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GIT-READ-TREE(1) | Git Manual | GIT-READ-TREE(1) |
NAME¶
git-read-tree - Reads tree information into the indexSYNOPSIS¶
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 index, but does not actually update any of the files it "caches". (see: git-checkout-index(1)) 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.OPTIONS¶
-mPerform 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 or 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:
--prefix=<prefix>/
•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.
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 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 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.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 $M command makes sure that no local
change is lost as the result of this "merge". Here are the
"carry forward" rules, where "I" denotes the index,
"clean" means that index and work tree coincide, and
"exists"/"nothing" refer to the presence of a path in the
specified commit:
I H M Result ------------------------------------------------------- 0 nothing nothing nothing (does not happen) 1 nothing nothing exists use M 2 nothing exists nothing remove path from index 3 nothing exists exists, use M if "initial checkout", H == M keep index otherwise exists, fail H != M
clean I==H I==M ------------------ 4 yes N/A N/A nothing nothing keep index 5 no N/A N/A nothing nothing keep index
6 yes N/A yes nothing exists keep index 7 no N/A yes nothing exists keep index 8 yes N/A no nothing exists fail 9 no N/A no nothing exists fail
10 yes yes N/A exists nothing remove path from index 11 no yes N/A exists nothing fail 12 yes no N/A exists nothing fail 13 no no N/A exists nothing fail
clean (H==M) ------ 14 yes exists exists keep index 15 no exists exists keep index
clean I==H I==M (H!=M) ------------------ 16 yes no no exists exists fail 17 no no no exists exists fail 18 yes no yes exists exists keep index 19 no no yes exists exists keep index 20 yes yes no exists exists use M 21 no yes no exists exists fail
3-Way Merge¶
Each "index" entry has two bits worth of "stage" state. stage 0 is the normal one, and is the only one you’d see in any kind of normal use. However, when you do git read-tree with three trees, the "stage" starts out at 1. This means that you can do$ git read-tree -m <tree1> <tree2> <tree3>
•stage 2 and 3 are the same; take one or the other
(it makes no 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 (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 (we did something while they did nothing)
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, 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).
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 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 policy" to determine how to remove the
non-0 stages, and insert a 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 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 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 committed last to your repository:
•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 ..
$ JC=`git rev-parse --verify "HEAD^0"` $ git checkout-index -f -u -a $JC
$ git fetch git://.... linus $ LT=`git rev-parse FETCH_HEAD`
$ 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
SPARSE CHECKOUT¶
"Sparse checkout" allows populating the working directory sparsely. It uses the skip-worktree bit (see 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
/*
SEE ALSO¶
git-write-tree(1); git-ls-files(1); gitignore(5)GIT¶
Part of the git(1) suite05/15/2017 | Git 2.11.0 |