Browsing Git Objects
We can ask git about particular objects with the cat-file command. Note that you can shorten the shas to only a few characters to save yourself typing all 40 hex digits:
$ git-cat-file -t 54196cc2
commit
$ git-cat-file commit 54196cc2
tree 92b8b694ffb1675e5975148e1121810081dbdffe
author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
initial commit
A tree can refer to one or more "blob" objects, each corresponding to a file. In addition, a tree can also refer to other tree objects, thus creating a directory hierarchy. You can examine the contents of any tree using ls-tree (remember that a long enough initial portion of the SHA1 will also work):
$ git ls-tree 92b8b694
100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt
Thus we see that this tree has one file in it. The SHA1 hash is a reference to that file's data:
$ git cat-file -t 3b18e512
blob
A "blob" is just file data, which we can also examine with cat-file:
$ git cat-file blob 3b18e512
hello world
Note that this is the old file data; so the object that git named in its response to the initial tree was a tree with a snapshot of the directory state that was recorded by the first commit.
All of these objects are stored under their SHA1 names inside the git directory:
$ find .git/objects/
.git/objects/
.git/objects/pack
.git/objects/info
.git/objects/3b
.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
.git/objects/92
.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
.git/objects/54
.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
.git/objects/a0
.git/objects/a0/423896973644771497bdc03eb99d5281615b51
.git/objects/d0
.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
.git/objects/c4
.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241
and the contents of these files is just the compressed data plus a header identifying their length and their type. The type is either a blob, a tree, a commit, or a tag.
The simplest commit to find is the HEAD commit, which we can find from .git/HEAD:
$ cat .git/HEAD
ref: refs/heads/master
As you can see, this tells us which branch we're currently on, and it tells us this by naming a file under the .git directory, which itself contains a SHA1 name referring to a commit object, which we can examine with cat-file:
$ cat .git/refs/heads/master
c4d59f390b9cfd4318117afde11d601c1085f241
$ git cat-file -t c4d59f39
commit
$ git cat-file commit c4d59f39
tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
add emphasis
The "tree" object here refers to the new state of the tree:
$ git ls-tree d0492b36
100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
$ git cat-file blob a0423896
hello world!
and the "parent" object refers to the previous commit:
$ git-cat-file commit 54196cc2
tree 92b8b694ffb1675e5975148e1121810081dbdffe
author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
