Documentation/git-read-tree.txt

   1 git-read-tree(1)
   2 ================
   3
   4 NAME
   5 ----
   6 git-read-tree - Reads tree information into the index
   7
   8
   9 SYNOPSIS
  10 --------
  11 'git-read-tree' (<tree-ish> | [[-m [--aggressive]| --reset] [-u | -i]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
  12
  13
  14 DESCRIPTION
  15 -----------
  16 Reads the tree information given by <tree-ish> into the index,
  17 but does not actually *update* any of the files it "caches". (see:
  18 gitlink:git-checkout-index[1])
  19
  20 Optionally, it can merge a tree into the index, perform a
  21 fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
  22 flag.  When used with `-m`, the `-u` flag causes it to also update
  23 the files in the work tree with the result of the merge.
  24
  25 Trivial merges are done by `git-read-tree` itself.  Only conflicting paths
  26 will be in unmerged state when `git-read-tree` returns.
  27
  28 OPTIONS
  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 --aggressive::
  54         Usually a three-way merge by `git-read-tree` resolves
  55         the merge for really trivial cases and leaves other
  56         cases unresolved in the index, so that Porcelains can
  57         implement different merge policies.  This flag makes the
  58         command to resolve a few more cases internally:
  59 +
  60 * when one side removes a path and the other side leaves the path
  61   unmodified.  The resolution is to remove that path.
  62 * when both sides remove a path.  The resolution is to remove that path.
  63 * when both sides adds a path identically.  The resolution
  64   is to add that path.
  65
  66 <tree-ish#>::
  67         The id of the tree object(s) to be read/merged.
  68
  69
  70 Merging
  71 -------
  72 If `-m` is specified, `git-read-tree` can perform 3 kinds of
  73 merge, a single tree merge if only 1 tree is given, a
  74 fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
  75 provided.
  76
  77
  78 Single Tree Merge
  79 ~~~~~~~~~~~~~~~~~
  80 If only 1 tree is specified, git-read-tree operates as if the user did not
  81 specify `-m`, except that if the original index has an entry for a
  82 given pathname, and the contents of the path matches with the tree
  83 being read, the stat info from the index is used. (In other words, the
  84 index's stat()s take precedence over the merged tree's).
  85
  86 That means that if you do a `git-read-tree -m <newtree>` followed by a
  87 `git-checkout-index -f -u -a`, the `git-checkout-index` only checks out
  88 the stuff that really changed.
  89
  90 This is used to avoid unnecessary false hits when `git-diff-files` is
  91 run after `git-read-tree`.
  92
  93
  94 Two Tree Merge
  95 ~~~~~~~~~~~~~~
  96
  97 Typically, this is invoked as `git-read-tree -m $H $M`, where $H
  98 is the head commit of the current repository, and $M is the head
  99 of a foreign tree, which is simply ahead of $H (i.e. we are in a
 100 fast forward situation).
 101
 102 When two trees are specified, the user is telling git-read-tree
 103 the following:
 104
 105      1. The current index and work tree is derived from $H, but
 106         the user may have local changes in them since $H;
 107
 108      2. The user wants to fast-forward to $M.
 109
 110 In this case, the `git-read-tree -m $H $M` command makes sure
 111 that no local change is lost as the result of this "merge".
 112 Here are the "carry forward" rules:
 113
 114         I (index)           H        M        Result
 115        -------------------------------------------------------
 116       0 nothing             nothing  nothing  (does not happen)
 117       1 nothing             nothing  exists   use M
 118       2 nothing             exists   nothing  remove path from index
 119       3 nothing             exists   exists   use M
 120
 121         clean I==H  I==M
 122        ------------------
 123       4 yes   N/A   N/A     nothing  nothing  keep index
 124       5 no    N/A   N/A     nothing  nothing  keep index
 125
 126       6 yes   N/A   yes     nothing  exists   keep index
 127       7 no    N/A   yes     nothing  exists   keep index
 128       8 yes   N/A   no      nothing  exists   fail
 129       9 no    N/A   no      nothing  exists   fail
 130
 131      10 yes   yes   N/A     exists   nothing  remove path from index
 132      11 no    yes   N/A     exists   nothing  fail
 133      12 yes   no    N/A     exists   nothing  fail
 134      13 no    no    N/A     exists   nothing  fail
 135
 136         clean (H=M)
 137        ------
 138      14 yes                 exists   exists   keep index
 139      15 no                  exists   exists   keep index
 140
 141         clean I==H  I==M (H!=M)
 142        ------------------
 143      16 yes   no    no      exists   exists   fail
 144      17 no    no    no      exists   exists   fail
 145      18 yes   no    yes     exists   exists   keep index
 146      19 no    no    yes     exists   exists   keep index
 147      20 yes   yes   no      exists   exists   use M
 148      21 no    yes   no      exists   exists   fail
 149
 150 In all "keep index" cases, the index entry stays as in the
 151 original index file.  If the entry were not up to date,
 152 git-read-tree keeps the copy in the work tree intact when
 153 operating under the -u flag.
 154
 155 When this form of git-read-tree returns successfully, you can
 156 see what "local changes" you made are carried forward by running
 157 `git-diff-index --cached $M`.  Note that this does not
 158 necessarily match `git-diff-index --cached $H` would have
 159 produced before such a two tree merge.  This is because of cases
 160 18 and 19 --- if you already had the changes in $M (e.g. maybe
 161 you picked it up via e-mail in a patch form), `git-diff-index
 162 --cached $H` would have told you about the change before this
 163 merge, but it would not show in `git-diff-index --cached $M`
 164 output after two-tree merge.
 165
 166
 167 3-Way Merge
 168 ~~~~~~~~~~~
 169 Each "index" entry has two bits worth of "stage" state. stage 0 is the
 170 normal one, and is the only one you'd see in any kind of normal use.
 171
 172 However, when you do `git-read-tree` with three trees, the "stage"
 173 starts out at 1.
 174
 175 This means that you can do
 176
 177 ----------------
 178 $ git-read-tree -m <tree1> <tree2> <tree3>
 179 ----------------
 180
 181 and you will end up with an index with all of the <tree1> entries in
 182 "stage1", all of the <tree2> entries in "stage2" and all of the
 183 <tree3> entries in "stage3".  When performing a merge of another
 184 branch into the current branch, we use the common ancestor tree
 185 as <tree1>, the current branch head as <tree2>, and the other
 186 branch head as <tree3>.
 187
 188 Furthermore, `git-read-tree` has special-case logic that says: if you see
 189 a file that matches in all respects in the following states, it
 190 "collapses" back to "stage0":
 191
 192    - stage 2 and 3 are the same; take one or the other (it makes no
 193      difference - the same work has been done on our branch in
 194      stage 2 and their branch in stage 3)
 195
 196    - stage 1 and stage 2 are the same and stage 3 is different; take
 197      stage 3 (our branch in stage 2 did not do anything since the
 198      ancestor in stage 1 while their branch in stage 3 worked on
 199      it)
 200
 201    - stage 1 and stage 3 are the same and stage 2 is different take
 202      stage 2 (we did something while they did nothing)
 203
 204 The `git-write-tree` command refuses to write a nonsensical tree, and it
 205 will complain about unmerged entries if it sees a single entry that is not
 206 stage 0.
 207
 208 Ok, this all sounds like a collection of totally nonsensical rules,
 209 but it's actually exactly what you want in order to do a fast
 210 merge. The different stages represent the "result tree" (stage 0, aka
 211 "merged"), the original tree (stage 1, aka "orig"), and the two trees
 212 you are trying to merge (stage 2 and 3 respectively).
 213
 214 The order of stages 1, 2 and 3 (hence the order of three
 215 <tree-ish> command line arguments) are significant when you
 216 start a 3-way merge with an index file that is already
 217 populated.  Here is an outline of how the algorithm works:
 218
 219 - if a file exists in identical format in all three trees, it will
 220   automatically collapse to "merged" state by git-read-tree.
 221
 222 - a file that has _any_ difference what-so-ever in the three trees
 223   will stay as separate entries in the index. It's up to "porcelain
 224   policy" to determine how to remove the non-0 stages, and insert a
 225   merged version.
 226
 227 - the index file saves and restores with all this information, so you
 228   can merge things incrementally, but as long as it has entries in
 229   stages 1/2/3 (ie "unmerged entries") you can't write the result. So
 230   now the merge algorithm ends up being really simple:
 231
 232   * you walk the index in order, and ignore all entries of stage 0,
 233     since they've already been done.
 234
 235   * if you find a "stage1", but no matching "stage2" or "stage3", you
 236     know it's been removed from both trees (it only existed in the
 237     original tree), and you remove that entry.
 238
 239   * if you find a matching "stage2" and "stage3" tree, you remove one
 240     of them, and turn the other into a "stage0" entry. Remove any
 241     matching "stage1" entry if it exists too.  .. all the normal
 242     trivial rules ..
 243
 244 You would normally use `git-merge-index` with supplied
 245 `git-merge-one-file` to do this last step.  The script updates
 246 the files in the working tree as it merges each path and at the
 247 end of a successful merge.
 248
 249 When you start a 3-way merge with an index file that is already
 250 populated, it is assumed that it represents the state of the
 251 files in your work tree, and you can even have files with
 252 changes unrecorded in the index file.  It is further assumed
 253 that this state is "derived" from the stage 2 tree.  The 3-way
 254 merge refuses to run if it finds an entry in the original index
 255 file that does not match stage 2.
 256
 257 This is done to prevent you from losing your work-in-progress
 258 changes, and mixing your random changes in an unrelated merge
 259 commit.  To illustrate, suppose you start from what has been
 260 commited last to your repository:
 261
 262 ----------------
 263 $ JC=`git-rev-parse --verify "HEAD^0"`
 264 $ git-checkout-index -f -u -a $JC
 265 ----------------
 266
 267 You do random edits, without running git-update-index.  And then
 268 you notice that the tip of your "upstream" tree has advanced
 269 since you pulled from him:
 270
 271 ----------------
 272 $ git-fetch git://.... linus
 273 $ LT=`cat .git/FETCH_HEAD`
 274 ----------------
 275
 276 Your work tree is still based on your HEAD ($JC), but you have
 277 some edits since.  Three-way merge makes sure that you have not
 278 added or modified index entries since $JC, and if you haven't,
 279 then does the right thing.  So with the following sequence:
 280
 281 ----------------
 282 $ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT
 283 $ git-merge-index git-merge-one-file -a
 284 $ echo "Merge with Linus" | \
 285   git-commit-tree `git-write-tree` -p $JC -p $LT
 286 ----------------
 287
 288 what you would commit is a pure merge between $JC and $LT without
 289 your work-in-progress changes, and your work tree would be
 290 updated to the result of the merge.
 291
 292 However, if you have local changes in the working tree that
 293 would be overwritten by this merge,`git-read-tree` will refuse
 294 to run to prevent your changes from being lost.
 295
 296 In other words, there is no need to worry about what exists only
 297 in the working tree.  When you have local changes in a part of
 298 the project that is not involved in the merge, your changes do
 299 not interfere with the merge, and are kept intact.  When they
 300 *do* interfere, the merge does not even start (`git-read-tree`
 301 complains loudly and fails without modifying anything).  In such
 302 a case, you can simply continue doing what you were in the
 303 middle of doing, and when your working tree is ready (i.e. you
 304 have finished your work-in-progress), attempt the merge again.
 305
 306
 307 See Also
 308 --------
 309 gitlink:git-write-tree[1]; gitlink:git-ls-files[1]
 310
 311
 312 Author
 313 ------
 314 Written by Linus Torvalds <torvalds@osdl.org>
 315
 316 Documentation
 317 --------------
 318 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
 319
 320 GIT
 321 ---
 322 Part of the gitlink:git[7] suite
 323