did, and why.
Every commit has a 40-hexdigit id, sometimes called the "object name" or the
-"SHA1 id", shown on the first line of the "git show" output. You can usually
+"SHA-1 id", shown on the first line of the "git show" output. You can usually
refer to a commit by a shorter name, such as a tag or a branch name, but this
longer name can also be useful. Most importantly, it is a globally unique
name for this commit: so if you tell somebody else the object name (for
HEAD is now at 427abfa... Linux v2.6.17
------------------------------------------------
-The HEAD then refers to the SHA1 of the commit instead of to a branch,
+The HEAD then refers to the SHA-1 of the commit instead of to a branch,
and git branch shows that you are no longer on a branch:
------------------------------------------------
-------------------------------------------------
Alternatively, you may often see this sort of thing done with the
-lower-level command linkgit:git-rev-list[1], which just lists the SHA1's
+lower-level command linkgit:git-rev-list[1], which just lists the SHA-1's
of all the given commits:
-------------------------------------------------
We already saw in <<understanding-commits>> that all commits are stored
under a 40-digit "object name". In fact, all the information needed to
represent the history of a project is stored in objects with such names.
-In each case the name is calculated by taking the SHA1 hash of the
-contents of the object. The SHA1 hash is a cryptographic hash function.
+In each case the name is calculated by taking the SHA-1 hash of the
+contents of the object. The SHA-1 hash is a cryptographic hash function.
What that means to us is that it is impossible to find two different
objects with the same name. This has a number of advantages; among
others:
same content stored in two repositories will always be stored under
the same name.
- Git can detect errors when it reads an object, by checking that the
- object's name is still the SHA1 hash of its contents.
+ object's name is still the SHA-1 hash of its contents.
(See <<object-details>> for the details of the object formatting and
-SHA1 calculation.)
+SHA-1 calculation.)
There are four different types of objects: "blob", "tree", "commit", and
"tag".
As you can see, a commit is defined by:
-- a tree: The SHA1 name of a tree object (as defined below), representing
+- a tree: The SHA-1 name of a tree object (as defined below), representing
the contents of a directory at a certain point in time.
-- parent(s): The SHA1 name of some number of commits which represent the
+- parent(s): The SHA-1 name of some number of commits which represent the
immediately previous step(s) in the history of the project. The
example above has one parent; merge commits may have more than
one. A commit with no parents is called a "root" commit, and
------------------------------------------------
As you can see, a tree object contains a list of entries, each with a
-mode, object type, SHA1 name, and name, sorted by name. It represents
+mode, object type, SHA-1 name, and name, sorted by name. It represents
the contents of a single directory tree.
The object type may be a blob, representing the contents of a file, or
another tree, representing the contents of a subdirectory. Since trees
-and blobs, like all other objects, are named by the SHA1 hash of their
-contents, two trees have the same SHA1 name if and only if their
+and blobs, like all other objects, are named by the SHA-1 hash of their
+contents, two trees have the same SHA-1 name if and only if their
contents (including, recursively, the contents of all subdirectories)
are identical. This allows git to quickly determine the differences
between two related tree objects, since it can ignore any entries with
Trust
~~~~~
-If you receive the SHA1 name of a blob from one source, and its contents
+If you receive the SHA-1 name of a blob from one source, and its contents
from another (possibly untrusted) source, you can still trust that those
-contents are correct as long as the SHA1 name agrees. This is because
-the SHA1 is designed so that it is infeasible to find different contents
+contents are correct as long as the SHA-1 name agrees. This is because
+the SHA-1 is designed so that it is infeasible to find different contents
that produce the same hash.
-Similarly, you need only trust the SHA1 name of a top-level tree object
+Similarly, you need only trust the SHA-1 name of a top-level tree object
to trust the contents of the entire directory that it refers to, and if
-you receive the SHA1 name of a commit from a trusted source, then you
+you receive the SHA-1 name of a commit from a trusted source, then you
can easily verify the entire history of commits reachable through
parents of that commit, and all of those contents of the trees referred
to by those commits.
commits tells others that they can trust the whole history.
In other words, you can easily validate a whole archive by just
-sending out a single email that tells the people the name (SHA1 hash)
+sending out a single email that tells the people the name (SHA-1 hash)
of the top commit, and digitally sign that email using something
like GPG/PGP.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Newly created objects are initially created in a file named after the
-object's SHA1 hash (stored in .git/objects).
+object's SHA-1 hash (stored in .git/objects).
Unfortunately this system becomes inefficient once a project has a
lot of objects. Try this on an old project:
------------------------------------------------
which will create and store a blob object with the contents of
-somedirectory/myfile, and output the sha1 of that object. if you're
+somedirectory/myfile, and output the SHA-1 of that object. if you're
extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200, in
which case you've guessed right, and the corruption is fixed!
-----------
The index is a binary file (generally kept in .git/index) containing a
-sorted list of path names, each with permissions and the SHA1 of a blob
+sorted list of path names, each with permissions and the SHA-1 of a blob
object; linkgit:git-ls-files[1] can show you the contents of the index:
-------------------------------------------------
index. Normal operation is just
-------------------------------------------------
-$ git read-tree <sha1 of tree>
+$ git read-tree <SHA-1 of tree>
-------------------------------------------------
and your index file will now be equivalent to the tree that you saved
------------------------------------------------
Each line of the `git ls-files --unmerged` output begins with
-the blob mode bits, blob SHA1, 'stage number', and the
+the blob mode bits, blob SHA-1, 'stage number', and the
filename. The 'stage number' is git's way to say which tree it
came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
tree, and stage3 `$target` tree.
Regardless of object type, all objects share the following
characteristics: they are all deflated with zlib, and have a header
that not only specifies their type, but also provides size information
-about the data in the object. It's worth noting that the SHA1 hash
+about the data in the object. It's worth noting that the SHA-1 hash
that is used to name the object is the hash of the original data
plus this header, so `sha1sum` 'file' does not match the object name
for 'file'.
(Historical note: in the dawn of the age of git the hash
-was the sha1 of the 'compressed' object.)
+was the SHA-1 of the 'compressed' object.)
As a result, the general consistency of an object can always be tested
independently of the contents or the type of the object: all objects can
projects / git.git / commitdiff