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 [-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 git-checkout-index)
  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.
  32
  33 --reset::
  34
  35         Same as -m except that unmerged entries will be silently ignored.
  36
  37 -u::
  38         After a successful merge, update the files in the work
  39         tree with the result of the merge.
  40
  41 -i::
  42         Usually a merge requires the index file as well as the
  43         files in the working tree are up to date with the
  44         current head commit, in order not to lose local
  45         changes.  This flag disables the check with the working
  46         tree and is meant to be used when creating a merge of
  47         trees that are not directly related to the current
  48         working tree status into a temporary index file.
  49
  50
  51 <tree-ish#>::
  52         The id of the tree object(s) to be read/merged.
  53
  54
  55 Merging
  56 -------
  57 If '-m' is specified, "git-read-tree" can perform 3 kinds of
  58 merge, a single tree merge if only 1 tree is given, a
  59 fast-forward merge with 2 trees, or a 3-way merge if 3 trees are
  60 provided.
  61
  62
  63 Single Tree Merge
  64 ~~~~~~~~~~~~~~~~~
  65 If only 1 tree is specified, git-read-tree operates as if the user did not
  66 specify '-m', except that if the original index has an entry for a
  67 given pathname, and the contents of the path matches with the tree
  68 being read, the stat info from the index is used. (In other words, the
  69 index's stat()s take precedence over the merged tree's).
  70
  71 That means that if you do a "git-read-tree -m <newtree>" followed by a
  72 "git-checkout-index -f -u -a", the "git-checkout-index" only checks out
  73 the stuff that really changed.
  74
  75 This is used to avoid unnecessary false hits when "git-diff-files" is
  76 run after git-read-tree.
  77
  78
  79 Two Tree Merge
  80 ~~~~~~~~~~~~~~
  81
  82 Typically, this is invoked as "git-read-tree -m $H $M", where $H
  83 is the head commit of the current repository, and $M is the head
  84 of a foreign tree, which is simply ahead of $H (i.e. we are in a
  85 fast forward situation).
  86
  87 When two trees are specified, the user is telling git-read-tree
  88 the following:
  89
  90      1. The current index and work tree is derived from $H, but
  91         the user may have local changes in them since $H;
  92
  93      2. The user wants to fast-forward to $M.
  94
  95 In this case, the "git-read-tree -m $H $M" command makes sure
  96 that no local change is lost as the result of this "merge".
  97 Here are the "carry forward" rules:
  98
  99         I (index)           H        M        Result
 100        -------------------------------------------------------
 101       0 nothing             nothing  nothing  (does not happen)
 102       1 nothing             nothing  exists   use M
 103       2 nothing             exists   nothing  remove path from index
 104       3 nothing             exists   exists   use M
 105
 106         clean I==H  I==M
 107        ------------------
 108       4 yes   N/A   N/A     nothing  nothing  keep index
 109       5 no    N/A   N/A     nothing  nothing  keep index
 110
 111       6 yes   N/A   yes     nothing  exists   keep index
 112       7 no    N/A   yes     nothing  exists   keep index
 113       8 yes   N/A   no      nothing  exists   fail
 114       9 no    N/A   no      nothing  exists   fail
 115
 116      10 yes   yes   N/A     exists   nothing  remove path from index
 117      11 no    yes   N/A     exists   nothing  fail
 118      12 yes   no    N/A     exists   nothing  fail
 119      13 no    no    N/A     exists   nothing  fail
 120
 121         clean (H=M)
 122        ------
 123      14 yes                 exists   exists   keep index
 124      15 no                  exists   exists   keep index
 125
 126         clean I==H  I==M (H!=M)
 127        ------------------
 128      16 yes   no    no      exists   exists   fail
 129      17 no    no    no      exists   exists   fail
 130      18 yes   no    yes     exists   exists   keep index
 131      19 no    no    yes     exists   exists   keep index
 132      20 yes   yes   no      exists   exists   use M
 133      21 no    yes   no      exists   exists   fail
 134
 135 In all "keep index" cases, the index entry stays as in the
 136 original index file.  If the entry were not up to date,
 137 git-read-tree keeps the copy in the work tree intact when
 138 operating under the -u flag.
 139
 140 When this form of git-read-tree returns successfully, you can
 141 see what "local changes" you made are carried forward by running
 142 "git-diff-index --cached $M".  Note that this does not
 143 necessarily match "git-diff-index --cached $H" would have
 144 produced before such a two tree merge.  This is because of cases
 145 18 and 19 --- if you already had the changes in $M (e.g. maybe
 146 you picked it up via e-mail in a patch form), "git-diff-index
 147 --cached $H" would have told you about the change before this
 148 merge, but it would not show in "git-diff-index --cached $M"
 149 output after two-tree merge.
 150
 151
 152 3-Way Merge
 153 ~~~~~~~~~~~
 154 Each "index" entry has two bits worth of "stage" state. stage 0 is the
 155 normal one, and is the only one you'd see in any kind of normal use.
 156
 157 However, when you do "git-read-tree" with three trees, the "stage"
 158 starts out at 1.
 159
 160 This means that you can do
 161
 162         git-read-tree -m <tree1> <tree2> <tree3>
 163
 164 and you will end up with an index with all of the <tree1> entries in
 165 "stage1", all of the <tree2> entries in "stage2" and all of the
 166 <tree3> entries in "stage3".
 167
 168 Furthermore, "git-read-tree" has special-case logic that says: if you see
 169 a file that matches in all respects in the following states, it
 170 "collapses" back to "stage0":
 171
 172    - stage 2 and 3 are the same; take one or the other (it makes no
 173      difference - the same work has been done on stage 2 and 3)
 174
 175    - stage 1 and stage 2 are the same and stage 3 is different; take
 176      stage 3 (some work has been done on stage 3)
 177
 178    - stage 1 and stage 3 are the same and stage 2 is different take
 179      stage 2 (some work has been done on stage 2)
 180
 181 The "git-write-tree" command refuses to write a nonsensical tree, and it
 182 will complain about unmerged entries if it sees a single entry that is not
 183 stage 0.
 184
 185 Ok, this all sounds like a collection of totally nonsensical rules,
 186 but it's actually exactly what you want in order to do a fast
 187 merge. The different stages represent the "result tree" (stage 0, aka
 188 "merged"), the original tree (stage 1, aka "orig"), and the two trees
 189 you are trying to merge (stage 2 and 3 respectively).
 190
 191 The order of stages 1, 2 and 3 (hence the order of three
 192 <tree-ish> command line arguments) are significant when you
 193 start a 3-way merge with an index file that is already
 194 populated.  Here is an outline of how the algorithm works:
 195
 196 - if a file exists in identical format in all three trees, it will
 197   automatically collapse to "merged" state by git-read-tree.
 198
 199 - a file that has _any_ difference what-so-ever in the three trees
 200   will stay as separate entries in the index. It's up to "porcelain
 201   policy" to determine how to remove the non-0 stages, and insert a
 202   merged version.
 203
 204 - the index file saves and restores with all this information, so you
 205   can merge things incrementally, but as long as it has entries in
 206   stages 1/2/3 (ie "unmerged entries") you can't write the result. So
 207   now the merge algorithm ends up being really simple:
 208
 209   * you walk the index in order, and ignore all entries of stage 0,
 210     since they've already been done.
 211
 212   * if you find a "stage1", but no matching "stage2" or "stage3", you
 213     know it's been removed from both trees (it only existed in the
 214     original tree), and you remove that entry.
 215
 216   * if you find a matching "stage2" and "stage3" tree, you remove one
 217     of them, and turn the other into a "stage0" entry. Remove any
 218     matching "stage1" entry if it exists too.  .. all the normal
 219     trivial rules ..
 220
 221 You would normally use "git-merge-index" with supplied
 222 "git-merge-one-file" to do this last step.  The script
 223 does not touch the files in the work tree, and the entire merge
 224 happens in the index file.  In other words, there is no need to
 225 worry about what is in the working directory, since it is never
 226 shown and never used.
 227
 228 When you start a 3-way merge with an index file that is already
 229 populated, it is assumed that it represents the state of the
 230 files in your work tree, and you can even have files with
 231 changes unrecorded in the index file.  It is further assumed
 232 that this state is "derived" from the stage 2 tree.  The 3-way
 233 merge refuses to run if it finds an entry in the original index
 234 file that does not match stage 2.
 235
 236 This is done to prevent you from losing your work-in-progress
 237 changes.  To illustrate, suppose you start from what has been
 238 commited last to your repository:
 239
 240     $ JC=`git-rev-parse --verify "HEAD^0"`
 241     $ git-checkout-index -f -u -a $JC
 242
 243 You do random edits, without running git-update-index.  And then
 244 you notice that the tip of your "upstream" tree has advanced
 245 since you pulled from him:
 246
 247     $ git-fetch rsync://.... linus
 248     $ LT=`cat .git/MERGE_HEAD`
 249
 250 Your work tree is still based on your HEAD ($JC), but you have
 251 some edits since.  Three-way merge makes sure that you have not
 252 added or modified index entries since $JC, and if you haven't,
 253 then does the right thing.  So with the following sequence:
 254
 255     $ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT
 256     $ git-merge-index git-merge-one-file -a
 257     $ echo "Merge with Linus" | \
 258       git-commit-tree `git-write-tree` -p $JC -p $LT
 259
 260 what you would commit is a pure merge between $JC and LT without
 261 your work-in-progress changes, and your work tree would be
 262 updated to the result of the merge.
 263
 264
 265 See Also
 266 --------
 267 gitlink:git-write-tree[1]; gitlink:git-ls-files[1]
 268
 269
 270 Author
 271 ------
 272 Written by Linus Torvalds <torvalds@osdl.org>
 273
 274 Documentation
 275 --------------
 276 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
 277
 278 GIT
 279 ---
 280 Part of the gitlink:git[7] suite
 281