git-read-tree

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



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

SYNOPSIS
       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
       -m
           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. When used with -u, updates leading to loss of working tree
           changes will not abort the operation.

       -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:

           •   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.

       --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-]recurse-submodules
           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.

       --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.

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

       <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 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 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
       "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


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

GIT
       Part of the git(1) suite



Git 2.30.0                         12/28/2020                   GIT-READ-TREE(1)