-
-
+////////////////////////////////////////////////////////////////
GIT - the stupid content tracker
-
+////////////////////////////////////////////////////////////////
"git" can mean anything, depending on your mood.
- random three-letter combination that is pronounceable, and not
- "goddamn idiotic truckload of sh*t": when it breaks
This is a stupid (but extremely fast) directory content manager. It
-doesn't do a whole lot, but what it _does_ do is track directory
+doesn't do a whole lot, but what it 'does' do is track directory
contents efficiently.
There are two object abstractions: the "object database", and the
"current directory cache" aka "index".
-
-
- The Object Database (GIT_OBJECT_DIRECTORY)
-
-
+The Object Database
+~~~~~~~~~~~~~~~~~~~
The object database is literally just a content-addressable collection
of objects. All objects are named by their content, which is
approximated by the SHA1 hash of the object itself. Objects may refer
-to other objects (by referencing their SHA1 hash), and so you can build
-up a hierarchy of objects.
+to other objects (by referencing their SHA1 hash), and so you can
+build up a hierarchy of objects.
All objects have a statically determined "type" aka "tag", which is
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).
-There are currently three different object types: "blob", "tree" and
-"commit".
+the object (i.e. how it is used, and how it can refer to other
+objects). There are currently four different object types: "blob",
+"tree", "commit" and "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
directory structure. In addition, a tree object can refer to other tree
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
(the directory hierarchy at the time of the commit). In addition, a
"commit" refers to one or more "parent" commit objects that describe the
just going to confuse people. So aim for the notion of "one root object
per project", even if git itself does not enforce that.
-Regardless of object type, all objects are share the following
-characteristics: they are all in deflated with zlib, and have a header
-that not only specifies their tag, but also size information about the
-data in the object. It's worth noting that the SHA1 hash that is used
-to name the object is always the hash of this _compressed_ object, not
-the original data.
+A "tag" object symbolically identifies and can be used to sign other
+objects. It contains the identifier and type of another object, a
+symbolic name (of course!) and, optionally, a signature.
+
+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 tag, but also provides size information
+about the data in the object. It's worth noting that the SHA1 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.)
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
forms a sequence of <ascii tag without space> + <space> + <ascii decimal
size> + <byte\0> + <binary object data>.
-The structured objects can further have their structure and connectivity
-to other objects verified. This is generally done with the "fsck-cache"
-program, which generates a full dependency graph of all objects, and
-verifies their internal consistency (in addition to just verifying their
-superficial consistency through the hash).
+The structured objects can further have their structure and
+connectivity to other objects verified. This is generally done with
+the `git-fsck-objects` program, which generates a full dependency graph
+of all objects, and verifies their internal consistency (in addition
+to just verifying their superficial consistency through the hash).
The object types in some more detail:
- BLOB: A "blob" object is nothing but a binary blob of data, and
- doesn't refer to anything else. There is no signature or any
- other verification of the data, so while the object is
- consistent (it _is_ indexed by its sha1 hash, so the data itself
- is certainly correct), it has absolutely no other attributes.
- No name associations, no permissions. It is purely a blob of
- data (i.e. normally "file contents").
-
- In particular, since the blob is entirely defined by its data,
- if two files in a directory tree (or in multiple different
- versions of the repository) have the same contents, they will
- share the same blob object. The object is toally independent
- of it's location in the directory tree, and renaming a file does
- not change the object that file is associated with in any way.
-
- TREE: The next hierarchical object type is the "tree" object. A tree
- object is a list of mode/name/blob data, sorted by name.
- Alternatively, the mode data may specify a directory mode, in
- which case instead of naming a blob, that name is associated
- with another TREE object.
-
- Like the "blob" object, a tree object is uniquely determined by
- the set contents, and so two separate but identical trees will
- always share the exact same object. This is true at all levels,
- i.e. it's true for a "leaf" tree (which does not refer to any
- other trees, only blobs) as well as for a whole subdirectory.
-
- For that reason a "tree" object is just a pure data abstraction:
- it has no history, no signatures, no verification of validity,
- except that since the contents are again protected by the hash
- itself, we can trust that the tree is immutable and its contents
- never change.
-
- So you can trust the contents of a tree to be valid, the same
- way you can trust the contents of a blob, but you don't know
- where those contents _came_ from.
-
- Side note on trees: since a "tree" object is a sorted list of
- "filename+content", you can create a diff between two trees
- without actually having to unpack two trees. Just ignore all
- common parts, and your diff will look right. In other words,
- you can effectively (and efficiently) tell the difference
- between any two random trees by O(n) where "n" is the size of
- the difference, rather than the size of the tree.
-
- Side note 2 on trees: since the name of a "blob" depends
- entirely and exclusively on its contents (i.e. there are no names
- or permissions involved), you can see trivial renames or
- permission changes by noticing that the blob stayed the same.
- However, renames with data changes need a smarter "diff" implementation.
-
-CHANGESET: The "changeset" object is an object that introduces the
- notion of history into the picture. In contrast to the other
- objects, it doesn't just describe the physical state of a tree,
- it describes how we got there, and why.
-
- A "changeset" 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 comment on what happened. Again, a changeset is
- not trusted per se: the contents are well-defined and "safe" due
- to the cryptographically strong signatures at all levels, but
- there is no reason to believe that the tree is "good" or that
- the merge information makes sense. The parents do not have to
- actually have any relationship with the result, for example.
-
- Note on changesets: unlike real SCM's, changesets do not contain
- rename information or file mode change information. All of that
- is implicit in the trees involved (the result tree, and the
- result trees of the parents), and describing that makes no sense
- in this idiotic file manager.
-
-TRUST: 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 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 history, with full contents. You can't
- later fake any step of the way once you have the name of a
- changeset.
-
- 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 name of a top-level changeset. Your digital
- signature shows others that you trust that changeset, and the
- immutability of the history of changesets 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) of the top changeset, and digitally sign that email using
- something like GPG/PGP.
-
- In particular, you can also have a separate archive of "trust
- points" 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 content
-integrity, the trust has to come from outside.
-
-
-
- The "index" aka "Current Directory Cache" (".git/index")
-
-
+Blob Object
+~~~~~~~~~~~
+A "blob" object is nothing but a binary blob of data, and doesn't
+refer to anything else. There is no signature or any other
+verification of the data, so while the object is consistent (it 'is'
+indexed by its sha1 hash, so the data itself is certainly correct), it
+has absolutely no other attributes. No name associations, no
+permissions. It is purely a blob of data (i.e. normally "file
+contents").
+
+In particular, since the blob is entirely defined by its data, if two
+files in a directory tree (or in multiple different versions of the
+repository) have the same contents, they will share the same blob
+object. The object is totally independent of its location in the
+directory tree, and renaming a file does not change the object that
+file is associated with in any way.
+
+A blob is typically created when gitlink:git-update-index[1]
+is run, and its data can be accessed by gitlink:git-cat-file[1].
+
+Tree Object
+~~~~~~~~~~~
+The next hierarchical object type is the "tree" object. A tree object
+is a list of mode/name/blob data, sorted by name. Alternatively, the
+mode data may specify a directory mode, in which case instead of
+naming a blob, that name is associated with another TREE object.
+
+Like the "blob" object, a tree object is uniquely determined by the
+set contents, and so two separate but identical trees will always
+share the exact same object. This is true at all levels, i.e. it's
+true for a "leaf" tree (which does not refer to any other trees, only
+blobs) as well as for a whole subdirectory.
+
+For that reason a "tree" object is just a pure data abstraction: it
+has no history, no signatures, no verification of validity, except
+that since the contents are again protected by the hash itself, we can
+trust that the tree is immutable and its contents never change.
+
+So you can trust the contents of a tree to be valid, the same way you
+can trust the contents of a blob, but you don't know where those
+contents 'came' from.
+
+Side note on trees: since a "tree" object is a sorted list of
+"filename+content", you can create a diff between two trees without
+actually having to unpack two trees. Just ignore all common parts,
+and your diff will look right. In other words, you can effectively
+(and efficiently) tell the difference between any two random trees by
+O(n) where "n" is the size of the difference, rather than the size of
+the tree.
+
+Side note 2 on trees: since the name of a "blob" depends entirely and
+exclusively on its contents (i.e. there are no names or permissions
+involved), you can see trivial renames or permission changes by
+noticing that the blob stayed the same. However, renames with data
+changes need a smarter "diff" implementation.
+
+A tree is created with gitlink:git-write-tree[1] and
+its data can be accessed by gitlink:git-ls-tree[1].
+Two trees can be compared with gitlink:git-diff-tree[1].
+
+Commit Object
+~~~~~~~~~~~~~
+The "commit" object is an object that introduces the notion of
+history into the picture. In contrast to the other objects, it
+doesn't just describe the physical state of a tree, it describes how
+we got there, and why.
+
+A "commit" is defined by the tree-object that it results in, the
+parent commits (zero, one or more) that led up to that point, and a
+comment on what happened. Again, a commit is not trusted per se:
+the contents are well-defined and "safe" due to the cryptographically
+strong signatures at all levels, but there is no reason to believe
+that the tree is "good" or that the merge information makes sense.
+The parents do not have to actually have any relationship with the
+result, for example.
+
+Note on commits: unlike real SCM's, commits do not contain
+rename information or file mode change information. All of that is
+implicit in the trees involved (the result tree, and the result trees
+of the parents), and describing that makes no sense in this idiotic
+file manager.
+
+A commit is created with gitlink:git-commit-tree[1] and
+its data can be accessed by gitlink:git-cat-file[1].
+
+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
+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
+way once you have the name of a commit.
+
+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
+name of a top-level commit. Your digital signature shows others
+that you trust that commit, and the immutability of the history of
+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)
+of the top commit, and digitally sign that email using something
+like GPG/PGP.
+
+To assist in this, git also provides the tag object...
+
+Tag Object
+~~~~~~~~~~
+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 gitlink:git-mktag[1],
+its data can be accessed by gitlink:git-cat-file[1],
+and the signature can be verified by
+gitlink:git-verify-tag[1].
+
+
+The "index" aka "Current Directory Cache"
+-----------------------------------------
The index is a simple binary file, which contains an efficient
representation of a virtual directory content at some random time. It
does so by a simple array that associates a set of names, dates,
permissions and content (aka "blob") objects together. The cache is
always kept ordered by name, and names are unique (with a few very
specific rules) at any point in time, but the cache has no long-term
-meaning, and can be partially updated at any time.
+meaning, and can be partially updated at any time.
In particular, the index certainly does not need to be consistent with
the current directory contents (in fact, most operations will depend on
-different ways to make the index _not_ be consistent with the directory
+different ways to make the index 'not' be consistent with the directory
hierarchy), but it has three very important attributes:
- (a) it can re-generate the full state it caches (not just the directory
- structure: it contains pointers to the "blob" objects so that it
- can regenerate the data too)
+'(a) it can re-generate the full state it caches (not just the
+directory structure: it contains pointers to the "blob" objects so
+that it can regenerate the data too)'
- As a special case, there is a clear and unambiguous one-way mapping
- from a current directory cache to a "tree object", which can be
- efficiently created from just the current directory cache without
- actually looking at any other data. So a directory cache at any
- one time uniquely specifies one and only one "tree" object (but
- has additional data to make it easy to match up that tree object
- with what has happened in the directory)
+As a special case, there is a clear and unambiguous one-way mapping
+from a current directory cache to a "tree object", which can be
+efficiently created from just the current directory cache without
+actually looking at any other data. So a directory cache at any one
+time uniquely specifies one and only one "tree" object (but has
+additional data to make it easy to match up that tree object with what
+has happened in the directory)
- (b) it has efficient methods for finding inconsistencies between that
- cached state ("tree object waiting to be instantiated") and the
- current state.
+'(b) it has efficient methods for finding inconsistencies between that
+cached state ("tree object waiting to be instantiated") and the
+current state.'
- (c) it can additionally efficiently represent information about merge
- conflicts between different tree objects, allowing each pathname to
- be associated with sufficient information about the trees involved
- that you can create a three-way merge between them.
+'(c) it can additionally efficiently represent information about merge
+conflicts between different tree objects, allowing each pathname to be
+associated with sufficient information about the trees involved that
+you can create a three-way merge between them.'
Those are the three ONLY things that the directory cache does. It's a
cache, and the normal operation is to re-generate it completely from a
haven't lost any information as long as you have the name of the tree
that it described.
-At the same time, the directory index is at the same time also the
+At the same time, the index is at the same time also the
staging area for creating new trees, and creating a new tree always
involves a controlled modification of the index file. In particular,
the index file can have the representation of an intermediate tree that
has not yet been instantiated. So the index can be thought of as a
write-back cache, which can contain dirty information that has not yet
-been written back to the backing store.
-
+been written back to the backing store.
- The Workflow
-
+The Workflow
+------------
Generally, all "git" operations work on the index file. Some operations
-work _purely_ on the index file (showing the current state of the
+work *purely* on the index file (showing the current state of the
index), but most operations move data to and from the index file. Either
from the database or from the working directory. Thus there are four
main combinations:
- 1) working directory -> index
-
- You update the index with information from the working directory
- with the "update-cache" command. You generally update the index
- information by just specifying the filename you want to update,
- like so:
-
- update-cache filename
-
- but to avoid common mistakes with filename globbing etc, the
- command will not normally add totally new entries or remove old
- entries, i.e. it will normally just update existing cache entryes.
-
- To tell git that yes, you really do realize that certain files
- no longer exist in the archive, or that new files should be
- added, you should use the "--remove" and "--add" flags
- respectively.
-
- NOTE! A "--remove" flag does _not_ mean that subsequent
- filenames will necessarily be removed: if the files still exist
- in your directory structure, the index will be updated with
- their new status, not 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 does not exist any more, it
- will update the index accordingly.
-
- As a special case, you can also do "update-cache --refresh",
- which will refresh the "stat" information of each index to match
- the current stat information. It will _not_ update the object
- status itself, and it wil only update the fields that are used
- to quickly test whether an object still matches its old backing
- store object.
-
- 2) index -> object database
-
- You write your current index file to a "tree" object with the
- program
-
- write-tree
-
- 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, and it will return the name of the resulting top-level
- tree. You can use that tree to re-generate the index at any time
- by going in the other direction:
-
- 3) object database -> index
-
- You read a "tree" file from the object database, and use that to
- populate (and overwrite - don't do this if your index contains
- any unsaved state that you might want to restore later!) your
- current index. Normal operation is just
-
- read-tree <sha1 of tree>
-
- and your index file will now be equivalent to the tree that you
- saved earlier. However, that is only your _index_ file: your
- working directory contents have not been modified.
-
- 4) index -> working directory
-
- You update your working directory from the index by "checking
- out" files. This is not a very common operation, since normally
- you'd just keep your files updated, and rather than write to
- your working directory, you'd tell the index files about the
- changes in your working directory (i.e. "update-cache").
-
- However, if you decide to jump to a new version, or check out
- somebody elses 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 with
-
- checkout-cache filename
-
- or, if you want to check out all of the index, use "-a".
-
- NOTE! checkout-cache normally refuses to overwrite old files, so
- if you have an old version of the tree already checked out, you
- will need to use the "-f" flag (_before_ the "-a" flag or the
- filename) to _force_ the checkout.
-
-
-Finally, there are a few odds and ends which are not purely moving from
-one representation to the other:
-
- 5) Tying it all together
-
- To commit a tree you have instantiated with "write-tree", you'd
- create a "commit" object that refers to that tree and the
- history behind it - most notably the "parent" commits that
- preceded it in history.
-
- Normally a "commit" has one parent: the previous state of the
- tree before a certain change was made. However, sometimes it can
- have two or more parent commits, in which case we call it a
- "merge", due to the fact that such a commit brings together
- ("merges") two or more previous states represented by other
- commits.
-
- In other words, while a "tree" represents a particular directory
- state of a working directory, a "commit" represents that state
- in "time", and explains how we got there.
-
- 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:
-
- commit-tree <tree> -p <parent> [-p <parent2> ..]
-
- 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).
-
- commit-tree will return the name of the object that represents
- that commit, and you should save it away for later use.
- Normally, you'd commit a new "HEAD" state, and while git doesn't
- care where you 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 last committed state was.
-
- 6) Examining the data
-
- You can examine the data represented in the object database and
- the index with various helper tools. For every object, you can
- use "cat-file" to examine details about the object:
-
- cat-file -t <objectname>
-
- 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
-
- cat-file blob|tree|commit <objectname>
-
- 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", which turns the binary content into a more
- easily readable form.
-
- It's especially instructive to look at "commit" objects, since
- those tend to be small and fairly self-explanatory. In
- particular, if you follow the convention of having the top
- commit name in ".git/HEAD", you can do
-
- cat-file commit $(cat .git/HEAD)
-
- to see what the top commit was.
-
- 7) Merging multiple trees
-
- Git helps you do a three-way merge, which you can expand to
- n-way by repeating the merge procedure arbitrary times until you
- finally "commit" the state. The normal situation is that you'd
- only do one three-way merge (two parents), and commit it, but if
- you like to, you can do multiple parents in one go.
-
- To do a three-way merge, you need the two sets of "commit"
- objects that you want to merge, use those to find the closest
- common parent (a third "commit" object), and then use those
- commit objects to find the state of the directory ("tree"
- object) at these points.
-
- To get the "base" for the merge, you first look up the common
- parent of two commits with
-
- merge-base <commit1> <commit2>
+1) working directory -> index
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- 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 do with (for example)
+You update the index with information from the working directory with
+the gitlink:git-update-index[1] command. You
+generally update the index information by just specifying the filename
+you want to update, like so:
- cat-file commit <commitname> | head -1
+ git-update-index filename
- since the tree object information is always the first line in a
- commit object.
+but to avoid common mistakes with filename globbing etc, the command
+will not normally add totally new entries or remove old entries,
+i.e. it will normally just update existing cache entries.
- Once you know the three trees you are going to merge (the one
- "original" tree, aka the common case, and the two "result" trees,
- aka the branches you want to merge), you do a "merge" read into
- the index. This will throw away your old index contents, so you
- should make sure that you've committed those - in fact you would
- normally always do a merge against your last commit (which
- should thus match what you have in your current index anyway).
- To do the merge, do
+To tell git that yes, you really do realize that certain files no
+longer exist in the archive, or that new files should be added, you
+should use the `--remove` and `--add` flags respectively.
- read-tree -m <origtree> <target1tree> <target2tree>
+NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
+necessarily be removed: if the files still exist in your directory
+structure, the index will be updated with their new status, not
+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
+does not exist any more, it will update the index accordingly.
- 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".
+As a special case, you can also do `git-update-index --refresh`, which
+will refresh the "stat" information of each index to match the current
+stat information. It will 'not' update the object status itself, and
+it will only update the fields that are used to quickly test whether
+an object still matches its old backing store object.
- NOTE! Because the merge is done in the index file, and not in
- 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 merge be seen in your working directory.
+2) index -> object database
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
- 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 same file, you will be left with an index tree that
- contains "merge entries" in it. Such an index tree can _NOT_ be
- written out to a tree object, and you will have to resolve any
- such merge clashes using other tools before you can write out
- the result.
+You write your current index file to a "tree" object with the program
+
+ git-write-tree
+
+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,
+and it will return the name of the resulting top-level tree. You can
+use that tree to re-generate the index at any time by going in the
+other direction:
+
+3) object database -> index
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You read a "tree" file from the object database, and use that to
+populate (and overwrite - don't do this if your index contains any
+unsaved state that you might want to restore later!) your current
+index. Normal operation is just
+
+ git-read-tree <sha1 of tree>
+
+and your index file will now be equivalent to the tree that you saved
+earlier. However, that is only your 'index' file: your working
+directory contents have not been modified.
+
+4) index -> working directory
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You update your working directory from the index by "checking out"
+files. This is not a very common operation, since normally you'd just
+keep your files updated, and rather than write to your working
+directory, you'd tell the index files about the changes in your
+working directory (i.e. `git-update-index`).
+
+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
+index file with read-tree, and then you need to check out the result
+with
+
+ git-checkout-index filename
+
+or, if you want to check out all of the index, use `-a`.
+
+NOTE! git-checkout-index normally refuses to overwrite old files, so
+if you have an old version of the tree already checked out, you will
+need to use the "-f" flag ('before' the "-a" flag or the filename) to
+'force' the checkout.
+
+
+Finally, there are a few odds and ends which are not purely moving
+from one representation to the other:
+
+5) Tying it all together
+~~~~~~~~~~~~~~~~~~~~~~~~
+To commit a tree you have instantiated with "git-write-tree", you'd
+create a "commit" object that refers to that tree and the history
+behind it - most notably the "parent" commits that preceded it in
+history.
+
+Normally a "commit" has one parent: the previous state of the tree
+before a certain change was made. However, sometimes it can have two
+or more parent commits, in which case we call it a "merge", due to the
+fact that such a commit brings together ("merges") two or more
+previous states represented by other commits.
+
+In other words, while a "tree" represents a particular directory state
+of a working directory, a "commit" represents that state in "time",
+and explains how we got there.
+
+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:
+
+ git-commit-tree <tree> -p <parent> [-p <parent2> ..]
+
+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).
+
+git-commit-tree will return the name of the object that represents
+that commit, and you should save it away for later use. Normally,
+you'd commit a new `HEAD` state, and while git doesn't care where you
+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
+last committed state was.
+
+Here is an ASCII art by Jon Loeliger that illustrates how
+various pieces fit together.
+
+------------
+
+ commit-tree
+ commit obj
+ +----+
+ | |
+ | |
+ V V
+ +-----------+
+ | Object DB |
+ | Backing |
+ | Store |
+ +-----------+
+ ^
+ write-tree | |
+ tree obj | |
+ | | read-tree
+ | | tree obj
+ V
+ +-----------+
+ | Index |
+ | "cache" |
+ +-----------+
+ update-index ^
+ blob obj | |
+ | |
+ checkout-index -u | | checkout-index
+ stat | | blob obj
+ V
+ +-----------+
+ | Working |
+ | Directory |
+ +-----------+
+
+------------
+
+
+6) Examining the data
+~~~~~~~~~~~~~~~~~~~~~
+
+You can examine the data represented in the object database and the
+index with various helper tools. For every object, you can use
+gitlink:git-cat-file[1] to examine details about the
+object:
+
+ git-cat-file -t <objectname>
+
+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
+
+ git-cat-file blob|tree|commit|tag <objectname>
+
+to show its contents. NOTE! Trees have binary content, and as a result
+there is a special helper for showing that content, called
+`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
+tend to be small and fairly self-explanatory. In particular, if you
+follow the convention of having the top commit name in `.git/HEAD`,
+you can do
+
+ git-cat-file commit $(cat .git/HEAD)
+
+to see what the top commit was.
+
+7) Merging multiple trees
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Git helps you do a three-way merge, which you can expand to n-way by
+repeating the merge procedure arbitrary times until you finally
+"commit" the state. The normal situation is that you'd only do one
+three-way merge (two parents), and commit it, but if you like to, you
+can do multiple parents in one go.
+
+To do a three-way merge, you need the two sets of "commit" objects
+that you want to merge, use those to find the closest common parent (a
+third "commit" object), and then use those commit objects to find the
+state of the directory ("tree" object) at these points.
+
+To get the "base" for the merge, you first look up the common parent
+of two commits with
+
+ git-merge-base <commit1> <commit2>
+
+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
+do with (for example)
+
+ git-cat-file commit <commitname> | head -1
+
+since the tree object information is always the first line in a commit
+object.
+
+Once you know the three trees you are going to merge (the one
+"original" tree, aka the common case, and the two "result" trees, aka
+the branches you want to merge), you do a "merge" read into the
+index. This will complain if it has to throw away your old index contents, so you should
+make sure that you've committed those - in fact you would normally
+always do a merge against your last commit (which should thus match
+what you have in your current index anyway).
+
+To do the merge, do
+
+ git-read-tree -m -u <origtree> <yourtree> <targettree>
+
+which will do all trivial merge operations for you directly in the
+index file, and you can just write the result out with
+`git-write-tree`.
+
+Historical note. We did not have `-u` facility when this
+section was first written, so we used to warn that
+the merge is done in the index file, not in your
+working directory, and your working directory will no longer match your
+index.
+
+
+8) Merging multiple trees, continued
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Sadly, many merges aren't trivial. If there are files that have
+been added.moved or removed, or if both branches have modified the
+same file, you will be left with an index tree that contains "merge
+entries" in it. Such an index tree can 'NOT' be written out to a tree
+object, and you will have to resolve any such merge clashes using
+other tools before you can write out the result.
+
+You can examine such index state with `git-ls-files --unmerged`
+command. An example:
- [ fixme: talk about resolving merges here ]
+------------------------------------------------
+$ git-read-tree -m $orig HEAD $target
+$ git-ls-files --unmerged
+100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c
+100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c
+100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c
+------------------------------------------------
+
+Each line of the `git-ls-files --unmerged` output begins with
+the blob mode bits, blob SHA1, '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.
+Earlier we said that trivial merges are done inside
+`git-read-tree -m`. For example, if the file did not change
+from `$orig` to `HEAD` nor `$target`, or if the file changed
+from `$orig` to `HEAD` and `$orig` to `$target` the same way,
+obviously the final outcome is what is in `HEAD`. What the
+above example shows is that file `hello.c` was changed from
+`$orig` to `HEAD` and `$orig` to `$target` in a different way.
+You could resolve this by running your favorite 3-way merge
+program, e.g. `diff3` or `merge`, on the blob objects from
+these three stages yourself, like this:
+
+------------------------------------------------
+$ git-cat-file blob 263414f... >hello.c~1
+$ git-cat-file blob 06fa6a2... >hello.c~2
+$ git-cat-file blob cc44c73... >hello.c~3
+$ merge hello.c~2 hello.c~1 hello.c~3
+------------------------------------------------
+
+This would leave the merge result in `hello.c~2` file, along
+with conflict markers if there are conflicts. After verifying
+the merge result makes sense, you can tell git what the final
+merge result for this file is by:
+
+ mv -f hello.c~2 hello.c
+ git-update-index hello.c
+
+When a path is in unmerged state, running `git-update-index` for
+that path tells git to mark the path resolved.
+
+The above is the description of a git merge at the lowest level,
+to help you understand what conceptually happens under the hood.
+In practice, nobody, not even git itself, uses three `git-cat-file`
+for this. There is `git-merge-index` program that extracts the
+stages to temporary files and calls a `merge` script on it
+
+ git-merge-index git-merge-one-file hello.c
+
+and that is what higher level `git resolve` is implemented with.