man pages section 1: User Commands

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Updated: July 2014

git-read-tree (1)


git-read-tree - Reads tree information into the index


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>]])


Git Manual                                       GIT-READ-TREE(1)

     git-read-tree - Reads tree information into the index

     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>]])

     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

     Trivial merges are done by git read-tree itself. Only
     conflicting paths will be in unmerged state when git
     read-tree returns.

         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

         Same as -m, except that unmerged entries are discarded
         instead of failing.

         After a successful merge, update the files in the work
         tree with the result of the merge.

         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.

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         Show the progress of checking files out.

         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.

         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:

         o   when one side removes a path and the other side
             leaves the path unmodified. The resolution is to
             remove that path.

         o   when both sides remove a path. The resolution is to
             remove that path.

         o   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.

         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.

         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

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         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.

         Disable sparse checkout support even if
         core.sparseCheckout is true.

         Instead of reading tree object(s) into the index, just
         empty it.

         The id of the tree object(s) to be read/merged.

     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

     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.

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      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

     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.

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     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
     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

  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>

     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". 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 a file that matches in all respects in the
     following states, it "collapses" back to "stage0":

     o   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)

     o   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

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         branch in stage 3 worked on it)

     o   stage 1 and stage 3 are the same and stage 2 is
         different take stage 2 (we did something while they did

     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:

     o   if a file exists in identical format in all three trees,
         it will automatically collapse to "merged" state by git

     o   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.

     o   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:

         o   you walk the index in order, and ignore all entries
             of stage 0, since they've already been done.

         o   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.

         o   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 ..

     You would normally use git merge-index with supplied git
     merge-one-file to do this last step. The script updates the

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     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:

         $ 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=`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 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

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     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" 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:


     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.

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     See attributes(5) for descriptions of the following

     |Availability   | developer/versioning/git |
     |Stability      | Uncommitted              |
     git-write-tree(1); git-ls-files(1); gitignore(4)

     Part of the git(1) suite

     This software was built from source available at  The original
     community source was downloaded from  http://git-

     Further information about this software can be found on the
     open source community website at

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