GIT-READ-TREE(1)                 Git Manual                 GIT-READ-TREE(1)

NAME         top

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

SYNOPSIS         top

       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         top

       Reads the tree information given by <tree-ish> into the index, but
       does not actually update any of the files it "caches". (see:

       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         top

           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.

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

           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
           or the files in the working tree for real.

           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

           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.

           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.

           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 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.

           Using --recurse-submodules will update the content of all
           initialized submodules according to the commit recorded in the
           superproject by calling read-tree recursively, also setting the
           submodules HEAD to be detached at that commit.

           Disable sparse checkout support even if core.sparseCheckout is

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

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

MERGING         top

       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 or more 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

        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

       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
       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 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

       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":

       ·   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 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 ..

       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:

           $ 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 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.

SEE ALSO         top

       git-write-tree(1); git-ls-files(1); gitignore(5)

GIT         top

       Part of the git(1) suite

COLOPHON         top

       This page is part of the git (Git distributed version control system)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug report for this manual page,
       see ⟨⟩.  This page was obtained from the
       project's upstream Git repository ⟨⟩ on
       2018-10-29.  (At that time, the date of the most recent commit that
       was found in the repository was 2018-10-26.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

Git           10/28/2018                 GIT-READ-TREE(1)

Pages that refer to this page: git(1)git-config(1)git-diff(1)git-ls-files(1)git-worktree(1)gitrepository-layout(5)