# git-read-tree

## NAME



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


## SYNOPSIS



           _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



           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.
    
           Only trivial merges are done by _git read-tree_ itself. Only
           conflicting paths will be in an 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 a 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 or untracked files or directories 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.
    
           --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



           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**
           command.
    
           "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** :
    
               /*
               !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.
    


## GIT



           Part of the git(1) suite
    


* * *