOME/V, I tried
> 
> env GIT_DIR=$HOME/V/.git git diff master@{midnight}
> 
> to get the same effect but, whilst I do get a diff output, it
> looks like a diff of the commit against an empty tree.

Yes, this is fully expected.

The @{time} notation accesses the _reflogs_, which are purely local 
beasts.  They are not transmitted when cloning.

The rationale: in a distributed environment, you cannot trust others' 
timestamps.  Therefore we don't.

We can only (to a certain extent, at least) trust our _own_ timestamp.  
That is why we have timestamp access to the reflogs (which are purely 
local, as I mentioned above), but we have no way to reference commits by 
timestamp otherwise.

Hth,
Dscho

Re: behaviour of git diff, GIT_DIR & checked out tree

[David Tweed]

On 9/24/07, Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:
> Yes, this is fully expected.
>
> The @{time} notation accesses the _reflogs_, which are purely local
> beasts.  They are not transmitted when cloning.

Thanks of your reply; however I suspect this isn't the problem
here since I just tried with just master and I get the same thing.
To be clear, I'm doing this on the same machine with the same
user account as the git directory, just from a directory outside
the git tree, eg, the git tracked tree is in $HOME/V and I'm
in $HOME/A running

env GIT_DIR=$HOME/V/.git git diff master

As a guess without looking at the code, what I imagine
is happening is that git diff limits showing changes to the directory
tree below $PWD, and when $PWD isn't actually within the git
dir that ends up somehow as an empty tree.

-- 
cheers, dave tweed__________________________
david.tweed@gmail.com
Rm 124, School of Systems Engineering, University of Reading.
"we had no idea that when we added templates we were adding a Turing-
complete compile-time language." -- C++ standardisation committee

Re: behaviour of git diff, GIT_DIR & checked out tree

[Junio C Hamano]

Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

> On Mon, 24 Sep 2007, David Tweed wrote:
>
>> ... In
>> a different directory, OUTSIDE of $HOME/V, I tried
>> 
>> env GIT_DIR=$HOME/V/.git git diff master@{midnight}
>> 
>> to get the same effect but, whilst I do get a diff output, it
>> looks like a diff of the commit against an empty tree.
>
> Yes, this is fully expected.
>
> The @{time} notation accesses the _reflogs_, which are purely local 
> beasts.  They are not transmitted when cloning.

Yeah, but my reading of the problem description suggests the two
cases refer to the same repository (hence the same reflogs).

The issue is that the second case runs "git diff <one-tree>" in
a random directory.  This form is about comparing the part of
work tree you are in with a given tree, and does not make _any_
sense when outside the work tree.  Usually without GIT_DIR
environment, the command would give you an error message.

With $GIT_DIR, but without $GIT_WORK_TREE, the user is telling
the command that it is being run at the top level of the work
tree and the repository metadata is not in the usual ".git"
subdirectory of the top level of the work tree (in this case, by
definition that is "$PWD/.git") but elsewhere where $GIT_DIR
specifies.  Because it is very likely that the files under the
random location does not share much resemblance to what are in
$HOME/V, it is not surprising that the output consisted of many
deletions.

It would be illustrative if David did the following.

	$ cd $HOME
        $ cp -a V W
        $ rm -fr W/.git
        $ cd W
        $ GIT_DIR=$HOME/V/.git git diff @{midnight}

Now we are in a random place outside of the work tree
(i.e. "W"), and we drive "git diff" with GIT_DIR specified,
telling it to pretend that we are at the top level of the work
tree.  So ~/W/frotz is compared with "frotz" at the top level of
the commit (which usually is compared with ~/V/frotz), etc.  But
the directory we happen to be in very much resembles the work
tree, so it would give identical results to

	$ cd $HOME/V
        $ git diff @{midnight}

Re: behaviour of git diff, GIT_DIR & checked out tree

[David Tweed]

On 9/24/07, Junio C Hamano <gitster@pobox.com> wrote:
> It would be illustrative if David did the following.
>
>         $ cd $HOME
>         $ cp -a V W
>         $ rm -fr W/.git
>         $ cd W
>         $ GIT_DIR=$HOME/V/.git git diff @{midnight}
>
> Now we are in a random place outside of the work tree
> (i.e. "W"), and we drive "git diff" with GIT_DIR specified,
> telling it to pretend that we are at the top level of the work
> tree.  So ~/W/frotz is compared with "frotz" at the top level of
> the commit (which usually is compared with ~/V/frotz), etc.  But
> the directory we happen to be in very much resembles the work
> tree, so it would give identical results to
>
>         $ cd $HOME/V
>         $ git diff @{midnight}

To separate the two issues, I did all the commands to the diff
above and then removed one directory from the tree in W.

GIT_DIR=$HOME/V/.git git diff master~1
(which ought not need reflogs, right) and it gave a diff between
the contents of W and master~1. I also tried with commit
master@{midnight} and that also gave the difference between
W and the last commit yesterday. So Junio's explanation of what's going on
is right. I'm just asking if there's a way to say
"the `working dir' associated with this git dir" explicitly (in the same
way I can use `HEAD' to refer to the topmost commit on the
current branch) when you're not somewhere within the 'working dir'.
If there's not I can alias around it with
"cd $HOME/V && command && cd -". (As I say, for reasons unrelated
to git usage I don't actually "do work" within the tracked tree.)

Many thanks,

-- 
cheers, dave tweed__________________________
david.tweed@gmail.com
Rm 124, School of Systems Engineering, University of Reading.
"we had no idea that when we added templates we were adding a Turing-
complete compile-time language." -- C++ standardisation committee

Re: behaviour of git diff, GIT_DIR & checked out tree

[Junio C Hamano]

"David Tweed" <david.tweed@gmail.com> writes:

> ... I'm just asking if there's a way to say
> "the `working dir' associated with this git dir" explicitly (in the same
> way I can use `HEAD' to refer to the topmost commit on the
> current branch) when you're not somewhere within the 'working dir'.

Not for "git diff <one-tree>" which is a way to diff the _part_
of work tree you are _currently in_ with the given tree, nor for
"git diff" which is to do the same with the index.  With these
commands (and others that error out when you run without GIT_DIR
outside the work tree), you are really expected to be _in_ the
subdirectory you are interested in.

GIT_WORK_TREE is somewhat related but is a different issue, by
the way.  It is to let you say where the top level of the work
tree is.  It does not change the fact that the way for you to
express which subdirectory in the work tree you are interested
in to work tree commands is by your $PWD relative to the top of
the work tree.