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 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. 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'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

   Part of the git(1) suite





Opportunity


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.





Free Software


Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.


Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.





Free Books


The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.


Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.





Education


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.


Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.