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[PATCH] Docs - tag object, git- prefix and s/changeset/commit/g

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Add docs for tag type
Rename commands to have git- prefix
Rename changeset to commit throughout

Signed-off-by: David Greaves <david@dgreaves.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
David Greaves 2005-05-22 18:44:17 +01:00 committed by Linus Torvalds
parent 6c84e2e0c7
commit 7096a645cd
1 changed files with 89 additions and 70 deletions
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README
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@ -31,9 +31,9 @@ build up a hierarchy of objects.
All objects have a statically determined "type" aka "tag", which is All objects have a statically determined "type" aka "tag", which is
determined at object creation time, and which identifies the format of determined at object creation time, and which identifies the format of
the object (i.e. how it is used, and how it can refer to other objects). the object (i.e. how it is used, and how it can refer to other
There are currently three different object types: "blob", "tree" and objects). There are currently four different object types: "blob",
"commit". "tree", "commit" and "tag".
A "blob" object cannot refer to any other object, and is, like the tag A "blob" object cannot refer to any other object, and is, like the tag
implies, a pure storage object containing some user data. It is used to implies, a pure storage object containing some user data. It is used to
@ -44,7 +44,7 @@ A "tree" object is an object that ties one or more "blob" objects into a
directory structure. In addition, a tree object can refer to other tree directory structure. In addition, a tree object can refer to other tree
objects, thus creating a directory hierarchy. objects, thus creating a directory hierarchy.
Finally, a "commit" object ties such directory hierarchies together into A "commit" object ties such directory hierarchies together into
a DAG of revisions - each "commit" is associated with exactly one tree a DAG of revisions - each "commit" is associated with exactly one tree
(the directory hierarchy at the time of the commit). In addition, a (the directory hierarchy at the time of the commit). In addition, a
"commit" refers to one or more "parent" commit objects that describe the "commit" refers to one or more "parent" commit objects that describe the
@ -79,9 +79,9 @@ size> + <byte\0> + <binary object data>.
The structured objects can further have their structure and The structured objects can further have their structure and
connectivity to other objects verified. This is generally done with connectivity to other objects verified. This is generally done with
the "fsck-cache" program, which generates a full dependency graph of the "git-fsck-cache" program, which generates a full dependency graph
all objects, and verifies their internal consistency (in addition to of all objects, and verifies their internal consistency (in addition
just verifying their superficial consistency through the hash). to just verifying their superficial consistency through the hash).
The object types in some more detail: The object types in some more detail:
@ -102,6 +102,9 @@ object. The object is totally independent of it's location in the
directory tree, and renaming a file does not change the object that directory tree, and renaming a file does not change the object that
file is associated with in any way. file is associated with in any way.
A blob is created with link:git-write-blob.html[git-write-blob] and
it's data can be accessed by link:git-cat-file.html[git-cat-file]
Tree Object Tree Object
~~~~~~~~~~~ ~~~~~~~~~~~
The next hierarchical object type is the "tree" object. A tree object The next hierarchical object type is the "tree" object. A tree object
@ -138,65 +141,79 @@ involved), you can see trivial renames or permission changes by
noticing that the blob stayed the same. However, renames with data noticing that the blob stayed the same. However, renames with data
changes need a smarter "diff" implementation. changes need a smarter "diff" implementation.
A tree is created with link:git-write-tree.html[git-write-tree] and
it's data can be accessed by link:git-ls-tree.html[git-ls-tree]
Changeset Object Commit Object
~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
The "changeset" object is an object that introduces the notion of The "commit" object is an object that introduces the notion of
history into the picture. In contrast to the other objects, it history into the picture. In contrast to the other objects, it
doesn't just describe the physical state of a tree, it describes how doesn't just describe the physical state of a tree, it describes how
we got there, and why. we got there, and why.
A "changeset" is defined by the tree-object that it results in, the A "commit" is defined by the tree-object that it results in, the
parent changesets (zero, one or more) that led up to that point, and a parent commits (zero, one or more) that led up to that point, and a
comment on what happened. Again, a changeset is not trusted per se: comment on what happened. Again, a commit is not trusted per se:
the contents are well-defined and "safe" due to the cryptographically the contents are well-defined and "safe" due to the cryptographically
strong signatures at all levels, but there is no reason to believe strong signatures at all levels, but there is no reason to believe
that the tree is "good" or that the merge information makes sense. that the tree is "good" or that the merge information makes sense.
The parents do not have to actually have any relationship with the The parents do not have to actually have any relationship with the
result, for example. result, for example.
Note on changesets: unlike real SCM's, changesets do not contain Note on commits: unlike real SCM's, commits do not contain
rename information or file mode change information. All of that is rename information or file mode chane information. All of that is
implicit in the trees involved (the result tree, and the result trees implicit in the trees involved (the result tree, and the result trees
of the parents), and describing that makes no sense in this idiotic of the parents), and describing that makes no sense in this idiotic
file manager. file manager.
Trust Object A commit is created with link:git-commit-tree.html[git-commit-tree] and
~~~~~~~~~~~~ it's data can be accessed by link:git-cat-file.html[git-cat-file]
The notion of "trust" is really outside the scope of "git", but it's
worth noting a few things. First off, since everything is hashed with
SHA1, you _can_ trust that an object is intact and has not been messed
with by external sources. So the name of an object uniquely
identifies a known state - just not a state that you may want to
trust.
Furthermore, since the SHA1 signature of a changeset refers to the Trust
~~~~~
An aside on the notion of "trust". Trust is really outside the scope
of "git", but it's worth noting a few things. First off, since
everything is hashed with SHA1, you _can_ trust that an object is
intact and has not been messed with by external sources. So the name
of an object uniquely identifies a known state - just not a state that
you may want to trust.
Furthermore, since the SHA1 signature of a commit refers to the
SHA1 signatures of the tree it is associated with and the signatures SHA1 signatures of the tree it is associated with and the signatures
of the parent, a single named changeset specifies uniquely a whole set of the parent, a single named commit specifies uniquely a whole set
of history, with full contents. You can't later fake any step of the of history, with full contents. You can't later fake any step of the
way once you have the name of a changeset. way once you have the name of a commit.
So to introduce some real trust in the system, the only thing you need So to introduce some real trust in the system, the only thing you need
to do is to digitally sign just _one_ special note, which includes the to do is to digitally sign just _one_ special note, which includes the
name of a top-level changeset. Your digital signature shows others name of a top-level commit. Your digital signature shows others
that you trust that changeset, and the immutability of the history of that you trust that commit, and the immutability of the history of
changesets tells others that they can trust the whole history. commits tells others that they can trust the whole history.
In other words, you can easily validate a whole archive by just 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 (SHA1 hash)
of the top changeset, and digitally sign that email using something of the top commit, and digitally sign that email using something
like GPG/PGP. like GPG/PGP.
In particular, you can also have a separate archive of "trust points" To assist in this, git also provides the tag object...
or tags, which document your (and other peoples) trust. You may, of
course, archive these "certificates of trust" using "git" itself, but
it's not something "git" does for you.
Another way of saying the last point: "git" itself only handles Tag Object
content integrity, the trust has to come from outside. ~~~~~~~~~~
Git provides the "tag" object to simplify creating, managing and
exchanging symbolic and signed tokens. The "tag" object at its
simplest simply symbolically identifies another object by containing
the sha1, type and symbolic name.
However it can optionally contain additional signature information
(which git doesn't care about as long as there's less than 8k of
it). This can then be verified externally to git.
Note that despite the tag features, "git" itself only handles content
integrity; the trust framework (and signature provision and
verification) has to come from outside.
A tag is created with link:git-mktag.html[git-mktag] and
it's data can be accessed by link:git-cat-file.html[git-cat-file]
The "index" aka "Current Directory Cache" The "index" aka "Current Directory Cache"
----------------------------------------- -----------------------------------------
@ -263,11 +280,11 @@ main combinations:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You update the index with information from the working directory with You update the index with information from the working directory with
the "update-cache" command. You generally update the index the link:git-update-cache.html[git-update-cache] command. You
information by just specifying the filename you want to update, like generally update the index information by just specifying the filename
so: you want to update, like so:
update-cache filename git-update-cache filename
but to avoid common mistakes with filename globbing etc, the command but to avoid common mistakes with filename globbing etc, the command
will not normally add totally new entries or remove old entries, will not normally add totally new entries or remove old entries,
@ -284,7 +301,7 @@ removed. The only thing "--remove" means is that update-cache will be
considering a removed file to be a valid thing, and if the file really considering a removed file to be a valid thing, and if the file really
does not exist any more, it will update the index accordingly. does not exist any more, it will update the index accordingly.
As a special case, you can also do "update-cache --refresh", which As a special case, you can also do "git-update-cache --refresh", which
will refresh the "stat" information of each index to match the current will refresh the "stat" information of each index to match the current
stat information. It will _not_ update the object status itself, and stat information. It will _not_ update the object status itself, and
it will only update the fields that are used to quickly test whether it will only update the fields that are used to quickly test whether
@ -295,7 +312,7 @@ an object still matches its old backing store object.
You write your current index file to a "tree" object with the program You write your current index file to a "tree" object with the program
write-tree git-write-tree
that doesn't come with any options - it will just write out the that doesn't come with any options - it will just write out the
current index into the set of tree objects that describe that state, current index into the set of tree objects that describe that state,
@ -311,7 +328,7 @@ populate (and overwrite - don't do this if your index contains any
unsaved state that you might want to restore later!) your current unsaved state that you might want to restore later!) your current
index. Normal operation is just index. Normal operation is just
read-tree <sha1 of tree> git-read-tree <sha1 of tree>
and your index file will now be equivalent to the tree that you saved and your index file will now be equivalent to the tree that you saved
earlier. However, that is only your _index_ file: your working earlier. However, that is only your _index_ file: your working
@ -324,20 +341,19 @@ You update your working directory from the index by "checking out"
files. This is not a very common operation, since normally you'd just files. This is not a very common operation, since normally you'd just
keep your files updated, and rather than write to your working keep your files updated, and rather than write to your working
directory, you'd tell the index files about the changes in your directory, you'd tell the index files about the changes in your
working directory (i.e. "update-cache"). working directory (i.e. "git-update-cache").
However, if you decide to jump to a new version, or check out somebody However, if you decide to jump to a new version, or check out somebody
else's version, or just restore a previous tree, you'd populate your else's version, or just restore a previous tree, you'd populate your
index file with read-tree, and then you need to check out the result index file with read-tree, and then you need to check out the result
with with
git-checkout-cache filename
checkout-cache filename
or, if you want to check out all of the index, use "-a". or, if you want to check out all of the index, use "-a".
NOTE! checkout-cache normally refuses to overwrite old files, so if NOTE! git-checkout-cache normally refuses to overwrite old files, so
you have an old version of the tree already checked out, you will need if you have an old version of the tree already checked out, you will
to use the "-f" flag (_before_ the "-a" flag or the filename) to need to use the "-f" flag (_before_ the "-a" flag or the filename) to
_force_ the checkout. _force_ the checkout.
@ -346,10 +362,10 @@ from one representation to the other:
5) Tying it all together 5) Tying it all together
~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~
To commit a tree you have instantiated with "git-write-tree", you'd
To commit a tree you have instantiated with "write-tree", you'd create create a "commit" object that refers to that tree and the history
a "commit" object that refers to that tree and the history behind it - behind it - most notably the "parent" commits that preceded it in
most notably the "parent" commits that preceded it in history. history.
Normally a "commit" has one parent: the previous state of the tree Normally a "commit" has one parent: the previous state of the tree
before a certain change was made. However, sometimes it can have two before a certain change was made. However, sometimes it can have two
@ -364,15 +380,15 @@ and explains how we got there.
You create a commit object by giving it the tree that describes the You create a commit object by giving it the tree that describes the
state at the time of the commit, and a list of parents: state at the time of the commit, and a list of parents:
commit-tree <tree> -p <parent> [-p <parent2> ..] git-commit-tree <tree> -p <parent> [-p <parent2> ..]
and then giving the reason for the commit on stdin (either through and then giving the reason for the commit on stdin (either through
redirection from a pipe or file, or by just typing it at the tty). redirection from a pipe or file, or by just typing it at the tty).
commit-tree will return the name of the object that represents that git-commit-tree will return the name of the object that represents
commit, and you should save it away for later use. Normally, you'd that commit, and you should save it away for later use. Normally,
commit a new "HEAD" state, and while git doesn't care where you save you'd commit a new "HEAD" state, and while git doesn't care where you
the note about that state, in practice we tend to just write the save the note about that state, in practice we tend to just write the
result to the file ".git/HEAD", so that we can always see what the result to the file ".git/HEAD", so that we can always see what the
last committed state was. last committed state was.
@ -381,25 +397,27 @@ last committed state was.
You can examine the data represented in the object database and the You can examine the data represented in the object database and the
index with various helper tools. For every object, you can use index with various helper tools. For every object, you can use
"cat-file" to examine details about the object: link:git-cat-file.html[git-cat-file] to examine details about the
object:
cat-file -t <objectname> git-cat-file -t <objectname>
shows the type of the object, and once you have the type (which is shows the type of the object, and once you have the type (which is
usually implicit in where you find the object), you can use usually implicit in where you find the object), you can use
cat-file blob|tree|commit <objectname> git-cat-file blob|tree|commit <objectname>
to show its contents. NOTE! Trees have binary content, and as a result to show its contents. NOTE! Trees have binary content, and as a result
there is a special helper for showing that content, called "ls-tree", there is a special helper for showing that content, called
which turns the binary content into a more easily readable form. "git-ls-tree", which turns the binary content into a more easily
readable form.
It's especially instructive to look at "commit" objects, since those It's especially instructive to look at "commit" objects, since those
tend to be small and fairly self-explanatory. In particular, if you tend to be small and fairly self-explanatory. In particular, if you
follow the convention of having the top commit name in ".git/HEAD", follow the convention of having the top commit name in ".git/HEAD",
you can do you can do
cat-file commit $(cat .git/HEAD) git-cat-file commit $(cat .git/HEAD)
to see what the top commit was. to see what the top commit was.
@ -420,13 +438,13 @@ state of the directory ("tree" object) at these points.
To get the "base" for the merge, you first look up the common parent To get the "base" for the merge, you first look up the common parent
of two commits with of two commits with
merge-base <commit1> <commit2> git-merge-base <commit1> <commit2>
which will return you the commit they are both based on. You should which will return you the commit they are both based on. You should
now look up the "tree" objects of those commits, which you can easily now look up the "tree" objects of those commits, which you can easily
do with (for example) do with (for example)
cat-file commit <commitname> | head -1 git-cat-file commit <commitname> | head -1
since the tree object information is always the first line in a commit since the tree object information is always the first line in a commit
object. object.
@ -441,15 +459,16 @@ what you have in your current index anyway).
To do the merge, do To do the merge, do
read-tree -m <origtree> <target1tree> <target2tree> git-read-tree -m <origtree> <target1tree> <target2tree>
which will do all trivial merge operations for you directly in the which will do all trivial merge operations for you directly in the
index file, and you can just write the result out with "write-tree". index file, and you can just write the result out with
"git-write-tree".
NOTE! Because the merge is done in the index file, and not in your NOTE! Because the merge is done in the index file, and not in your
working directory, your working directory will no longer match your working directory, your working directory will no longer match your
index. You can use "checkout-cache -f -a" to make the effect of the index. You can use "git-checkout-cache -f -a" to make the effect of
merge be seen in your working directory. the merge be seen in your working directory.
NOTE2! Sadly, many merges aren't trivial. If there are files that have NOTE2! Sadly, many merges aren't trivial. If there are files that have
been added.moved or removed, or if both branches have modified the been added.moved or removed, or if both branches have modified the