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 [-u]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
+[verse]
+'git read-tree' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
+ [-u [--exclude-per-directory=<gitignore>] | -i]]
+ [--index-output=<file>] [--no-sparse-checkout]
+ (--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])
DESCRIPTION
-----------
-Reads the tree information given by <tree-ish> 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-index)
+linkgit:git-checkout-index[1])
-Optionally, it can merge a tree into the cache, 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
+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 to be 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.
+
+-n::
+--dry-run::
+ Check if the command would error out, without updating the index
+ nor the files in the working tree for real.
+
+-v::
+ Show the progress of checking files out.
+
+--trivial::
+ Restrict three-way merge by 'git read-tree' to happen
+ only if there is no file-level merging required, instead
+ of resolving merge for trivial cases and leaving
+ conflicting files unresolved in the index.
+
+--aggressive::
+ Usually a three-way merge by 'git read-tree' resolves
+ the merge for really trivial cases and leaves other
+ cases unresolved in the index, so that porcelains can
+ implement different merge policies. This flag makes the
+ command resolve a few more cases internally:
++
+* when one side removes a path and the other side leaves the path
+ unmodified. The resolution is to remove that path.
+* when both sides remove a path. The resolution is to remove that path.
+* when both sides add a path identically. The resolution
+ is to add that path.
+
+--prefix=<prefix>/::
+ Keep the current index contents, and read the contents
+ of the named tree-ish under the directory at `<prefix>`.
+ The command will refuse to overwrite entries that already
+ existed in the original index file. Note that the `<prefix>/`
+ value must end with a slash.
+
+--exclude-per-directory=<gitignore>::
+ When running the command with `-u` and `-m` options, the
+ merge result may need to overwrite paths that are not
+ tracked in the current branch. The command usually
+ refuses to proceed with the merge to avoid losing such a
+ path. However this safety valve sometimes gets in the
+ way. For example, it often happens that the other
+ branch added a file that used to be a generated file in
+ your branch, and the safety valve triggers when you try
+ to switch to that branch after you ran `make` but before
+ running `make clean` to remove the generated file. This
+ option tells the command to read per-directory exclude
+ file (usually '.gitignore') and allows such an untracked
+ but explicitly ignored file to be overwritten.
+
+--index-output=<file>::
+ Instead of writing the results out to `$GIT_INDEX_FILE`,
+ write the resulting index in the named file. While the
+ command is operating, the original index file is locked
+ with the same mechanism as usual. The file must allow
+ to be rename(2)ed into from a temporary file that is
+ created next to the usual index file; typically this
+ means it needs to be on the same filesystem as the index
+ file itself, and you need write permission to the
+ directories the index file and index output file are
+ located in.
+
+--no-sparse-checkout::
+ Disable sparse checkout support even if `core.sparseCheckout`
+ is true.
+
+--empty::
+ Instead of reading tree object(s) into the index, just empty
+ it.
+
<tree-ish#>::
The id of the tree object(s) to be read/merged.
Merging
-------
-If '-m' is specified, "git-read-tree" can perform 3 kinds of
+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).
-
-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
+If only 1 tree is specified, 'git read-tree' operates as if the user did not
+specify `-m`, except that if the original index has an entry for a
+given pathname, and the contents of the path match 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-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
+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).
+fast-forward situation).
-When two trees are specified, the user is telling git-read-tree
+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;
+ 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.
+ 2. The user wants to fast-forward to $M.
-In this case, the "git-read-tree -m $H $M" command makes sure
+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:
+Here are the "carry forward" rules, where "I" denotes the index,
+"clean" means that index and work tree coincide, and "exists"/"nothing"
+refer to the presence of a path in the specified commit:
- I (index) H M Result
+ I H M Result
-------------------------------------------------------
- 0 nothing nothing nothing (does not happen)
- 1 nothing nothing exists use M
- 2 nothing exists nothing remove path from cache
- 3 nothing exists exists use M
+ 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 if "initial checkout",
+ H == M keep index otherwise
+ exists, fail
+ H != 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
+ 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
+ 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 cache
+ 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)
+ clean (H==M)
------
14 yes exists exists keep index
15 no 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 cache 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
+In all "keep index" cases, the index entry stays as in the
+original index file. If the entry is 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
+When this form of 'git read-tree' returns successfully, you can
+see which of the "local changes" that you made were carried forward by running
+`git diff-index --cached $M`. Note that this does not
+necessarily match what `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.
-
+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 the two-tree merge.
+
+Case 3 is slightly tricky and needs explanation. The result from this
+rule logically should be to remove the path if the user staged the removal
+of the path and then switching to a new branch. That however will prevent
+the initial checkout from happening, so the rule is modified to use M (new
+tree) only when the content of the index is empty. Otherwise the removal
+of the path is kept as long as $H and $M are the same.
3-Way Merge
~~~~~~~~~~~
Each "index" entry has two bits worth of "stage" state. stage 0 is the
normal one, and is the only one you'd see in any kind of normal use.
-However, when you do "git-read-tree" with three trees, the "stage"
+However, when you do 'git read-tree' with three trees, the "stage"
starts out at 1.
This means that you can do
- git-read-tree -m <tree1> <tree2> <tree3>
+----------------
+$ 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.
-Ok, this all sounds like a collection of totally nonsensical rules,
+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
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 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 "porcelain
- 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
+ stages 1/2/3 (i.e., "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,
matching "stage1" entry if it exists too. .. all the normal
trivial rules ..
-You would normally use "git-merge-index" with supplied
-"git-merge-one-file" to do this last step. The script
-does not touch the files in the work tree, and the entire merge
-happens in the index file. In other words, 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
file that does not match stage 2.
This is done to prevent you from losing your work-in-progress
-changes. To illustrate, suppose you start from what has been
-commited last to your repository:
+changes, and mixing your random changes in an unrelated merge
+commit. To illustrate, suppose you start from what has been
+committed last to your repository:
- $ JC=`cat .git/HEAD`
- $ git-checkout-index -f -u -a $JC
+----------------
+$ 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 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 rsync://.... linus
- $ LT=`cat .git/MERGE_HEAD`
+----------------
+$ git fetch git://.... linus
+$ LT=`git rev-parse 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 cache entries since $JC, and if you haven't,
+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 $LT
+----------------
+$ 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
+what you would commit is a pure merge between $JC and $LT without
your work-in-progress changes, and your work tree would be
updated to the result of the merge.
-
-See Also
+However, if you have local changes in the working tree that
+would be overwritten by this merge, 'git read-tree' will refuse
+to run to prevent your changes from being lost.
+
+In other words, there is no need to worry about what exists only
+in the working tree. When you have local changes in a part of
+the project that is not involved in the merge, your changes do
+not interfere with the merge, and are kept intact. When they
+*do* interfere, the merge does not even start ('git read-tree'
+complains loudly and fails without modifying anything). In such
+a case, you can simply continue doing what you were in the
+middle of doing, and when your working tree is ready (i.e. you
+have finished your work-in-progress), attempt the merge again.
+
+
+Sparse checkout
+---------------
+
+"Sparse checkout" allows populating the working directory sparsely.
+It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell
+Git whether a file in the working directory is worth looking at.
+
+'git read-tree' and other merge-based commands ('git merge', 'git
+checkout'...) can help maintaining the skip-worktree bitmap and working
+directory update. `$GIT_DIR/info/sparse-checkout` is used to
+define the skip-worktree reference bitmap. When 'git read-tree' needs
+to update the working directory, it resets the skip-worktree bit in the index
+based on this file, which uses the same syntax as .gitignore files.
+If an entry matches a pattern in this file, skip-worktree will not be
+set on that entry. Otherwise, skip-worktree will be set.
+
+Then it compares the new skip-worktree value with the previous one. If
+skip-worktree turns from set to unset, it will add the corresponding
+file back. If it turns from unset to set, that file will be removed.
+
+While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
+files are in, you can also specify what files are _not_ in, using
+negate patterns. For example, to remove the file `unwanted`:
+
+----------------
+/*
+!unwanted
+----------------
+
+Another tricky thing is fully repopulating the working directory when you
+no longer want sparse checkout. You cannot just disable "sparse
+checkout" because skip-worktree bits are still in the index and your working
+directory is still sparsely populated. You should re-populate the working
+directory with the `$GIT_DIR/info/sparse-checkout` file content as
+follows:
+
+----------------
+/*
+----------------
+
+Then you can disable sparse checkout. Sparse checkout support in 'git
+read-tree' and similar commands is disabled by default. You need to
+turn `core.sparseCheckout` on in order to have sparse checkout
+support.
+
+
+SEE ALSO
--------
-link:git-write-tree.html[git-write-tree]; link:git-ls-files.html[git-ls-files]
-
-
-Author
-------
-Written by Linus Torvalds <torvalds@osdl.org>
-
-Documentation
---------------
-Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
+linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
+linkgit:gitignore[5]
GIT
---
-Part of the link:git.html[git] suite
-
+Part of the linkgit:git[1] suite