Reference documentation for the core git commands.

Signed-off-by: David Greaves <david@dgreaves.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

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+This file contains reference information for the core git commands.
+It is actually based on the source from Petr Baudis' tree and may
+therefore contain a few 'extras' that may or may not make it upstream.
+
+The README contains much useful definition and clarification info -
+read that first.  And of the commands, I suggest reading
+'update-cache' and 'read-tree' first - I wish I had!
+
+Thanks to original email authors and proof readers esp Junio C Hamano
+<junkio@cox.net>
+
+David Greaves <david@dgreaves.com>
+24/4/05
+
+Identifier terminology used:
+
+<object>
+	Indicates any object sha1 identifier
+
+<blob>
+	Indicates a blob object sha1 identifier
+
+<tree>
+	Indicates a tree object sha1 identifier
+
+<commit>
+	Indicates a commit object sha1 identifier
+
+<tree/commit>
+	Indicates a tree or commit object sha1 identifier (usually
+	because the command can read the <tree> a <commit> contains).
+	[Eventually may be replaced with <tree> if <tree> means
+	<tree/commit> in all commands]
+
+<type>
+	Indicates that an object type is required.
+	Currently one of: blob/tree/commit
+
+<file>
+	Indicates a filename - often includes leading path
+
+<path>
+	Indicates the path of a file (is this ever useful?)
+
+
+
+################################################################
+cat-file
+	cat-file (-t | <type>) <object>
+
+Provide contents or type of objects in the repository. The type is
+required if -t is not being used to find the object type.
+
+<object>
+	The sha1 identifier of the object.
+
+-t
+	show the object type identified by <object>
+
+<type>
+	One of: blob/tree/commit
+
+Output
+
+If -t is specified, one of:
+        blob/tree/commit
+
+Otherwise the raw (though uncompressed) contents of the <object> will
+be returned.
+
+
+################################################################
+check-files
+	check-files <file>...
+
+Check that a list of files are up-to-date between the filesystem and
+the cache. Used to verify a patch target before doing a patch.
+
+Files that do not exist on the filesystem are considered up-to-date
+(whether or not they are in the cache).
+
+Emits an error message on failure.
+preparing to update existing file <file> not in cache
+	  <file> exists but is not in the cache
+
+preparing to update file <file> not uptodate in cache
+	  <file> on disk is not up-to-date with the cache
+
+exits with a status code indicating success if all files are
+up-to-date.
+
+see also: update-cache
+
+
+################################################################
+checkout-cache
+	checkout-cache [-q] [-a] [-f] [-n] [--prefix=<string>]
+		       [--] <file>...
+
+Will copy all files listed from the cache to the working directory
+(not overwriting existing files). Note that the file contents are
+restored - NOT the file permissions.
+??? l 58 checkout-cache.c says restore executable bit.
+
+-q
+	be quiet if files exist or are not in the cache
+
+-f
+	forces overwrite of existing files
+
+-a
+	checks out all files in the cache (will then continue to
+	process listed files).
+-n
+	Don't checkout new files, only refresh files already checked
+	out.
+
+--prefix=<string>
+	When creating files, prepend <string> (usually a directory
+	including a trailing /)
+
+--
+	Do not interpret any more arguments as options.
+
+Note that the order of the flags matters:
+
+	checkout-cache -a -f file.c
+
+will first check out all files listed in the cache (but not overwrite
+any old ones), and then force-checkout file.c a second time (ie that
+one _will_ overwrite any old contents with the same filename).
+
+Also, just doing "checkout-cache" does nothing. You probably meant
+"checkout-cache -a". And if you want to force it, you want
+"checkout-cache -f -a".
+
+Intuitiveness is not the goal here. Repeatability is. The reason for
+the "no arguments means no work" thing is that from scripts you are
+supposed to be able to do things like
+
+	find . -name '*.h' -print0 | xargs -0 checkout-cache -f --
+
+which will force all existing *.h files to be replaced with their
+cached copies. If an empty command line implied "all", then this would
+force-refresh everything in the cache, which was not the point.
+
+To update and refresh only the files already checked out:
+
+   checkout-cache -n -f -a && update-cache --ignore-missing --refresh
+
+Oh, and the "--" is just a good idea when you know the rest will be
+filenames. Just so that you wouldn't have a filename of "-a" causing
+problems (not possible in the above example, but get used to it in
+scripting!).
+
+The prefix ability basically makes it trivial to use checkout-cache as
+a "export as tree" function. Just read the desired tree into the
+index, and do a
+  
+        checkout-cache --prefix=export-dir/ -a
+  
+and checkout-cache will "export" the cache into the specified
+directory.
+  
+NOTE! The final "/" is important. The exported name is literally just
+prefixed with the specified string, so you can also do something like
+  
+        checkout-cache --prefix=.merged- Makefile
+  
+to check out the currently cached copy of "Makefile" into the file
+".merged-Makefile".
+
+
+################################################################
+commit-tree
+	commit-tree <tree> [-p <parent commit>]*   < changelog
+
+Creates a new commit object based on the provided tree object and
+emits the new commit object id on stdout. If no parent is given then
+it is considered to be an initial tree.
+
+A commit object usually has 1 parent (a commit after a change) or up
+to 16 parents.  More than one parent represents a merge of branches
+that led to them.
+
+While a tree represents a particular directory state of a working
+directory, a commit represents that state in "time", and explains how
+to get there.
+
+Normally a commit would identify 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.
+
+Options
+
+<tree>
+	An existing tree object
+
+-p <parent commit>
+	Each -p indicates a the id of a parent commit object.
+	
+
+Commit Information
+
+A commit encapsulates:
+	all parent object ids
+	author name, email and date
+	committer name and email and the commit time.
+
+If not provided, commit-tree uses your name, hostname and domain to
+provide author and committer info. This can be overridden using the
+following environment variables.
+	AUTHOR_NAME
+	AUTHOR_EMAIL
+	AUTHOR_DATE
+	COMMIT_AUTHOR_NAME
+	COMMIT_AUTHOR_EMAIL
+(nb <,> and '\n's are stripped)
+
+A commit comment is read from stdin (max 999 chars). If a changelog
+entry is not provided via '<' redirection, commit-tree will just wait
+for one to be entered and terminated with ^D
+
+see also: write-tree
+
+
+################################################################
+diff-cache
+	diff-cache [-p] [-r] [-z] [--cached] <tree/commit>
+
+Compares the content and mode of the blobs found via a tree object
+with the content of the current cache and, optionally ignoring the
+stat state of the file on disk.
+
+<tree/commit>
+	The id of a tree or commit object to diff against.
+
+-p
+	generate patch (see section on generating patches)
+
+-r
+	recurse
+
+-z
+	\0 line termination on output
+
+--cached
+	do not consider the on-disk file at all
+
+Output format:
+
+See "Output format from diff-cache, diff-tree and show-diff" section.
+
+Operating Modes
+
+You can choose whether you want to trust the index file entirely
+(using the "--cached" flag) or ask the diff logic to show any files
+that don't match the stat state as being "tentatively changed".  Both
+of these operations are very useful indeed.
+
+Cached Mode
+
+If --cached is specified, it allows you to ask:
+	show me the differences between HEAD and the current index
+	contents (the ones I'd write with a "write-tree")
+
+For example, let's say that you have worked on your index file, and are
+ready to commit. You want to see eactly _what_ you are going to commit is
+without having to write a new tree object and compare it that way, and to
+do that, you just do
+
+	diff-cache --cached $(cat .git/HEAD)
+
+Example: let's say I had renamed "commit.c" to "git-commit.c", and I had 
+done an "upate-cache" to make that effective in the index file. 
+"show-diff" wouldn't show anything at all, since the index file matches 
+my working directory. But doing a diff-cache does:
+	torvalds@ppc970:~/git> diff-cache --cached $(cat .git/HEAD)
+	-100644 blob    4161aecc6700a2eb579e842af0b7f22b98443f74        commit.c
+	+100644 blob    4161aecc6700a2eb579e842af0b7f22b98443f74        git-commit.c
+
+And as you can see, the output matches "diff-tree -r" output (we
+always do equivalent of "-r", since the index is flat).
+You can trivially see that the above is a rename.
+
+In fact, "diff-cache --cached" _should_ always be entirely equivalent to
+actually doing a "write-tree" and comparing that. Except this one is much
+nicer for the case where you just want to check where you are.
+
+So doing a "diff-cache --cached" is basically very useful when you are 
+asking yourself "what have I already marked for being committed, and 
+what's the difference to a previous tree".
+
+Non-cached Mode
+
+The "non-cached" mode takes a different approach, and is potentially
+the even more useful of the two in that what it does can't be emulated
+with a "write-tree + diff-tree". Thus that's the default mode.  The
+non-cached version asks the question
+
+   "show me the differences between HEAD and the currently checked out 
+    tree - index contents _and_ files that aren't up-to-date"
+
+which is obviously a very useful question too, since that tells you what
+you _could_ commit. Again, the output matches the "diff-tree -r" output to
+a tee, but with a twist.
+
+The twist is that if some file doesn't match the cache, we don't have a
+backing store thing for it, and we use the magic "all-zero" sha1 to show
+that. So let's say that you have edited "kernel/sched.c", but have not
+actually done an update-cache on it yet - there is no "object" associated
+with the new state, and you get:
+
+	torvalds@ppc970:~/v2.6/linux> diff-cache $(cat .git/HEAD )
+	*100644->100664 blob    7476bbcfe5ef5a1dd87d745f298b831143e4d77e->0000000000000000000000000000000000000000      kernel/sched.c
+
+ie it shows that the tree has changed, and that "kernel/sched.c" has is
+not up-to-date and may contain new stuff. The all-zero sha1 means that to
+get the real diff, you need to look at the object in the working directory
+directly rather than do an object-to-object diff.
+
+NOTE! As with other commands of this type, "diff-cache" does not actually 
+look at the contents of the file at all. So maybe "kernel/sched.c" hasn't 
+actually changed, and it's just that you touched it. In either case, it's 
+a note that you need to upate-cache it to make the cache be in sync.
+
+NOTE 2! You can have a mixture of files show up as "has been updated" and
+"is still dirty in the working directory" together. You can always tell
+which file is in which state, since the "has been updated" ones show a
+valid sha1, and the "not in sync with the index" ones will always have the
+special all-zero sha1.
+
+################################################################
+diff-tree
+	diff-tree [-p] [-r] [-z] <tree/commit> <tree/commit> [<pattern>]*
+
+Compares the content and mode of the blobs found via two tree objects.
+
+Note that diff-tree can use the tree encapsulated in a commit object.
+
+<tree sha1>
+	The id of a tree or commit object.
+
+<pattern>
+
+	If provided, the results are limited to a subset of files
+	matching one of these prefix strings.
+	ie file matches /^<pattern1>|<pattern2>|.../
+	Note that pattern does not provide any wildcard or regexp features.
+
+-p
+	generate patch (see section on generating patches)
+
+-r
+	recurse
+
+-z
+	\0 line termination on output
+
+Limiting Output
+
+If you're only interested in differences in a subset of files, for
+example some architecture-specific files, you might do:
+
+	diff-tree -r <tree/commit> <tree/commit> arch/ia64 include/asm-ia64
+
+and it will only show you what changed in those two directories.
+
+Or if you are searching for what changed in just kernel/sched.c, just do
+
+	diff-tree -r <tree/commit> <tree/commit> kernel/sched.c
+
+and it will ignore all differences to other files.
+
+The pattern is always the prefix, and is matched exactly (ie there are no
+wildcards - although matching a directory, which it does support, can
+obviously be seen as a "wildcard" for all the files under that directory).
+
+Output format:
+
+See "Output format from diff-cache, diff-tree and show-diff" section.
+
+An example of normal usage is:
+
+	torvalds@ppc970:~/git> diff-tree 5319e4d609cdd282069cc4dce33c1db559539b03 b4e628ea30d5ab3606119d2ea5caeab141d38df7
+	*100664->100664 blob    ac348b7d5278e9d04e3a1cd417389379c32b014f->a01513ed4d4d565911a60981bfb4173311ba3688      fsck-cache.c
+
+which tells you that the last commit changed just one file (it's from
+this one:
+
+	commit 3c6f7ca19ad4043e9e72fa94106f352897e651a8
+	tree 5319e4d609cdd282069cc4dce33c1db559539b03
+	parent b4e628ea30d5ab3606119d2ea5caeab141d38df7
+	author Linus Torvalds <torvalds@ppc970.osdl.org> Sat Apr 9 12:02:30 2005
+	committer Linus Torvalds <torvalds@ppc970.osdl.org> Sat Apr 9 12:02:30 2005
+
+	Make "fsck-cache" print out all the root commits it finds.
+
+	Once I do the reference tracking, I'll also make it print out all the
+	HEAD commits it finds, which is even more interesting.
+
+in case you care).
+
+################################################################
+diff-tree-helper
+	diff-tree-helper [-z]
+
+Reads output from diff-cache, diff-tree and show-diff and
+generates patch format output.
+
+-z
+	\0 line termination on input
+
+See also the section on generating patches.
+
+################################################################
+fsck-cache
+	fsck-cache [[--unreachable] <commit>*]
+
+Verifies the connectivity and validity of the objects in the database.
+
+<commit>
+	A commit object to treat as the head of an unreachability
+	trace
+
+--unreachable
+	print out objects that exist but that aren't readable from any
+	of the specified root nodes
+
+It tests SHA1 and general object sanity, but it does full tracking of
+the resulting reachability and everything else. It prints out any
+corruption it finds (missing or bad objects), and if you use the
+"--unreachable" flag it will also print out objects that exist but
+that aren't readable from any of the specified root nodes.
+
+So for example
+
+	fsck-cache --unreachable $(cat .git/HEAD)
+
+or, for Cogito users:
+
+	fsck-cache --unreachable $(cat .git/heads/*)
+
+will do quite a _lot_ of verification on the tree. There are a few
+extra validity tests to be added (make sure that tree objects are
+sorted properly etc), but on the whole if "fsck-cache" is happy, you
+do have a valid tree.
+
+Any corrupt objects you will have to find in backups or other archives
+(ie you can just remove them and do an "rsync" with some other site in
+the hopes that somebody else has the object you have corrupted).
+
+Of course, "valid tree" doesn't mean that it wasn't generated by some
+evil person, and the end result might be crap. Git is a revision
+tracking system, not a quality assurance system ;)
+
+Extracted Diagnostics
+
+expect dangling commits - potential heads - due to lack of head information
+	You haven't specified any nodes as heads so it won't be
+	possible to differentiate between un-parented commits and
+	root nodes.
+
+missing sha1 directory '<dir>'
+	The directory holding the sha1 objects is missing.
+
+unreachable <type> <object>
+	The <type> object <object>, isn't actually referred to directly
+	or indirectly in any of the trees or commits seen. This can
+	mean that there's another root na SHA1_ode that you're not specifying
+	or that the tree is corrupt. If you haven't missed a root node
+	then you might as well delete unreachable nodes since they
+	can't be used.
+
+missing <type> <object>
+	The <type> object <object>, is referred to but isn't present in
+	the database.
+
+dangling <type> <object>
+	The <type> object <object>, is present in the database but never
+	_directly_ used. A dangling commit could be a root node.
+
+warning: fsck-cache: tree <tree> has full pathnames in it
+	And it shouldn't...
+
+sha1 mismatch <object>
+	The database has an object who's sha1 doesn't match the
+	database value.
+	This indicates a ??serious?? data integrity problem.
+	(note: this error occured during early git development when
+	the database format changed.)
+
+Environment Variables
+
+SHA1_FILE_DIRECTORY
+	used to specify the object database root (usually .git/objects)
+
+################################################################
+git-export
+	git-export top [base]
+
+probably deprecated:
+On Wed, 20 Apr 2005, Petr Baudis wrote:
+>> I will probably not buy git-export, though. (That is, it is merged, but
+>> I won't make git frontend for it.) My "git export" already does
+>> something different, but more importantly, "git patch" of mine already
+>> does effectively the same thing as you do, just for a single patch; so I
+>> will probably just extend it to do it for an (a,b] range of patches.
+
+
+That's fine. It was a quick hack, just to show that if somebody wants to, 
+the data is trivially exportable.
+
+		Linus
+
+Although in Linus' distribution, git-export is not part of 'core' git.
+
+################################################################
+init-db
+	init-db
+
+This simply creates an empty git object database - basically a .git
+directory.
+
+If the object storage directory is specified via the
+SHA1_FILE_DIRECTORY environment variable then the sha1 directories are
+created underneath - otherwise the default .git/objects directory is
+used.
+
+init-db won't hurt an existing repository.
+
+
+################################################################
+ls-tree
+	ls-tree [-r] [-z] <tree/commit>
+
+convert the tree object to a human readable (and script
+processable) form.
+
+<tree/commit>
+	Id of a tree or commit object.
+-r
+	recurse into sub-trees
+
+-z
+	\0 line termination on output
+
+Output Format
+<mode>\t	<type>\t	<object>\t	<path><file>	
+
+
+################################################################
+merge-base
+	merge-base <commit> <commit>
+
+merge-base finds as good a common ancestor as possible. Given a
+selection of equally good common ancestors it should not be relied on
+to decide in any particular way.
+
+The merge-base algorithm is still in flux - use the source...
+
+
+################################################################
+merge-cache
+	merge-cache <merge-program> (-a | -- | <file>*) 
+
+This looks up the <file>(s) in the cache and, if there are any merge
+entries, unpacks all of them (which may be just one file, of course)
+into up to three separate temporary files, and then executes the
+supplied <merge-program> with those three files as arguments 1,2,3
+(empty argument if no file), and <file> as argument 4.
+
+--
+	Interpret all future arguments as filenames
+
+-a
+	Run merge against all files in the cache that need merging.
+
+If merge-cache is called with multiple <file>s (or -a) then it
+processes them in turn only stopping if merge returns a non-zero exit
+code.
+
+Typically this is run with the a script calling the merge command from
+the RCS package.
+
+A sample script called git-merge-one-file-script is included in the
+ditribution.
+
+ALERT ALERT ALERT! The git "merge object order" is different from the
+RCS "merge" program merge object order. In the above ordering, the
+original is first. But the argument order to the 3-way merge program
+"merge" is to have the original in the middle. Don't ask me why.
+
+Examples:
+
+	torvalds@ppc970:~/merge-test> merge-cache cat MM
+	This is MM from the original tree.			# original
+	This is modified MM in the branch A.			# merge1
+	This is modified MM in the branch B.			# merge2
+	This is modified MM in the branch B.			# current contents
+
+or 
+
+	torvalds@ppc970:~/merge-test> merge-cache cat AA MM
+	cat: : No such file or directory
+	This is added AA in the branch A.
+	This is added AA in the branch B.
+	This is added AA in the branch B.
+	fatal: merge program failed
+
+where the latter example shows how "merge-cache" will stop trying to
+merge once anything has returned an error (ie "cat" returned an error
+for the AA file, because it didn't exist in the original, and thus
+"merge-cache" didn't even try to merge the MM thing).
+
+
+################################################################
+read-tree
+	read-tree (<tree/commit> | -m <tree/commit1> [<tree/commit2> <tree/commit3>])"
+
+Reads the tree information given by <tree> into the directory cache,
+but does not actually _update_ any of the files it "caches". (see:
+checkout-cache)
+
+Optionally, it can merge a tree into the cache or perform a 3-way
+merge.
+
+Trivial merges are done by read-tree itself.  Only conflicting paths
+will be in unmerged state when read-tree returns.
+
+-m
+	Perform a merge, not just a read
+
+<tree#>
+	The id of the tree object(s) to be read/merged.
+
+
+Merging
+If -m is specified, read-tree performs 2 kinds of merge, a single tree
+merge if only 1 tree is given or a 3-way merge if 3 trees are
+provided.
+
+Single Tree Merge
+If only 1 tree is specified, read-tree operates as if the user did not
+specify "-m", except that if the original cache has an entry for a
+given pathname; and the contents of the path matches with the tree
+being read, the stat info from the cache is used. (In other words, the
+cache's stat()s take precedence over the merged tree's)
+
+That means that if you do a "read-tree -m <newtree>" followed by a
+"checkout-cache -f -a", the checkout-cache only checks out the stuff
+that really changed.
+
+This is used to avoid unnecessary false hits when show-diff is
+run after read-tree.
+
+3-Way Merge
+Each "index" entry has two bits worth of "stage" state. stage 0 is the
+normal one, and is the only one you'd see in any kind of normal use.
+
+However, when you do "read-tree" with multiple trees, the "stage"
+starts out at 0, but increments for each tree you read. And in
+particular, the "-m" flag means "start at stage 1" instead.
+
+This means that you can do
+
+	read-tree -m <tree1> <tree2> <tree3>
+
+and you will end up with an index with all of the <tree1> entries in
+"stage1", all of the <tree2> entries in "stage2" and all of the
+<tree3> entries in "stage3".
+
+Furthermore, "read-tree" has special-case logic that says: if you see
+a file that matches in all respects in the following states, it
+"collapses" back to "stage0":
+
+   - stage 2 and 3 are the same; take one or the other (it makes no
+     difference - the same work has been done on stage 2 and 3)
+
+   - stage 1 and stage 2 are the same and stage 3 is different; take
+     stage 3 (some work has been done on stage 3)
+
+   - stage 1 and stage 3 are the same and stage 2 is different take
+     stage 2 (some work has been done on stage 2)
+
+Write-tree refuses to write a nonsensical tree, so write-tree will
+complain about unmerged entries if it sees a single entry that is not
+stage 0".
+
+Ok, this all sounds like a collection of totally nonsensical rules,
+but it's actually exactly what you want in order to do a fast
+merge. The different stages represent the "result tree" (stage 0, aka
+"merged"), the original tree (stage 1, aka "orig"), and the two trees
+you are trying to merge (stage 2 and 3 respectively).
+
+In fact, the way "read-tree" works, it's entirely agnostic about how
+you assign the stages, and you could really assign them any which way,
+and the above is just a suggested way to do it (except since
+"write-tree" refuses to write anything but stage0 entries, it makes
+sense to always consider stage 0 to be the "full merge" state).
+
+So what happens? Try it out. Select the original tree, and two trees
+to merge, and look how it works:
+
+ - if a file exists in identical format in all three trees, it will 
+   automatically collapse to "merged" state by the new read-tree.
+
+ - a file that has _any_ difference what-so-ever in the three trees
+   will stay as separate entries in the index. It's up to "script
+   policy" to determine how to remove the non-0 stages, and insert a
+   merged version.  But since the index is always sorted, they're easy
+   to find: they'll be clustered together.
+
+ - the index file saves and restores with all this information, so you
+   can merge things incrementally, but as long as it has entries in
+   stages 1/2/3 (ie "unmerged entries") you can't write the result.
+
+So now the merge algorithm ends up being really simple:
+
+ - you walk the index in order, and ignore all entries of stage 0,
+   since they've already been done.
+
+ - if you find a "stage1", but no matching "stage2" or "stage3", you
+   know it's been removed from both trees (it only existed in the
+   original tree), and you remove that entry.  - if you find a
+   matching "stage2" and "stage3" tree, you remove one of them, and
+   turn the other into a "stage0" entry. Remove any matching "stage1"
+   entry if it exists too.  .. all the normal trivial rules ..
+
+Incidentally - it also means that you don't even have to have a separate 
+subdirectory for this. All the information literally is in the index file, 
+which is a temporary thing anyway. There is no need to worry about what is in 
+the working directory, since it is never shown and never used.
+
+see also:
+write-tree
+show-files
+
+
+################################################################
+rev-list <commit>
+
+Lists commit objects in reverse chronological order starting at the
+given commit, taking ancestry relationship into account.  This is
+useful to produce human-readable log output.
+
+
+################################################################
+rev-tree
+	rev-tree [--edges] [--cache <cache-file>] [^]<commit> [[^]<commit>]
+
+Provides the revision tree for one or more commits.
+
+--edges
+	Show edges (ie places where the marking changes between parent
+	and child)
+
+--cache <cache-file>
+	Use the specified file as a cache. [Not implemented yet]
+
+[^]<commit>
+	The commit id to trace (a leading caret means to ignore this
+	commit-id and below)
+
+Output:
+<date> <commit>:<flags> [<parent-commit>:<flags> ]*
+
+<date>
+	Date in 'seconds since epoch'
+
+<commit>
+	id of commit object
+
+<parent-commit>
+	id of each parent commit object (>1 indicates a merge)
+
+<flags>
+
+	The flags are read as a bitmask representing each commit
+	provided on the commandline. eg: given the command:
+
+		 $ rev-tree <com1> <com2> <com3>
+
+	The output:
+
+	    <date> <commit>:5
+
+	 means that <commit> is reachable from <com1>(1) and <com3>(4)
+	
+A revtree can get quite large. rev-tree will eventually allow you to
+cache previous state so that you don't have to follow the whole thing
+down.
+
+So the change difference between two commits is literally
+
+	rev-tree [commit-id1]  > commit1-revtree
+	rev-tree [commit-id2]  > commit2-revtree
+	join -t : commit1-revtree commit2-revtree > common-revisions
+
+(this is also how to find the most common parent - you'd look at just
+the head revisions - the ones that aren't referred to by other
+revisions - in "common-revision", and figure out the best one. I
+think.)
+
+
+################################################################
+show-diff
+	show-diff [-p] [-q] [-s] [-z] [paths...]
+
+Compares the files in the working tree and the cache.  When paths
+are specified, compares only those named paths.  Otherwise all
+entries in the cache are compared.  The output format is the
+same as diff-cache and diff-tree.
+
+-p
+	generate patch (see section on generating patches)
+
+-q
+	Remain silent even on nonexisting files
+
+-s
+	Does not do anything other than what -q does.
+
+Output format:
+
+See "Output format from diff-cache, diff-tree and show-diff" section.
+
+################################################################
+show-files
+	show-files [-z] [-t]
+		(--[cached|deleted|others|ignored|stage|unmerged])*
+		(-[c|d|o|i|s|u])*
+		[-x <pattern>|--exclude=<pattern>]
+		[-X <file>|--exclude-from=<file>]
+
+This merges the file listing in the directory cache index with the
+actual working directory list, and shows different combinations of the
+two.
+
+One or more of the options below may be used to determine the files
+shown:
+
+-c|--cached
+	Show cached files in the output (default)
+
+-d|--deleted
+	Show deleted files in the output
+
+-o|--others
+	Show other files in the output
+
+-i|--ignored
+	Show ignored files in the output
+	Note the this also reverses any exclude list present.
+
+-s|--stage
+	Show stage files in the output
+
+-u|--unmerged
+	Show unmerged files in the output (forces --stage)
+
+#-t [not in Linus' tree (yet?)]
+#	Identify the file status with the following tags (followed by
+#	a space) at the start of each line:
+#	H	cached
+#	M	unmerged
+#	R	removed/deleted
+#	?	other
+
+-z
+	\0 line termination on output
+
+-x|--exclude=<pattern>
+	Skips files matching pattern.
+	Note that pattern is a shell wildcard pattern.
+
+-X|--exclude-from=<file>
+	exclude patterns are read from <file>; 1 per line.
+	Allows the use of the famous dontdiff file as follows to find
+	out about uncommitted files just as dontdiff is used with
+	the diff command:
+	     show-files --others --exclude-from=dontdiff
+
+Output
+show files just outputs the filename unless --stage is specified in
+which case it outputs:
+
+[<tag> ]<mode> <object> <stage> <file>
+
+show-files --unmerged" and "show-files --stage " can be used to examine
+detailed information on unmerged paths.
+
+For an unmerged path, instead of recording a single mode/SHA1 pair,
+the dircache records up to three such pairs; one from tree O in stage
+1, A in stage 2, and B in stage 3.  This information can be used by
+the user (or Cogito) to see what should eventually be recorded at the
+path. (see read-cache for more information on state)
+
+see also:
+read-cache
+
+
+################################################################
+unpack-file
+	unpack-file <blob>
+
+Creates a file holding the contents of the blob specified by sha1. It
+returns the name of the temporary file in the following format:
+	.merge_file_XXXXX
+
+<blob>
+	Must be a blob id
+
+################################################################
+update-cache
+	update-cache [--add] [--remove] [--refresh [--ignore-missing]]
+		     [--cacheinfo <mode> <object> <path>]*
+		     [--] [<file>]*
+
+Modifies the index or directory cache. Each file mentioned is updated
+into the cache and any 'unmerged' or 'needs updating' state is
+cleared.
+
+The way update-cache handles files it is told about can be modified
+using the various options:
+
+--add
+	If a specified file isn't in the cache already then it's
+	added.
+	Default behaviour is to ignore new files.
+
+--remove
+	If a specified file is in the cache but is missing then it's
+	removed.
+	Default behaviour is to ignore removed file.
+
+--refresh
+	Looks at the current cache and checks to see if merges or
+	updates are needed by checking stat() information.
+
+--ignore-missing
+	Ignores missing files during a --refresh
+
+--cacheinfo <mode> <object> <path>
+	Directly insert the specified info into the cache.
+	
+--
+	Do not interpret any more arguments as options.
+
+<file>
+	Files to act on.
+	Note that files begining with '.' are discarded. This includes
+	"./file" and "dir/./file". If you don't want this, then use	
+	cleaner names.
+	The same applies to directories ending '/' and paths with '//'
+
+
+Using --refresh
+
+--refresh does not calculate a new sha1 file or bring the cache
+up-to-date for mode/content changes. But what it _does_ do is to
+"re-match" the stat information of a file with the cache, so that you
+can refresh the cache for a file that hasn't been changed but where
+the stat entry is out of date.
+
+For example, you'd want to do this after doing a "read-tree", to link
+up the stat cache details with the proper files.
+
+Using --cacheinfo
+--cacheinfo is used to register a file that is not in the current
+working directory.  This is useful for minimum-checkout merging.
+
+To pretend you have a file with mode and sha1 at path, say:
+
+ $ update-cache --cacheinfo mode sha1 path
+
+To update and refresh only the files already checked out:
+
+   checkout-cache -n -f -a && update-cache --ignore-missing --refresh
+
+
+################################################################
+write-tree
+	write-tree
+
+Creates a tree object using the current cache.
+
+The cache must be merged.
+
+Conceptually, write-tree sync()s the current directory cache contents
+into a set of tree files.
+In order to have that match what is actually in your directory right
+now, you need to have done a "update-cache" phase before you did the
+"write-tree".
+
+
+################################################################
+
+Output format from diff-cache, diff-tree and show-diff.
+
+These commands all compare two sets of things; what are
+compared are different:
+
+    diff-cache <tree/commit>
+
+        compares the <tree/commit> and the files on the filesystem.
+
+    diff-cache --cached <tree/commit>
+
+        compares the <tree/commit> and the cache.
+
+    diff-tree [-r] <tree/commit-1> <tree/commit-2> [paths...]
+
+        compares the trees named by the two arguments.
+
+    show-diff [paths...]
+
+        compares the cache and the files on the filesystem.
+
+The following desription uses "old" and "new" to mean those
+compared entities.
+
+For files in old but not in new (i.e. removed):
+-<mode> \t <type> \t <object> \t <path>
+
+For files not in old but in new (i.e. added):
++<mode> \t <type> \t <object> \t <path>
+
+For files that differ:
+*<old-mode>-><new-mode> \t <type> \t <old-sha1>-><new-sha1> \t <path>
+
+<new-sha1> is shown as all 0's if new is a file on the
+filesystem and it is out of sync with the cache.  Example:
+
+    *100644->100660 blob    5be4a414b32cf4204f889469942986d3d783da84->0000000000000000000000000000000000000000      file.c
+
+################################################################
+
+Generating patches
+
+When diff-cache, diff-tree, or show-diff are run with a -p
+option, they do not produce the output described in "Output
+format from diff-cache, diff-tree and show-diff" section.  It
+instead produces a patch file.
+
+The patch generation can be customized at two levels.  This
+customization also applies to diff-tree-helper.
+
+1. When the environment variable GIT_EXTERNAL_DIFF is not set,
+   these commands internally invoke diff like this:
+
+   diff -L k/<path> -L l/<path> -pu <old> <new>
+
+   For added files, /dev/null is used for <old>.  For removed
+   files, /dev/null is used for <new>
+
+   The first part of the above command-line can be customized via
+   the environment variable GIT_DIFF_CMD.  For example, if you
+   do not want to show the extra level of leading path, you can
+   say this:
+
+   GIT_DIFF_CMD="diff -L'%s' -L'%s'" show-diff -p
+
+   Caution:  Do not use more than two '%s' in GIT_DIFF_CMD.
+
+   The diff formatting options can be customized via the
+   environment variable GIT_DIFF_OPTS.  For example, if you
+   prefer context diff:
+
+   GIT_DIFF_OPTS=-c diff-cache -p $(cat .git/HEAD)
+
+
+2. When the environment variable GIT_EXTERNAL_DIFF is set, the
+   program named by it is called, instead of the diff invocation
+   described above.
+
+   For a path that is added, removed, or modified,
+   GIT_EXTERNAL_DIFF is called with 7 parameters:
+
+     path old-file old-hex old-mode new-file new-hex new-mode
+
+   where
+     <old|new>-file are files GIT_EXTERNAL_DIFF can use to read the
+                    contents of <old|ne>,
+     <old|new>-hex are the 40-hexdigit SHA1 hashes,
+     <old|new>-mode are the octal representation of the file modes.
+
+   The file parameters can point at the user's working file
+   (e.g. new-file in show-diff), /dev/null (e.g. old-file when a
+   new file is added), or a temporary file (e.g. old-file in the
+   cache).  GIT_EXTERNAL_DIFF should not worry about
+   unlinking the temporary file --- it is removed when
+   GIT_EXTERNAL_DIFF exits.
+
+   For a path that is unmerged, GIT_EXTERNAL_DIFF is called with
+   1 parameter, path.
+
+################################################################
+
+Terminology: - see README for description
+Each line contains terms used interchangeably
+
+object database, .git directory
+directory cache, index
+id, sha1, sha1-id, sha1 hash
+type, tag
+blob, blob object
+tree, tree object
+commit, commit object
+parent
+root object
+changeset
+
+
+git Environment Variables
+AUTHOR_NAME
+AUTHOR_EMAIL
+AUTHOR_DATE
+COMMIT_AUTHOR_NAME
+COMMIT_AUTHOR_EMAIL
+GIT_DIFF_CMD
+GIT_DIFF_OPTS
+GIT_EXTERNAL_DIFF
+GIT_INDEX_FILE
+SHA1_FILE_DIRECTORY
+
+