git-read-tree(1)
================
-v0.1, May 2005
NAME
----
-git-read-tree - Reads tree information into the directory cache
+git-read-tree - Reads tree information into the index
SYNOPSIS
--------
-'git-read-tree' (<tree-ish> | -m <tree-ish1> [<tree-ish2> <tree-ish3>])"
+'git-read-tree' (<tree-ish> | [-m [-u|-i]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
+
DESCRIPTION
-----------
-Reads the tree information given by <tree> into the directory cache,
+Reads the tree information given by <tree-ish> into the index,
but does not actually *update* any of the files it "caches". (see:
-git-checkout-cache)
+gitlink:git-checkout-index[1])
-Optionally, it can merge a tree into the cache or perform a 3-way
-merge.
+Optionally, it can merge a tree into the index, perform a
+fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
+flag. When used with `-m`, the `-u` flag causes it to also update
+the files in the work tree with the result of the merge.
-Trivial merges are done by "git-read-tree" itself. Only conflicting paths
-will be in unmerged state when "git-read-tree" returns.
+Trivial merges are done by `git-read-tree` itself. Only conflicting paths
+will be in unmerged state when `git-read-tree` returns.
OPTIONS
-------
-m::
- Perform a merge, not just a read
+ Perform a merge, not just a read. The command will
+ refuse to run if your index file has unmerged entries,
+ indicating that you have not finished previous merge you
+ started.
+
+--reset::
+ Same as -m, except that unmerged entries are discarded
+ instead of failing.
+
+-u::
+ After a successful merge, update the files in the work
+ tree with the result of the merge.
+
+-i::
+ Usually a merge requires the index file as well as the
+ files in the working tree are up to date with the
+ current head commit, in order not to lose local
+ changes. This flag disables the check with the working
+ tree and is meant to be used when creating a merge of
+ trees that are not directly related to the current
+ working tree status into a temporary index file.
<tree-ish#>::
The id of the tree object(s) to be read/merged.
Merging
-------
-If '-m' is specified, "git-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
+If `-m` is specified, `git-read-tree` can perform 3 kinds of
+merge, a single tree merge if only 1 tree is given, a
+fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
provided.
+
Single Tree Merge
~~~~~~~~~~~~~~~~~
If only 1 tree is specified, git-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)
+specify `-m`, except that if the original index has an entry for a
+given pathname, and the contents of the path matches with the tree
+being read, the stat info from the index is used. (In other words, the
+index's stat()s take precedence over the merged tree's).
-That means that if you do a "git-read-tree -m <newtree>" followed by a
-"git-checkout-cache -f -a", the "git-checkout-cache" only checks out
+That means that if you do a `git-read-tree -m <newtree>` followed by a
+`git-checkout-index -f -u -a`, the `git-checkout-index` only checks out
the stuff that really changed.
-This is used to avoid unnecessary false hits when "git-diff-files" is
-run after git-read-tree.
+This is used to avoid unnecessary false hits when `git-diff-files` is
+run after `git-read-tree`.
+
+
+Two Tree Merge
+~~~~~~~~~~~~~~
+
+Typically, this is invoked as `git-read-tree -m $H $M`, where $H
+is the head commit of the current repository, and $M is the head
+of a foreign tree, which is simply ahead of $H (i.e. we are in a
+fast forward situation).
+
+When two trees are specified, the user is telling git-read-tree
+the following:
+
+ 1. The current index and work tree is derived from $H, but
+ the user may have local changes in them since $H;
+
+ 2. The user wants to fast-forward to $M.
+
+In this case, the `git-read-tree -m $H $M` command makes sure
+that no local change is lost as the result of this "merge".
+Here are the "carry forward" rules:
+
+ I (index) H M Result
+ -------------------------------------------------------
+ 0 nothing nothing nothing (does not happen)
+ 1 nothing nothing exists use M
+ 2 nothing exists nothing remove path from index
+ 3 nothing exists exists use M
+
+ clean I==H I==M
+ ------------------
+ 4 yes N/A N/A nothing nothing keep index
+ 5 no N/A N/A nothing nothing keep index
+
+ 6 yes N/A yes nothing exists keep index
+ 7 no N/A yes nothing exists keep index
+ 8 yes N/A no nothing exists fail
+ 9 no N/A no nothing exists fail
+
+ 10 yes yes N/A exists nothing remove path from index
+ 11 no yes N/A exists nothing fail
+ 12 yes no N/A exists nothing fail
+ 13 no no N/A exists nothing fail
+
+ clean (H=M)
+ ------
+ 14 yes exists exists keep index
+ 15 no exists exists keep index
+
+ clean I==H I==M (H!=M)
+ ------------------
+ 16 yes no no exists exists fail
+ 17 no no no exists exists fail
+ 18 yes no yes exists exists keep index
+ 19 no no yes exists exists keep index
+ 20 yes yes no exists exists use M
+ 21 no yes no exists exists fail
+
+In all "keep index" cases, the index entry stays as in the
+original index file. If the entry were not up to date,
+git-read-tree keeps the copy in the work tree intact when
+operating under the -u flag.
+
+When this form of git-read-tree returns successfully, you can
+see what "local changes" you made are carried forward by running
+`git-diff-index --cached $M`. Note that this does not
+necessarily match `git-diff-index --cached $H` would have
+produced before such a two tree merge. This is because of cases
+18 and 19 --- if you already had the changes in $M (e.g. maybe
+you picked it up via e-mail in a patch form), `git-diff-index
+--cached $H` would have told you about the change before this
+merge, but it would not show in `git-diff-index --cached $M`
+output after two-tree merge.
+
3-Way Merge
~~~~~~~~~~~
Each "index" entry has two bits worth of "stage" state. stage 0 is the
normal one, and is the only one you'd see in any kind of normal use.
-However, when you do "git-read-tree" with three trees, the "stage"
+However, when you do `git-read-tree` with three trees, the "stage"
starts out at 1.
This means that you can do
- git-read-tree -m <tree1> <tree2> <tree3>
+----------------
+$ git-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".
+<tree3> entries in "stage3". When performing a merge of another
+branch into the current branch, we use the common ancestor tree
+as <tree1>, the current branch head as <tree2>, and the other
+branch head as <tree3>.
-Furthermore, "git-read-tree" has special-case logic that says: if you see
+Furthermore, `git-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)
+ difference - the same work has been done on our branch in
+ stage 2 and their branch in stage 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 3 (our branch in stage 2 did not do anything since the
+ ancestor in stage 1 while their branch in stage 3 worked on
+ it)
- stage 1 and stage 3 are the same and stage 2 is different take
- stage 2 (some work has been done on stage 2)
+ stage 2 (we did something while they did nothing)
-The "git-write-tree" command refuses to write a nonsensical tree, and it
+The `git-write-tree` command refuses to write a nonsensical tree, and it
will complain about unmerged entries if it sees a single entry that is not
stage 0.
"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 "git-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
-"git-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:
+The order of stages 1, 2 and 3 (hence the order of three
+<tree-ish> command line arguments) are significant when you
+start a 3-way merge with an index file that is already
+populated. Here is an outline of how the algorithm works:
- if a file exists in identical format in all three trees, it will
- automatically collapse to "merged" state by the new git-read-tree.
+ automatically collapse to "merged" state by git-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
+ will stay as separate entries in the index. It's up to "porcelain
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.
+ merged version.
- the index file saves and restores with all this information, so you
can merge things incrementally, but as long as it has entries in
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.
+You would normally use `git-merge-index` with supplied
+`git-merge-one-file` to do this last step. The script updates
+the files in the working tree as it merges each path and at the
+end of a successful merge.
+
+When you start a 3-way merge with an index file that is already
+populated, it is assumed that it represents the state of the
+files in your work tree, and you can even have files with
+changes unrecorded in the index file. It is further assumed
+that this state is "derived" from the stage 2 tree. The 3-way
+merge refuses to run if it finds an entry in the original index
+file that does not match stage 2.
+
+This is done to prevent you from losing your work-in-progress
+changes, and mixing your random changes in an unrelated merge
+commit. To illustrate, suppose you start from what has been
+commited last to your repository:
+
+----------------
+$ JC=`git-rev-parse --verify "HEAD^0"`
+$ git-checkout-index -f -u -a $JC
+----------------
+
+You do random edits, without running git-update-index. And then
+you notice that the tip of your "upstream" tree has advanced
+since you pulled from him:
+
+----------------
+$ git-fetch git://.... linus
+$ LT=`cat .git/FETCH_HEAD`
+----------------
+
+Your work tree is still based on your HEAD ($JC), but you have
+some edits since. Three-way merge makes sure that you have not
+added or modified index entries since $JC, and if you haven't,
+then does the right thing. So with the following sequence:
+
+----------------
+$ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT
+$ git-merge-index git-merge-one-file -a
+$ echo "Merge with Linus" | \
+ git-commit-tree `git-write-tree` -p $JC -p $LT
+----------------
+
+what you would commit is a pure merge between $JC and $LT without
+your work-in-progress changes, and your work tree would be
+updated to the result of the merge.
+
+However, if you have local changes in the working tree that
+would be overwritten by this merge,`git-read-tree` will refuse
+to run to prevent your changes from being lost.
+
+In other words, there is no need to worry about what exists only
+in the working tree. When you have local changes in a part of
+the project that is not involved in the merge, your changes do
+not interfere with the merge, and are kept intact. When they
+*do* interfere, the merge does not even start (`git-read-tree`
+complains loudly and fails without modifying anything). In such
+a case, you can simply continue doing what you were in the
+middle of doing, and when your working tree is ready (i.e. you
+have finished your work-in-progress), attempt the merge again.
+
See Also
--------
-link:git-write-tree.html[git-write-tree]; link:git-ls-files.html[git-ls-files]
+gitlink:git-write-tree[1]; gitlink:git-ls-files[1]
Author
GIT
---
-Part of the link:git.html[git] suite
+Part of the gitlink:git[7] suite