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Update git environment variable docs Update first section of command docs (Manipulation commands section) Signed-off-by: David Greaves <david@dgreaves.com>
153 lines
5.2 KiB
Plaintext
153 lines
5.2 KiB
Plaintext
git-read-tree(1)
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================
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v0.1, May 2005
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NAME
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----
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git-read-tree - Reads tree information into the directory cache
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SYNOPSIS
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--------
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'git-read-tree' (<tree-ish> | -m <tree-ish1> [<tree-ish2> <tree-ish3>])"
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DESCRIPTION
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-----------
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Reads the tree information given by <tree> into the directory cache,
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but does not actually *update* any of the files it "caches". (see:
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git-checkout-cache)
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Optionally, it can merge a tree into the cache or perform a 3-way
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merge.
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Trivial merges are done by "git-read-tree" itself. Only conflicting paths
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will be in unmerged state when "git-read-tree" returns.
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OPTIONS
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-------
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-m::
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Perform a merge, not just a read
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<tree-ish#>::
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The id of the tree object(s) to be read/merged.
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Merging
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-------
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If '-m' is specified, "git-read-tree" performs 2 kinds of merge, a single tree
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merge if only 1 tree is given or a 3-way merge if 3 trees are
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provided.
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Single Tree Merge
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~~~~~~~~~~~~~~~~~
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If only 1 tree is specified, git-read-tree operates as if the user did not
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specify '-m', except that if the original cache has an entry for a
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given pathname; and the contents of the path matches with the tree
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being read, the stat info from the cache is used. (In other words, the
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cache's stat()s take precedence over the merged tree's)
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That means that if you do a "git-read-tree -m <newtree>" followed by a
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"git-checkout-cache -f -a", the "git-checkout-cache" only checks out
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the stuff that really changed.
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This is used to avoid unnecessary false hits when "git-diff-files" is
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run after git-read-tree.
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3-Way Merge
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~~~~~~~~~~~
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Each "index" entry has two bits worth of "stage" state. stage 0 is the
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normal one, and is the only one you'd see in any kind of normal use.
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However, when you do "git-read-tree" with three trees, the "stage"
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starts out at 1.
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This means that you can do
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git-read-tree -m <tree1> <tree2> <tree3>
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and you will end up with an index with all of the <tree1> entries in
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"stage1", all of the <tree2> entries in "stage2" and all of the
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<tree3> entries in "stage3".
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Furthermore, "git-read-tree" has special-case logic that says: if you see
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a file that matches in all respects in the following states, it
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"collapses" back to "stage0":
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- stage 2 and 3 are the same; take one or the other (it makes no
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difference - the same work has been done on stage 2 and 3)
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- stage 1 and stage 2 are the same and stage 3 is different; take
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stage 3 (some work has been done on stage 3)
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- stage 1 and stage 3 are the same and stage 2 is different take
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stage 2 (some work has been done on stage 2)
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The "git-write-tree" command refuses to write a nonsensical tree, and it
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will complain about unmerged entries if it sees a single entry that is not
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stage 0.
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Ok, this all sounds like a collection of totally nonsensical rules,
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but it's actually exactly what you want in order to do a fast
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merge. The different stages represent the "result tree" (stage 0, aka
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"merged"), the original tree (stage 1, aka "orig"), and the two trees
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you are trying to merge (stage 2 and 3 respectively).
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In fact, the way "git-read-tree" works, it's entirely agnostic about how
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you assign the stages, and you could really assign them any which way,
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and the above is just a suggested way to do it (except since
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"git-write-tree" refuses to write anything but stage0 entries, it makes
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sense to always consider stage 0 to be the "full merge" state).
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So what happens? Try it out. Select the original tree, and two trees
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to merge, and look how it works:
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- if a file exists in identical format in all three trees, it will
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automatically collapse to "merged" state by the new git-read-tree.
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- a file that has _any_ difference what-so-ever in the three trees
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will stay as separate entries in the index. It's up to "script
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policy" to determine how to remove the non-0 stages, and insert a
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merged version. But since the index is always sorted, they're easy
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to find: they'll be clustered together.
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- the index file saves and restores with all this information, so you
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can merge things incrementally, but as long as it has entries in
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stages 1/2/3 (ie "unmerged entries") you can't write the result. So
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now the merge algorithm ends up being really simple:
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* you walk the index in order, and ignore all entries of stage 0,
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since they've already been done.
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* if you find a "stage1", but no matching "stage2" or "stage3", you
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know it's been removed from both trees (it only existed in the
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original tree), and you remove that entry.
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* if you find a matching "stage2" and "stage3" tree, you remove one
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of them, and turn the other into a "stage0" entry. Remove any
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matching "stage1" entry if it exists too. .. all the normal
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trivial rules ..
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Incidentally - it also means that you don't even have to have a
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separate subdirectory for this. All the information literally is in
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the index file, which is a temporary thing anyway. There is no need to
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worry about what is in the working directory, since it is never shown
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and never used.
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See Also
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--------
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link:git-write-tree.html[git-write-tree]; link:git-ls-files.html[git-ls-files]
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Author
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------
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Written by Linus Torvalds <torvalds@osdl.org>
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Documentation
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--------------
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Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
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GIT
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---
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Part of the link:git.html[git] suite
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