GIT-READ-TREE(1)                                              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.


       -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 are 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.


       -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 to 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 adds a path identically. The resolution is to
                 add that path.

       --prefix=<prefix>/
              Keep the current index contents, and read the contents of named
              tree-ish under directory at <prefix>. The original index file
              cannot have anything at the path <prefix> itself, and have
              nothing in <prefix>/ directory. Note that the <prefix>/ value
              must end with a slash.


       --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-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.


       <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 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&#8217;s stat()s take precedence over the merged tree&#8217;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&#8217;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 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 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&#8217;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 read-tree.

       o  a file that has any difference what-so-ever in the three trees will
          stay as separate entries in the index. It&#8217;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&#8217;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&#8217;ve already been done.

          o  if you find a "stage1", but no matching "stage2" or "stage3", you
             know it&#8217;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 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=`cat .git/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&#8217;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 to sparsely populate working directory. It
       uses skip-worktree bit (see git-update-index(1)) to tell Git whether a
       file on working directory is worth looking at.


       "git read-tree" and other merge-based commands ("git merge", "git
       checkout"...) can help maintaining 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
       working directory, it will reset skip-worktree bit in 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 be set on that
       entry. Otherwise, skip-worktree will be unset.


       Then it compares the new skip-worktree value with the previous one. If
       skip-worktree turns from unset to set, it will add the corresponding
       file back. If it turns from set to unset, 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 file "unwanted":

       *
       !unwanted





       Another tricky thing is fully repopulating working directory when you
       no longer want sparse checkout. You cannot just disable "sparse
       checkout" because skip-worktree are still in the index and you working
       directory is still sparsely populated. You should re-populate 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
       Part of the git(1) suite




                                                              GIT-READ-TREE(1)
