Documentation / git-read-tree.txton commit Merge branch 'maint' (bf31115)
   1git-read-tree(1)
   2================
   3
   4NAME
   5----
   6git-read-tree - Reads tree information into the index
   7
   8
   9SYNOPSIS
  10--------
  11'git read-tree' (<tree-ish> | [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] [-u | -i]] [--exclude-per-directory=<gitignore>] [--index-output=<file>] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
  12
  13
  14DESCRIPTION
  15-----------
  16Reads the tree information given by <tree-ish> into the index,
  17but does not actually *update* any of the files it "caches". (see:
  18linkgit:git-checkout-index[1])
  19
  20Optionally, it can merge a tree into the index, perform a
  21fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
  22flag.  When used with `-m`, the `-u` flag causes it to also update
  23the files in the work tree with the result of the merge.
  24
  25Trivial merges are done by 'git-read-tree' itself.  Only conflicting paths
  26will be in unmerged state when 'git-read-tree' returns.
  27
  28OPTIONS
  29-------
  30-m::
  31        Perform a merge, not just a read.  The command will
  32        refuse to run if your index file has unmerged entries,
  33        indicating that you have not finished previous merge you
  34        started.
  35
  36--reset::
  37        Same as -m, except that unmerged entries are discarded
  38        instead of failing.
  39
  40-u::
  41        After a successful merge, update the files in the work
  42        tree with the result of the merge.
  43
  44-i::
  45        Usually a merge requires the index file as well as the
  46        files in the working tree are up to date with the
  47        current head commit, in order not to lose local
  48        changes.  This flag disables the check with the working
  49        tree and is meant to be used when creating a merge of
  50        trees that are not directly related to the current
  51        working tree status into a temporary index file.
  52
  53-v::
  54        Show the progress of checking files out.
  55
  56--trivial::
  57        Restrict three-way merge by 'git-read-tree' to happen
  58        only if there is no file-level merging required, instead
  59        of resolving merge for trivial cases and leaving
  60        conflicting files unresolved in the index.
  61
  62--aggressive::
  63        Usually a three-way merge by 'git-read-tree' resolves
  64        the merge for really trivial cases and leaves other
  65        cases unresolved in the index, so that Porcelains can
  66        implement different merge policies.  This flag makes the
  67        command to resolve a few more cases internally:
  68+
  69* when one side removes a path and the other side leaves the path
  70  unmodified.  The resolution is to remove that path.
  71* when both sides remove a path.  The resolution is to remove that path.
  72* when both sides adds a path identically.  The resolution
  73  is to add that path.
  74
  75--prefix=<prefix>/::
  76        Keep the current index contents, and read the contents
  77        of named tree-ish under directory at `<prefix>`.  The
  78        original index file cannot have anything at the path
  79        `<prefix>` itself, and have nothing in `<prefix>/`
  80        directory.  Note that the `<prefix>/` value must end
  81        with a slash.
  82
  83--exclude-per-directory=<gitignore>::
  84        When running the command with `-u` and `-m` options, the
  85        merge result may need to overwrite paths that are not
  86        tracked in the current branch.  The command usually
  87        refuses to proceed with the merge to avoid losing such a
  88        path.  However this safety valve sometimes gets in the
  89        way.  For example, it often happens that the other
  90        branch added a file that used to be a generated file in
  91        your branch, and the safety valve triggers when you try
  92        to switch to that branch after you ran `make` but before
  93        running `make clean` to remove the generated file.  This
  94        option tells the command to read per-directory exclude
  95        file (usually '.gitignore') and allows such an untracked
  96        but explicitly ignored file to be overwritten.
  97
  98--index-output=<file>::
  99        Instead of writing the results out to `$GIT_INDEX_FILE`,
 100        write the resulting index in the named file.  While the
 101        command is operating, the original index file is locked
 102        with the same mechanism as usual.  The file must allow
 103        to be rename(2)ed into from a temporary file that is
 104        created next to the usual index file; typically this
 105        means it needs to be on the same filesystem as the index
 106        file itself, and you need write permission to the
 107        directories the index file and index output file are
 108        located in.
 109
 110<tree-ish#>::
 111        The id of the tree object(s) to be read/merged.
 112
 113
 114Merging
 115-------
 116If `-m` is specified, 'git-read-tree' can perform 3 kinds of
 117merge, a single tree merge if only 1 tree is given, a
 118fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
 119provided.
 120
 121
 122Single Tree Merge
 123~~~~~~~~~~~~~~~~~
 124If only 1 tree is specified, 'git-read-tree' operates as if the user did not
 125specify `-m`, except that if the original index has an entry for a
 126given pathname, and the contents of the path matches with the tree
 127being read, the stat info from the index is used. (In other words, the
 128index's stat()s take precedence over the merged tree's).
 129
 130That means that if you do a `git read-tree -m <newtree>` followed by a
 131`git checkout-index -f -u -a`, the 'git-checkout-index' only checks out
 132the stuff that really changed.
 133
 134This is used to avoid unnecessary false hits when 'git-diff-files' is
 135run after 'git-read-tree'.
 136
 137
 138Two Tree Merge
 139~~~~~~~~~~~~~~
 140
 141Typically, this is invoked as `git read-tree -m $H $M`, where $H
 142is the head commit of the current repository, and $M is the head
 143of a foreign tree, which is simply ahead of $H (i.e. we are in a
 144fast forward situation).
 145
 146When two trees are specified, the user is telling 'git-read-tree'
 147the following:
 148
 149     1. The current index and work tree is derived from $H, but
 150        the user may have local changes in them since $H;
 151
 152     2. The user wants to fast-forward to $M.
 153
 154In this case, the `git read-tree -m $H $M` command makes sure
 155that no local change is lost as the result of this "merge".
 156Here are the "carry forward" rules:
 157
 158        I (index)           H        M        Result
 159       -------------------------------------------------------
 160      0 nothing             nothing  nothing  (does not happen)
 161      1 nothing             nothing  exists   use M
 162      2 nothing             exists   nothing  remove path from index
 163      3 nothing             exists   exists,  use M if "initial checkout"
 164                                     H == M   keep index otherwise
 165                                     exists   fail
 166                                     H != M
 167
 168        clean I==H  I==M
 169       ------------------
 170      4 yes   N/A   N/A     nothing  nothing  keep index
 171      5 no    N/A   N/A     nothing  nothing  keep index
 172
 173      6 yes   N/A   yes     nothing  exists   keep index
 174      7 no    N/A   yes     nothing  exists   keep index
 175      8 yes   N/A   no      nothing  exists   fail
 176      9 no    N/A   no      nothing  exists   fail
 177
 178     10 yes   yes   N/A     exists   nothing  remove path from index
 179     11 no    yes   N/A     exists   nothing  fail
 180     12 yes   no    N/A     exists   nothing  fail
 181     13 no    no    N/A     exists   nothing  fail
 182
 183        clean (H=M)
 184       ------
 185     14 yes                 exists   exists   keep index
 186     15 no                  exists   exists   keep index
 187
 188        clean I==H  I==M (H!=M)
 189       ------------------
 190     16 yes   no    no      exists   exists   fail
 191     17 no    no    no      exists   exists   fail
 192     18 yes   no    yes     exists   exists   keep index
 193     19 no    no    yes     exists   exists   keep index
 194     20 yes   yes   no      exists   exists   use M
 195     21 no    yes   no      exists   exists   fail
 196
 197In all "keep index" cases, the index entry stays as in the
 198original index file.  If the entry were not up to date,
 199'git-read-tree' keeps the copy in the work tree intact when
 200operating under the -u flag.
 201
 202When this form of 'git-read-tree' returns successfully, you can
 203see what "local changes" you made are carried forward by running
 204`git diff-index --cached $M`.  Note that this does not
 205necessarily match `git diff-index --cached $H` would have
 206produced before such a two tree merge.  This is because of cases
 20718 and 19 --- if you already had the changes in $M (e.g. maybe
 208you picked it up via e-mail in a patch form), `git diff-index
 209--cached $H` would have told you about the change before this
 210merge, but it would not show in `git diff-index --cached $M`
 211output after two-tree merge.
 212
 213Case #3 is slightly tricky and needs explanation.  The result from this
 214rule logically should be to remove the path if the user staged the removal
 215of the path and then switching to a new branch.  That however will prevent
 216the initial checkout from happening, so the rule is modified to use M (new
 217tree) only when the contents of the index is empty.  Otherwise the removal
 218of the path is kept as long as $H and $M are the same.
 219
 2203-Way Merge
 221~~~~~~~~~~~
 222Each "index" entry has two bits worth of "stage" state. stage 0 is the
 223normal one, and is the only one you'd see in any kind of normal use.
 224
 225However, when you do 'git-read-tree' with three trees, the "stage"
 226starts out at 1.
 227
 228This means that you can do
 229
 230----------------
 231$ git read-tree -m <tree1> <tree2> <tree3>
 232----------------
 233
 234and you will end up with an index with all of the <tree1> entries in
 235"stage1", all of the <tree2> entries in "stage2" and all of the
 236<tree3> entries in "stage3".  When performing a merge of another
 237branch into the current branch, we use the common ancestor tree
 238as <tree1>, the current branch head as <tree2>, and the other
 239branch head as <tree3>.
 240
 241Furthermore, 'git-read-tree' has special-case logic that says: if you see
 242a file that matches in all respects in the following states, it
 243"collapses" back to "stage0":
 244
 245   - stage 2 and 3 are the same; take one or the other (it makes no
 246     difference - the same work has been done on our branch in
 247     stage 2 and their branch in stage 3)
 248
 249   - stage 1 and stage 2 are the same and stage 3 is different; take
 250     stage 3 (our branch in stage 2 did not do anything since the
 251     ancestor in stage 1 while their branch in stage 3 worked on
 252     it)
 253
 254   - stage 1 and stage 3 are the same and stage 2 is different take
 255     stage 2 (we did something while they did nothing)
 256
 257The 'git-write-tree' command refuses to write a nonsensical tree, and it
 258will complain about unmerged entries if it sees a single entry that is not
 259stage 0.
 260
 261OK, this all sounds like a collection of totally nonsensical rules,
 262but it's actually exactly what you want in order to do a fast
 263merge. The different stages represent the "result tree" (stage 0, aka
 264"merged"), the original tree (stage 1, aka "orig"), and the two trees
 265you are trying to merge (stage 2 and 3 respectively).
 266
 267The order of stages 1, 2 and 3 (hence the order of three
 268<tree-ish> command line arguments) are significant when you
 269start a 3-way merge with an index file that is already
 270populated.  Here is an outline of how the algorithm works:
 271
 272- if a file exists in identical format in all three trees, it will
 273  automatically collapse to "merged" state by 'git-read-tree'.
 274
 275- a file that has _any_ difference what-so-ever in the three trees
 276  will stay as separate entries in the index. It's up to "porcelain
 277  policy" to determine how to remove the non-0 stages, and insert a
 278  merged version.
 279
 280- the index file saves and restores with all this information, so you
 281  can merge things incrementally, but as long as it has entries in
 282  stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
 283  now the merge algorithm ends up being really simple:
 284
 285  * you walk the index in order, and ignore all entries of stage 0,
 286    since they've already been done.
 287
 288  * if you find a "stage1", but no matching "stage2" or "stage3", you
 289    know it's been removed from both trees (it only existed in the
 290    original tree), and you remove that entry.
 291
 292  * if you find a matching "stage2" and "stage3" tree, you remove one
 293    of them, and turn the other into a "stage0" entry. Remove any
 294    matching "stage1" entry if it exists too.  .. all the normal
 295    trivial rules ..
 296
 297You would normally use 'git-merge-index' with supplied
 298'git-merge-one-file' to do this last step.  The script updates
 299the files in the working tree as it merges each path and at the
 300end of a successful merge.
 301
 302When you start a 3-way merge with an index file that is already
 303populated, it is assumed that it represents the state of the
 304files in your work tree, and you can even have files with
 305changes unrecorded in the index file.  It is further assumed
 306that this state is "derived" from the stage 2 tree.  The 3-way
 307merge refuses to run if it finds an entry in the original index
 308file that does not match stage 2.
 309
 310This is done to prevent you from losing your work-in-progress
 311changes, and mixing your random changes in an unrelated merge
 312commit.  To illustrate, suppose you start from what has been
 313committed last to your repository:
 314
 315----------------
 316$ JC=`git rev-parse --verify "HEAD^0"`
 317$ git checkout-index -f -u -a $JC
 318----------------
 319
 320You do random edits, without running 'git-update-index'.  And then
 321you notice that the tip of your "upstream" tree has advanced
 322since you pulled from him:
 323
 324----------------
 325$ git fetch git://.... linus
 326$ LT=`cat .git/FETCH_HEAD`
 327----------------
 328
 329Your work tree is still based on your HEAD ($JC), but you have
 330some edits since.  Three-way merge makes sure that you have not
 331added or modified index entries since $JC, and if you haven't,
 332then does the right thing.  So with the following sequence:
 333
 334----------------
 335$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
 336$ git merge-index git-merge-one-file -a
 337$ echo "Merge with Linus" | \
 338  git commit-tree `git write-tree` -p $JC -p $LT
 339----------------
 340
 341what you would commit is a pure merge between $JC and $LT without
 342your work-in-progress changes, and your work tree would be
 343updated to the result of the merge.
 344
 345However, if you have local changes in the working tree that
 346would be overwritten by this merge, 'git-read-tree' will refuse
 347to run to prevent your changes from being lost.
 348
 349In other words, there is no need to worry about what exists only
 350in the working tree.  When you have local changes in a part of
 351the project that is not involved in the merge, your changes do
 352not interfere with the merge, and are kept intact.  When they
 353*do* interfere, the merge does not even start ('git-read-tree'
 354complains loudly and fails without modifying anything).  In such
 355a case, you can simply continue doing what you were in the
 356middle of doing, and when your working tree is ready (i.e. you
 357have finished your work-in-progress), attempt the merge again.
 358
 359
 360SEE ALSO
 361--------
 362linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
 363linkgit:gitignore[5]
 364
 365
 366Author
 367------
 368Written by Linus Torvalds <torvalds@osdl.org>
 369
 370Documentation
 371--------------
 372Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
 373
 374GIT
 375---
 376Part of the linkgit:git[1] suite