Documentation / git-read-tree.txton commit ssh: introduce a 'simple' ssh variant (94b8ae5)
   1git-read-tree(1)
   2================
   3
   4NAME
   5----
   6git-read-tree - Reads tree information into the index
   7
   8
   9SYNOPSIS
  10--------
  11[verse]
  12'git read-tree' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
  13                [-u [--exclude-per-directory=<gitignore>] | -i]]
  14                [--index-output=<file>] [--no-sparse-checkout]
  15                (--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])
  16
  17
  18DESCRIPTION
  19-----------
  20Reads the tree information given by <tree-ish> into the index,
  21but does not actually *update* any of the files it "caches". (see:
  22linkgit:git-checkout-index[1])
  23
  24Optionally, it can merge a tree into the index, perform a
  25fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
  26flag.  When used with `-m`, the `-u` flag causes it to also update
  27the files in the work tree with the result of the merge.
  28
  29Trivial merges are done by 'git read-tree' itself.  Only conflicting paths
  30will be in unmerged state when 'git read-tree' returns.
  31
  32OPTIONS
  33-------
  34-m::
  35        Perform a merge, not just a read.  The command will
  36        refuse to run if your index file has unmerged entries,
  37        indicating that you have not finished previous merge you
  38        started.
  39
  40--reset::
  41        Same as -m, except that unmerged entries are discarded
  42        instead of failing.
  43
  44-u::
  45        After a successful merge, update the files in the work
  46        tree with the result of the merge.
  47
  48-i::
  49        Usually a merge requires the index file as well as the
  50        files in the working tree to be up to date with the
  51        current head commit, in order not to lose local
  52        changes.  This flag disables the check with the working
  53        tree and is meant to be used when creating a merge of
  54        trees that are not directly related to the current
  55        working tree status into a temporary index file.
  56
  57-n::
  58--dry-run::
  59        Check if the command would error out, without updating the index
  60        or the files in the working tree for real.
  61
  62-v::
  63        Show the progress of checking files out.
  64
  65--trivial::
  66        Restrict three-way merge by 'git read-tree' to happen
  67        only if there is no file-level merging required, instead
  68        of resolving merge for trivial cases and leaving
  69        conflicting files unresolved in the index.
  70
  71--aggressive::
  72        Usually a three-way merge by 'git read-tree' resolves
  73        the merge for really trivial cases and leaves other
  74        cases unresolved in the index, so that porcelains can
  75        implement different merge policies.  This flag makes the
  76        command resolve a few more cases internally:
  77+
  78* when one side removes a path and the other side leaves the path
  79  unmodified.  The resolution is to remove that path.
  80* when both sides remove a path.  The resolution is to remove that path.
  81* when both sides add a path identically.  The resolution
  82  is to add that path.
  83
  84--prefix=<prefix>/::
  85        Keep the current index contents, and read the contents
  86        of the named tree-ish under the directory at `<prefix>`.
  87        The command will refuse to overwrite entries that already
  88        existed in the original index file. Note that the `<prefix>/`
  89        value must end with a slash.
  90
  91--exclude-per-directory=<gitignore>::
  92        When running the command with `-u` and `-m` options, the
  93        merge result may need to overwrite paths that are not
  94        tracked in the current branch.  The command usually
  95        refuses to proceed with the merge to avoid losing such a
  96        path.  However this safety valve sometimes gets in the
  97        way.  For example, it often happens that the other
  98        branch added a file that used to be a generated file in
  99        your branch, and the safety valve triggers when you try
 100        to switch to that branch after you ran `make` but before
 101        running `make clean` to remove the generated file.  This
 102        option tells the command to read per-directory exclude
 103        file (usually '.gitignore') and allows such an untracked
 104        but explicitly ignored file to be overwritten.
 105
 106--index-output=<file>::
 107        Instead of writing the results out to `$GIT_INDEX_FILE`,
 108        write the resulting index in the named file.  While the
 109        command is operating, the original index file is locked
 110        with the same mechanism as usual.  The file must allow
 111        to be rename(2)ed into from a temporary file that is
 112        created next to the usual index file; typically this
 113        means it needs to be on the same filesystem as the index
 114        file itself, and you need write permission to the
 115        directories the index file and index output file are
 116        located in.
 117
 118--[no-]recurse-submodules::
 119        Using --recurse-submodules will update the content of all initialized
 120        submodules according to the commit recorded in the superproject by
 121        calling read-tree recursively, also setting the submodules HEAD to be
 122        detached at that commit.
 123
 124--no-sparse-checkout::
 125        Disable sparse checkout support even if `core.sparseCheckout`
 126        is true.
 127
 128--empty::
 129        Instead of reading tree object(s) into the index, just empty
 130        it.
 131
 132<tree-ish#>::
 133        The id of the tree object(s) to be read/merged.
 134
 135
 136Merging
 137-------
 138If `-m` is specified, 'git read-tree' can perform 3 kinds of
 139merge, a single tree merge if only 1 tree is given, a
 140fast-forward merge with 2 trees, or a 3-way merge if 3 or more trees are
 141provided.
 142
 143
 144Single Tree Merge
 145~~~~~~~~~~~~~~~~~
 146If only 1 tree is specified, 'git read-tree' operates as if the user did not
 147specify `-m`, except that if the original index has an entry for a
 148given pathname, and the contents of the path match with the tree
 149being read, the stat info from the index is used. (In other words, the
 150index's stat()s take precedence over the merged tree's).
 151
 152That means that if you do a `git read-tree -m <newtree>` followed by a
 153`git checkout-index -f -u -a`, the 'git checkout-index' only checks out
 154the stuff that really changed.
 155
 156This is used to avoid unnecessary false hits when 'git diff-files' is
 157run after 'git read-tree'.
 158
 159
 160Two Tree Merge
 161~~~~~~~~~~~~~~
 162
 163Typically, this is invoked as `git read-tree -m $H $M`, where $H
 164is the head commit of the current repository, and $M is the head
 165of a foreign tree, which is simply ahead of $H (i.e. we are in a
 166fast-forward situation).
 167
 168When two trees are specified, the user is telling 'git read-tree'
 169the following:
 170
 171     1. The current index and work tree is derived from $H, but
 172        the user may have local changes in them since $H.
 173
 174     2. The user wants to fast-forward to $M.
 175
 176In this case, the `git read-tree -m $H $M` command makes sure
 177that no local change is lost as the result of this "merge".
 178Here are the "carry forward" rules, where "I" denotes the index,
 179"clean" means that index and work tree coincide, and "exists"/"nothing"
 180refer to the presence of a path in the specified commit:
 181
 182        I                   H        M        Result
 183       -------------------------------------------------------
 184     0  nothing             nothing  nothing  (does not happen)
 185     1  nothing             nothing  exists   use M
 186     2  nothing             exists   nothing  remove path from index
 187     3  nothing             exists   exists,  use M if "initial checkout",
 188                                     H == M   keep index otherwise
 189                                     exists,  fail
 190                                     H != M
 191
 192        clean I==H  I==M
 193       ------------------
 194     4  yes   N/A   N/A     nothing  nothing  keep index
 195     5  no    N/A   N/A     nothing  nothing  keep index
 196
 197     6  yes   N/A   yes     nothing  exists   keep index
 198     7  no    N/A   yes     nothing  exists   keep index
 199     8  yes   N/A   no      nothing  exists   fail
 200     9  no    N/A   no      nothing  exists   fail
 201
 202     10 yes   yes   N/A     exists   nothing  remove path from index
 203     11 no    yes   N/A     exists   nothing  fail
 204     12 yes   no    N/A     exists   nothing  fail
 205     13 no    no    N/A     exists   nothing  fail
 206
 207        clean (H==M)
 208       ------
 209     14 yes                 exists   exists   keep index
 210     15 no                  exists   exists   keep index
 211
 212        clean I==H  I==M (H!=M)
 213       ------------------
 214     16 yes   no    no      exists   exists   fail
 215     17 no    no    no      exists   exists   fail
 216     18 yes   no    yes     exists   exists   keep index
 217     19 no    no    yes     exists   exists   keep index
 218     20 yes   yes   no      exists   exists   use M
 219     21 no    yes   no      exists   exists   fail
 220
 221In all "keep index" cases, the index entry stays as in the
 222original index file.  If the entry is not up to date,
 223'git read-tree' keeps the copy in the work tree intact when
 224operating under the -u flag.
 225
 226When this form of 'git read-tree' returns successfully, you can
 227see which of the "local changes" that you made were carried forward by running
 228`git diff-index --cached $M`.  Note that this does not
 229necessarily match what `git diff-index --cached $H` would have
 230produced before such a two tree merge.  This is because of cases
 23118 and 19 --- if you already had the changes in $M (e.g. maybe
 232you picked it up via e-mail in a patch form), `git diff-index
 233--cached $H` would have told you about the change before this
 234merge, but it would not show in `git diff-index --cached $M`
 235output after the two-tree merge.
 236
 237Case 3 is slightly tricky and needs explanation.  The result from this
 238rule logically should be to remove the path if the user staged the removal
 239of the path and then switching to a new branch.  That however will prevent
 240the initial checkout from happening, so the rule is modified to use M (new
 241tree) only when the content of the index is empty.  Otherwise the removal
 242of the path is kept as long as $H and $M are the same.
 243
 2443-Way Merge
 245~~~~~~~~~~~
 246Each "index" entry has two bits worth of "stage" state. stage 0 is the
 247normal one, and is the only one you'd see in any kind of normal use.
 248
 249However, when you do 'git read-tree' with three trees, the "stage"
 250starts out at 1.
 251
 252This means that you can do
 253
 254----------------
 255$ git read-tree -m <tree1> <tree2> <tree3>
 256----------------
 257
 258and you will end up with an index with all of the <tree1> entries in
 259"stage1", all of the <tree2> entries in "stage2" and all of the
 260<tree3> entries in "stage3".  When performing a merge of another
 261branch into the current branch, we use the common ancestor tree
 262as <tree1>, the current branch head as <tree2>, and the other
 263branch head as <tree3>.
 264
 265Furthermore, 'git read-tree' has special-case logic that says: if you see
 266a file that matches in all respects in the following states, it
 267"collapses" back to "stage0":
 268
 269   - stage 2 and 3 are the same; take one or the other (it makes no
 270     difference - the same work has been done on our branch in
 271     stage 2 and their branch in stage 3)
 272
 273   - stage 1 and stage 2 are the same and stage 3 is different; take
 274     stage 3 (our branch in stage 2 did not do anything since the
 275     ancestor in stage 1 while their branch in stage 3 worked on
 276     it)
 277
 278   - stage 1 and stage 3 are the same and stage 2 is different take
 279     stage 2 (we did something while they did nothing)
 280
 281The 'git write-tree' command refuses to write a nonsensical tree, and it
 282will complain about unmerged entries if it sees a single entry that is not
 283stage 0.
 284
 285OK, this all sounds like a collection of totally nonsensical rules,
 286but it's actually exactly what you want in order to do a fast
 287merge. The different stages represent the "result tree" (stage 0, aka
 288"merged"), the original tree (stage 1, aka "orig"), and the two trees
 289you are trying to merge (stage 2 and 3 respectively).
 290
 291The order of stages 1, 2 and 3 (hence the order of three
 292<tree-ish> command-line arguments) are significant when you
 293start a 3-way merge with an index file that is already
 294populated.  Here is an outline of how the algorithm works:
 295
 296- if a file exists in identical format in all three trees, it will
 297  automatically collapse to "merged" state by 'git read-tree'.
 298
 299- a file that has _any_ difference what-so-ever in the three trees
 300  will stay as separate entries in the index. It's up to "porcelain
 301  policy" to determine how to remove the non-0 stages, and insert a
 302  merged version.
 303
 304- the index file saves and restores with all this information, so you
 305  can merge things incrementally, but as long as it has entries in
 306  stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
 307  now the merge algorithm ends up being really simple:
 308
 309  * you walk the index in order, and ignore all entries of stage 0,
 310    since they've already been done.
 311
 312  * if you find a "stage1", but no matching "stage2" or "stage3", you
 313    know it's been removed from both trees (it only existed in the
 314    original tree), and you remove that entry.
 315
 316  * if you find a matching "stage2" and "stage3" tree, you remove one
 317    of them, and turn the other into a "stage0" entry. Remove any
 318    matching "stage1" entry if it exists too.  .. all the normal
 319    trivial rules ..
 320
 321You would normally use 'git merge-index' with supplied
 322'git merge-one-file' to do this last step.  The script updates
 323the files in the working tree as it merges each path and at the
 324end of a successful merge.
 325
 326When you start a 3-way merge with an index file that is already
 327populated, it is assumed that it represents the state of the
 328files in your work tree, and you can even have files with
 329changes unrecorded in the index file.  It is further assumed
 330that this state is "derived" from the stage 2 tree.  The 3-way
 331merge refuses to run if it finds an entry in the original index
 332file that does not match stage 2.
 333
 334This is done to prevent you from losing your work-in-progress
 335changes, and mixing your random changes in an unrelated merge
 336commit.  To illustrate, suppose you start from what has been
 337committed last to your repository:
 338
 339----------------
 340$ JC=`git rev-parse --verify "HEAD^0"`
 341$ git checkout-index -f -u -a $JC
 342----------------
 343
 344You do random edits, without running 'git update-index'.  And then
 345you notice that the tip of your "upstream" tree has advanced
 346since you pulled from him:
 347
 348----------------
 349$ git fetch git://.... linus
 350$ LT=`git rev-parse FETCH_HEAD`
 351----------------
 352
 353Your work tree is still based on your HEAD ($JC), but you have
 354some edits since.  Three-way merge makes sure that you have not
 355added or modified index entries since $JC, and if you haven't,
 356then does the right thing.  So with the following sequence:
 357
 358----------------
 359$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
 360$ git merge-index git-merge-one-file -a
 361$ echo "Merge with Linus" | \
 362  git commit-tree `git write-tree` -p $JC -p $LT
 363----------------
 364
 365what you would commit is a pure merge between $JC and $LT without
 366your work-in-progress changes, and your work tree would be
 367updated to the result of the merge.
 368
 369However, if you have local changes in the working tree that
 370would be overwritten by this merge, 'git read-tree' will refuse
 371to run to prevent your changes from being lost.
 372
 373In other words, there is no need to worry about what exists only
 374in the working tree.  When you have local changes in a part of
 375the project that is not involved in the merge, your changes do
 376not interfere with the merge, and are kept intact.  When they
 377*do* interfere, the merge does not even start ('git read-tree'
 378complains loudly and fails without modifying anything).  In such
 379a case, you can simply continue doing what you were in the
 380middle of doing, and when your working tree is ready (i.e. you
 381have finished your work-in-progress), attempt the merge again.
 382
 383
 384Sparse checkout
 385---------------
 386
 387"Sparse checkout" allows populating the working directory sparsely.
 388It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell
 389Git whether a file in the working directory is worth looking at.
 390
 391'git read-tree' and other merge-based commands ('git merge', 'git
 392checkout'...) can help maintaining the skip-worktree bitmap and working
 393directory update. `$GIT_DIR/info/sparse-checkout` is used to
 394define the skip-worktree reference bitmap. When 'git read-tree' needs
 395to update the working directory, it resets the skip-worktree bit in the index
 396based on this file, which uses the same syntax as .gitignore files.
 397If an entry matches a pattern in this file, skip-worktree will not be
 398set on that entry. Otherwise, skip-worktree will be set.
 399
 400Then it compares the new skip-worktree value with the previous one. If
 401skip-worktree turns from set to unset, it will add the corresponding
 402file back. If it turns from unset to set, that file will be removed.
 403
 404While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
 405files are in, you can also specify what files are _not_ in, using
 406negate patterns. For example, to remove the file `unwanted`:
 407
 408----------------
 409/*
 410!unwanted
 411----------------
 412
 413Another tricky thing is fully repopulating the working directory when you
 414no longer want sparse checkout. You cannot just disable "sparse
 415checkout" because skip-worktree bits are still in the index and your working
 416directory is still sparsely populated. You should re-populate the working
 417directory with the `$GIT_DIR/info/sparse-checkout` file content as
 418follows:
 419
 420----------------
 421/*
 422----------------
 423
 424Then you can disable sparse checkout. Sparse checkout support in 'git
 425read-tree' and similar commands is disabled by default. You need to
 426turn `core.sparseCheckout` on in order to have sparse checkout
 427support.
 428
 429
 430SEE ALSO
 431--------
 432linkgit:git-write-tree[1]; linkgit:git-ls-files[1];
 433linkgit:gitignore[5]
 434
 435GIT
 436---
 437Part of the linkgit:git[1] suite