git-read-tree(1) — Linux manual page


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 | -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.

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

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 a previous merge you started.

           Same as -m, except that unmerged entries are discarded
           instead of failing. When used with -u, updates leading to
           loss of working tree changes or untracked files or
           directories will not abort the operation.

           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 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:

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

           •   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.

           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
           active 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 true.

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

       -q, --quiet
           Quiet, suppress feedback messages.

           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 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

       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 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":

       •   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

       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

       •   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

           •   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.


       Note: The skip-worktree capabilities in git-update-index(1) and
       read-tree predated the introduction of git-sparse-checkout(1).
       Users are encouraged to use the sparse-checkout command in
       preference to these plumbing commands for
       sparse-checkout/skip-worktree related needs. However, the
       information below might be useful to users trying to understand
       the pattern style used in non-cone mode of the 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, or the entry corresponds to a file present in the
       working tree, then 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 2023-12-22.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2023-12-20.)  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         2023-12-20               GIT-READ-TREE(1)

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