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           git read-tree [[-m [--trivial] [--aggressive] | --reset |
           --prefix=<prefix>] [-u [--exclude-per-directory=<gitignore>] | -i]]
           [--index-output=<file>] [--no-sparse-checkout] <tree-ish1> [<tree-ish2>


           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-
           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.


               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 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.
               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
               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.
               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.
               Disable sparse checkout support even if core.sparseCheckout is
               The id of the tree object(s) to be read/merged.


           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'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.
            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.
           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:
                   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
               $ 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'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


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


           Written by Linus Torvalds <[1]>


           Documentation by David Greaves, Junio C Hamano and the git-list



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