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' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
  12                 [-u [--exclude-per-directory=<gitignore>] | -i]]
  13                 [--index-output=<file>] [--no-sparse-checkout]
  14                 <tree-ish1> [<tree-ish2> [<tree-ish3>]]
  15
  16
  17 DESCRIPTION
  18 -----------
  19 Reads the tree information given by <tree-ish> into the index,
  20 but does not actually *update* any of the files it "caches". (see:
  21 linkgit:git-checkout-index[1])
  22
  23 Optionally, it can merge a tree into the index, perform a
  24 fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
  25 flag.  When used with `-m`, the `-u` flag causes it to also update
  26 the files in the work tree with the result of the merge.
  27
  28 Trivial merges are done by 'git read-tree' itself.  Only conflicting paths
  29 will be in unmerged state when 'git read-tree' returns.
  30
  31 OPTIONS
  32 -------
  33 -m::
  34         Perform a merge, not just a read.  The command will
  35         refuse to run if your index file has unmerged entries,
  36         indicating that you have not finished previous merge you
  37         started.
  38
  39 --reset::
  40         Same as -m, except that unmerged entries are discarded
  41         instead of failing.
  42
  43 -u::
  44         After a successful merge, update the files in the work
  45         tree with the result of the merge.
  46
  47 -i::
  48         Usually a merge requires the index file as well as the
  49         files in the working tree are up to date with the
  50         current head commit, in order not to lose local
  51         changes.  This flag disables the check with the working
  52         tree and is meant to be used when creating a merge of
  53         trees that are not directly related to the current
  54         working tree status into a temporary index file.
  55
  56 -v::
  57         Show the progress of checking files out.
  58
  59 --trivial::
  60         Restrict three-way merge by 'git read-tree' to happen
  61         only if there is no file-level merging required, instead
  62         of resolving merge for trivial cases and leaving
  63         conflicting files unresolved in the index.
  64
  65 --aggressive::
  66         Usually a three-way merge by 'git read-tree' resolves
  67         the merge for really trivial cases and leaves other
  68         cases unresolved in the index, so that Porcelains can
  69         implement different merge policies.  This flag makes the
  70         command to resolve a few more cases internally:
  71 +
  72 * when one side removes a path and the other side leaves the path
  73   unmodified.  The resolution is to remove that path.
  74 * when both sides remove a path.  The resolution is to remove that path.
  75 * when both sides adds a path identically.  The resolution
  76   is to add that path.
  77
  78 --prefix=<prefix>/::
  79         Keep the current index contents, and read the contents
  80         of named tree-ish under directory at `<prefix>`.  The
  81         original index file cannot have anything at the path
  82         `<prefix>` itself, and have nothing in `<prefix>/`
  83         directory.  Note that the `<prefix>/` value must end
  84         with a slash.
  85
  86 --exclude-per-directory=<gitignore>::
  87         When running the command with `-u` and `-m` options, the
  88         merge result may need to overwrite paths that are not
  89         tracked in the current branch.  The command usually
  90         refuses to proceed with the merge to avoid losing such a
  91         path.  However this safety valve sometimes gets in the
  92         way.  For example, it often happens that the other
  93         branch added a file that used to be a generated file in
  94         your branch, and the safety valve triggers when you try
  95         to switch to that branch after you ran `make` but before
  96         running `make clean` to remove the generated file.  This
  97         option tells the command to read per-directory exclude
  98         file (usually '.gitignore') and allows such an untracked
  99         but explicitly ignored file to be overwritten.
 100
 101 --index-output=<file>::
 102         Instead of writing the results out to `$GIT_INDEX_FILE`,
 103         write the resulting index in the named file.  While the
 104         command is operating, the original index file is locked
 105         with the same mechanism as usual.  The file must allow
 106         to be rename(2)ed into from a temporary file that is
 107         created next to the usual index file; typically this
 108         means it needs to be on the same filesystem as the index
 109         file itself, and you need write permission to the
 110         directories the index file and index output file are
 111         located in.
 112
 113 --no-sparse-checkout::
 114         Disable sparse checkout support even if `core.sparseCheckout`
 115         is true.
 116
 117 <tree-ish#>::
 118         The id of the tree object(s) to be read/merged.
 119
 120
 121 Merging
 122 -------
 123 If `-m` is specified, 'git read-tree' can perform 3 kinds of
 124 merge, a single tree merge if only 1 tree is given, a
 125 fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
 126 provided.
 127
 128
 129 Single Tree Merge
 130 ~~~~~~~~~~~~~~~~~
 131 If only 1 tree is specified, 'git read-tree' operates as if the user did not
 132 specify `-m`, except that if the original index has an entry for a
 133 given pathname, and the contents of the path matches with the tree
 134 being read, the stat info from the index is used. (In other words, the
 135 index's stat()s take precedence over the merged tree's).
 136
 137 That means that if you do a `git read-tree -m <newtree>` followed by a
 138 `git checkout-index -f -u -a`, the 'git checkout-index' only checks out
 139 the stuff that really changed.
 140
 141 This is used to avoid unnecessary false hits when 'git diff-files' is
 142 run after 'git read-tree'.
 143
 144
 145 Two Tree Merge
 146 ~~~~~~~~~~~~~~
 147
 148 Typically, this is invoked as `git read-tree -m $H $M`, where $H
 149 is the head commit of the current repository, and $M is the head
 150 of a foreign tree, which is simply ahead of $H (i.e. we are in a
 151 fast-forward situation).
 152
 153 When two trees are specified, the user is telling 'git read-tree'
 154 the following:
 155
 156      1. The current index and work tree is derived from $H, but
 157         the user may have local changes in them since $H;
 158
 159      2. The user wants to fast-forward to $M.
 160
 161 In this case, the `git read-tree -m $H $M` command makes sure
 162 that no local change is lost as the result of this "merge".
 163 Here are the "carry forward" rules:
 164
 165         I (index)           H        M        Result
 166        -------------------------------------------------------
 167       0 nothing             nothing  nothing  (does not happen)
 168       1 nothing             nothing  exists   use M
 169       2 nothing             exists   nothing  remove path from index
 170       3 nothing             exists   exists,  use M if "initial checkout"
 171                                      H == M   keep index otherwise
 172                                      exists   fail
 173                                      H != M
 174
 175         clean I==H  I==M
 176        ------------------
 177       4 yes   N/A   N/A     nothing  nothing  keep index
 178       5 no    N/A   N/A     nothing  nothing  keep index
 179
 180       6 yes   N/A   yes     nothing  exists   keep index
 181       7 no    N/A   yes     nothing  exists   keep index
 182       8 yes   N/A   no      nothing  exists   fail
 183       9 no    N/A   no      nothing  exists   fail
 184
 185      10 yes   yes   N/A     exists   nothing  remove path from index
 186      11 no    yes   N/A     exists   nothing  fail
 187      12 yes   no    N/A     exists   nothing  fail
 188      13 no    no    N/A     exists   nothing  fail
 189
 190         clean (H=M)
 191        ------
 192      14 yes                 exists   exists   keep index
 193      15 no                  exists   exists   keep index
 194
 195         clean I==H  I==M (H!=M)
 196        ------------------
 197      16 yes   no    no      exists   exists   fail
 198      17 no    no    no      exists   exists   fail
 199      18 yes   no    yes     exists   exists   keep index
 200      19 no    no    yes     exists   exists   keep index
 201      20 yes   yes   no      exists   exists   use M
 202      21 no    yes   no      exists   exists   fail
 203
 204 In all "keep index" cases, the index entry stays as in the
 205 original index file.  If the entry were not up to date,
 206 'git read-tree' keeps the copy in the work tree intact when
 207 operating under the -u flag.
 208
 209 When this form of 'git read-tree' returns successfully, you can
 210 see what "local changes" you made are carried forward by running
 211 `git diff-index --cached $M`.  Note that this does not
 212 necessarily match `git diff-index --cached $H` would have
 213 produced before such a two tree merge.  This is because of cases
 214 18 and 19 --- if you already had the changes in $M (e.g. maybe
 215 you picked it up via e-mail in a patch form), `git diff-index
 216 --cached $H` would have told you about the change before this
 217 merge, but it would not show in `git diff-index --cached $M`
 218 output after two-tree merge.
 219
 220 Case #3 is slightly tricky and needs explanation.  The result from this
 221 rule logically should be to remove the path if the user staged the removal
 222 of the path and then switching to a new branch.  That however will prevent
 223 the initial checkout from happening, so the rule is modified to use M (new
 224 tree) only when the contents of the index is empty.  Otherwise the removal
 225 of the path is kept as long as $H and $M are the same.
 226
 227 3-Way Merge
 228 ~~~~~~~~~~~
 229 Each "index" entry has two bits worth of "stage" state. stage 0 is the
 230 normal one, and is the only one you'd see in any kind of normal use.
 231
 232 However, when you do 'git read-tree' with three trees, the "stage"
 233 starts out at 1.
 234
 235 This means that you can do
 236
 237 ----------------
 238 $ git read-tree -m <tree1> <tree2> <tree3>
 239 ----------------
 240
 241 and you will end up with an index with all of the <tree1> entries in
 242 "stage1", all of the <tree2> entries in "stage2" and all of the
 243 <tree3> entries in "stage3".  When performing a merge of another
 244 branch into the current branch, we use the common ancestor tree
 245 as <tree1>, the current branch head as <tree2>, and the other
 246 branch head as <tree3>.
 247
 248 Furthermore, 'git read-tree' has special-case logic that says: if you see
 249 a file that matches in all respects in the following states, it
 250 "collapses" back to "stage0":
 251
 252    - stage 2 and 3 are the same; take one or the other (it makes no
 253      difference - the same work has been done on our branch in
 254      stage 2 and their branch in stage 3)
 255
 256    - stage 1 and stage 2 are the same and stage 3 is different; take
 257      stage 3 (our branch in stage 2 did not do anything since the
 258      ancestor in stage 1 while their branch in stage 3 worked on
 259      it)
 260
 261    - stage 1 and stage 3 are the same and stage 2 is different take
 262      stage 2 (we did something while they did nothing)
 263
 264 The 'git write-tree' command refuses to write a nonsensical tree, and it
 265 will complain about unmerged entries if it sees a single entry that is not
 266 stage 0.
 267
 268 OK, this all sounds like a collection of totally nonsensical rules,
 269 but it's actually exactly what you want in order to do a fast
 270 merge. The different stages represent the "result tree" (stage 0, aka
 271 "merged"), the original tree (stage 1, aka "orig"), and the two trees
 272 you are trying to merge (stage 2 and 3 respectively).
 273
 274 The order of stages 1, 2 and 3 (hence the order of three
 275 <tree-ish> command line arguments) are significant when you
 276 start a 3-way merge with an index file that is already
 277 populated.  Here is an outline of how the algorithm works:
 278
 279 - if a file exists in identical format in all three trees, it will
 280   automatically collapse to "merged" state by 'git read-tree'.
 281
 282 - a file that has _any_ difference what-so-ever in the three trees
 283   will stay as separate entries in the index. It's up to "porcelain
 284   policy" to determine how to remove the non-0 stages, and insert a
 285   merged version.
 286
 287 - the index file saves and restores with all this information, so you
 288   can merge things incrementally, but as long as it has entries in
 289   stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
 290   now the merge algorithm ends up being really simple:
 291
 292   * you walk the index in order, and ignore all entries of stage 0,
 293     since they've already been done.
 294
 295   * if you find a "stage1", but no matching "stage2" or "stage3", you
 296     know it's been removed from both trees (it only existed in the
 297     original tree), and you remove that entry.
 298
 299   * if you find a matching "stage2" and "stage3" tree, you remove one
 300     of them, and turn the other into a "stage0" entry. Remove any
 301     matching "stage1" entry if it exists too.  .. all the normal
 302     trivial rules ..
 303
 304 You would normally use 'git merge-index' with supplied
 305 'git merge-one-file' to do this last step.  The script updates
 306 the files in the working tree as it merges each path and at the
 307 end of a successful merge.
 308
 309 When you start a 3-way merge with an index file that is already
 310 populated, it is assumed that it represents the state of the
 311 files in your work tree, and you can even have files with
 312 changes unrecorded in the index file.  It is further assumed
 313 that this state is "derived" from the stage 2 tree.  The 3-way
 314 merge refuses to run if it finds an entry in the original index
 315 file that does not match stage 2.
 316
 317 This is done to prevent you from losing your work-in-progress
 318 changes, and mixing your random changes in an unrelated merge
 319 commit.  To illustrate, suppose you start from what has been
 320 committed last to your repository:
 321
 322 ----------------
 323 $ JC=`git rev-parse --verify "HEAD^0"`
 324 $ git checkout-index -f -u -a $JC
 325 ----------------
 326
 327 You do random edits, without running 'git update-index'.  And then
 328 you notice that the tip of your "upstream" tree has advanced
 329 since you pulled from him:
 330
 331 ----------------
 332 $ git fetch git://.... linus
 333 $ LT=`cat .git/FETCH_HEAD`
 334 ----------------
 335
 336 Your work tree is still based on your HEAD ($JC), but you have
 337 some edits since.  Three-way merge makes sure that you have not
 338 added or modified index entries since $JC, and if you haven't,
 339 then does the right thing.  So with the following sequence:
 340
 341 ----------------
 342 $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
 343 $ git merge-index git-merge-one-file -a
 344 $ echo "Merge with Linus" | \
 345   git commit-tree `git write-tree` -p $JC -p $LT
 346 ----------------
 347
 348 what you would commit is a pure merge between $JC and $LT without
 349 your work-in-progress changes, and your work tree would be
 350 updated to the result of the merge.
 351
 352 However, if you have local changes in the working tree that
 353 would be overwritten by this merge, 'git read-tree' will refuse
 354 to run to prevent your changes from being lost.
 355
 356 In other words, there is no need to worry about what exists only
 357 in the working tree.  When you have local changes in a part of
 358 the project that is not involved in the merge, your changes do
 359 not interfere with the merge, and are kept intact.  When they
 360 *do* interfere, the merge does not even start ('git read-tree'
 361 complains loudly and fails without modifying anything).  In such
 362 a case, you can simply continue doing what you were in the
 363 middle of doing, and when your working tree is ready (i.e. you
 364 have finished your work-in-progress), attempt the merge again.
 365
 366
 367 Sparse checkout
 368 ---------------
 369
 370 "Sparse checkout" allows to sparsely populate working directory.
 371 It uses skip-worktree bit (see linkgit:git-update-index[1]) to tell
 372 Git whether a file on working directory is worth looking at.
 373
 374 "git read-tree" and other merge-based commands ("git merge", "git
 375 checkout"...) can help maintaining skip-worktree bitmap and working
 376 directory update. `$GIT_DIR/info/sparse-checkout` is used to
 377 define the skip-worktree reference bitmap. When "git read-tree" needs
 378 to update working directory, it will reset skip-worktree bit in index
 379 based on this file, which uses the same syntax as .gitignore files.
 380 If an entry matches a pattern in this file, skip-worktree will be
 381 set on that entry. Otherwise, skip-worktree will be unset.
 382
 383 Then it compares the new skip-worktree value with the previous one. If
 384 skip-worktree turns from unset to set, it will add the corresponding
 385 file back. If it turns from set to unset, that file will be removed.
 386
 387 While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
 388 files are in. You can also specify what files are _not_ in, using
 389 negate patterns. For example, to remove file "unwanted":
 390
 391 ----------------
 392 *
 393 !unwanted
 394 ----------------
 395
 396 Another tricky thing is fully repopulating working directory when you
 397 no longer want sparse checkout. You cannot just disable "sparse
 398 checkout" because skip-worktree are still in the index and you working
 399 directory is still sparsely populated. You should re-populate working
 400 directory with the `$GIT_DIR/info/sparse-checkout` file content as
 401 follows:
 402
 403 ----------------
 404 *
 405 ----------------
 406
 407 Then you can disable sparse checkout. Sparse checkout support in "git
 408 read-tree" and similar commands is disabled by default. You need to
 409 turn `core.sparseCheckout` on in order to have sparse checkout
 410 support.
 411
 412
 413 SEE ALSO
 414 --------
 415 linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
 416 linkgit:gitignore[5]
 417
 418
 419 Author
 420 ------
 421 Written by Linus Torvalds <torvalds@osdl.org>
 422
 423 Documentation
 424 --------------
 425 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
 426
 427 GIT
 428 ---
 429 Part of the linkgit:git[1] suite