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 [--aggressive] | --reset | --prefix=<prefix>] [-u | -i]] <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: 18gitlink: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--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--prefix=<prefix>/:: 67 Keep the current index contents, and read the contents 68 of named tree-ish under directory at `<prefix>`. The 69 original index file cannot have anything at the path 70 `<prefix>` itself, and have nothing in `<prefix>/` 71 directory. Note that the `<prefix>/` value must end 72 with a slash. 73 74 75<tree-ish#>:: 76 The id of the tree object(s) to be read/merged. 77 78 79Merging 80------- 81If `-m` is specified, `git-read-tree` can perform 3 kinds of 82merge, a single tree merge if only 1 tree is given, a 83fast-forward merge with 2 trees, or a 3-way merge if 3 trees are 84provided. 85 86 87Single Tree Merge 88~~~~~~~~~~~~~~~~~ 89If only 1 tree is specified, git-read-tree operates as if the user did not 90specify `-m`, except that if the original index has an entry for a 91given pathname, and the contents of the path matches with the tree 92being read, the stat info from the index is used. (In other words, the 93index's stat()s take precedence over the merged tree's). 94 95That means that if you do a `git-read-tree -m <newtree>` followed by a 96`git-checkout-index -f -u -a`, the `git-checkout-index` only checks out 97the stuff that really changed. 98 99This is used to avoid unnecessary false hits when `git-diff-files` is 100run after `git-read-tree`. 101 102 103Two Tree Merge 104~~~~~~~~~~~~~~ 105 106Typically, this is invoked as `git-read-tree -m $H $M`, where $H 107is the head commit of the current repository, and $M is the head 108of a foreign tree, which is simply ahead of $H (i.e. we are in a 109fast forward situation). 110 111When two trees are specified, the user is telling git-read-tree 112the following: 113 114 1. The current index and work tree is derived from $H, but 115 the user may have local changes in them since $H; 116 117 2. The user wants to fast-forward to $M. 118 119In this case, the `git-read-tree -m $H $M` command makes sure 120that no local change is lost as the result of this "merge". 121Here are the "carry forward" rules: 122 123 I (index) H M Result 124 ------------------------------------------------------- 125 0 nothing nothing nothing (does not happen) 126 1 nothing nothing exists use M 127 2 nothing exists nothing remove path from index 128 3 nothing exists exists use M 129 130 clean I==H I==M 131 ------------------ 132 4 yes N/A N/A nothing nothing keep index 133 5 no N/A N/A nothing nothing keep index 134 135 6 yes N/A yes nothing exists keep index 136 7 no N/A yes nothing exists keep index 137 8 yes N/A no nothing exists fail 138 9 no N/A no nothing exists fail 139 140 10 yes yes N/A exists nothing remove path from index 141 11 no yes N/A exists nothing fail 142 12 yes no N/A exists nothing fail 143 13 no no N/A exists nothing fail 144 145 clean (H=M) 146 ------ 147 14 yes exists exists keep index 148 15 no exists exists keep index 149 150 clean I==H I==M (H!=M) 151 ------------------ 152 16 yes no no exists exists fail 153 17 no no no exists exists fail 154 18 yes no yes exists exists keep index 155 19 no no yes exists exists keep index 156 20 yes yes no exists exists use M 157 21 no yes no exists exists fail 158 159In all "keep index" cases, the index entry stays as in the 160original index file. If the entry were not up to date, 161git-read-tree keeps the copy in the work tree intact when 162operating under the -u flag. 163 164When this form of git-read-tree returns successfully, you can 165see what "local changes" you made are carried forward by running 166`git-diff-index --cached $M`. Note that this does not 167necessarily match `git-diff-index --cached $H` would have 168produced before such a two tree merge. This is because of cases 16918 and 19 --- if you already had the changes in $M (e.g. maybe 170you picked it up via e-mail in a patch form), `git-diff-index 171--cached $H` would have told you about the change before this 172merge, but it would not show in `git-diff-index --cached $M` 173output after two-tree merge. 174 175 1763-Way Merge 177~~~~~~~~~~~ 178Each "index" entry has two bits worth of "stage" state. stage 0 is the 179normal one, and is the only one you'd see in any kind of normal use. 180 181However, when you do `git-read-tree` with three trees, the "stage" 182starts out at 1. 183 184This means that you can do 185 186---------------- 187$ git-read-tree -m <tree1> <tree2> <tree3> 188---------------- 189 190and you will end up with an index with all of the <tree1> entries in 191"stage1", all of the <tree2> entries in "stage2" and all of the 192<tree3> entries in "stage3". When performing a merge of another 193branch into the current branch, we use the common ancestor tree 194as <tree1>, the current branch head as <tree2>, and the other 195branch head as <tree3>. 196 197Furthermore, `git-read-tree` has special-case logic that says: if you see 198a file that matches in all respects in the following states, it 199"collapses" back to "stage0": 200 201 - stage 2 and 3 are the same; take one or the other (it makes no 202 difference - the same work has been done on our branch in 203 stage 2 and their branch in stage 3) 204 205 - stage 1 and stage 2 are the same and stage 3 is different; take 206 stage 3 (our branch in stage 2 did not do anything since the 207 ancestor in stage 1 while their branch in stage 3 worked on 208 it) 209 210 - stage 1 and stage 3 are the same and stage 2 is different take 211 stage 2 (we did something while they did nothing) 212 213The `git-write-tree` command refuses to write a nonsensical tree, and it 214will complain about unmerged entries if it sees a single entry that is not 215stage 0. 216 217Ok, this all sounds like a collection of totally nonsensical rules, 218but it's actually exactly what you want in order to do a fast 219merge. The different stages represent the "result tree" (stage 0, aka 220"merged"), the original tree (stage 1, aka "orig"), and the two trees 221you are trying to merge (stage 2 and 3 respectively). 222 223The order of stages 1, 2 and 3 (hence the order of three 224<tree-ish> command line arguments) are significant when you 225start a 3-way merge with an index file that is already 226populated. Here is an outline of how the algorithm works: 227 228- if a file exists in identical format in all three trees, it will 229 automatically collapse to "merged" state by git-read-tree. 230 231- a file that has _any_ difference what-so-ever in the three trees 232 will stay as separate entries in the index. It's up to "porcelain 233 policy" to determine how to remove the non-0 stages, and insert a 234 merged version. 235 236- the index file saves and restores with all this information, so you 237 can merge things incrementally, but as long as it has entries in 238 stages 1/2/3 (ie "unmerged entries") you can't write the result. So 239 now the merge algorithm ends up being really simple: 240 241 * you walk the index in order, and ignore all entries of stage 0, 242 since they've already been done. 243 244 * if you find a "stage1", but no matching "stage2" or "stage3", you 245 know it's been removed from both trees (it only existed in the 246 original tree), and you remove that entry. 247 248 * if you find a matching "stage2" and "stage3" tree, you remove one 249 of them, and turn the other into a "stage0" entry. Remove any 250 matching "stage1" entry if it exists too. .. all the normal 251 trivial rules .. 252 253You would normally use `git-merge-index` with supplied 254`git-merge-one-file` to do this last step. The script updates 255the files in the working tree as it merges each path and at the 256end of a successful merge. 257 258When you start a 3-way merge with an index file that is already 259populated, it is assumed that it represents the state of the 260files in your work tree, and you can even have files with 261changes unrecorded in the index file. It is further assumed 262that this state is "derived" from the stage 2 tree. The 3-way 263merge refuses to run if it finds an entry in the original index 264file that does not match stage 2. 265 266This is done to prevent you from losing your work-in-progress 267changes, and mixing your random changes in an unrelated merge 268commit. To illustrate, suppose you start from what has been 269commited last to your repository: 270 271---------------- 272$ JC=`git-rev-parse --verify "HEAD^0"` 273$ git-checkout-index -f -u -a $JC 274---------------- 275 276You do random edits, without running git-update-index. And then 277you notice that the tip of your "upstream" tree has advanced 278since you pulled from him: 279 280---------------- 281$ git-fetch git://.... linus 282$ LT=`cat .git/FETCH_HEAD` 283---------------- 284 285Your work tree is still based on your HEAD ($JC), but you have 286some edits since. Three-way merge makes sure that you have not 287added or modified index entries since $JC, and if you haven't, 288then does the right thing. So with the following sequence: 289 290---------------- 291$ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT 292$ git-merge-index git-merge-one-file -a 293$ echo "Merge with Linus" | \ 294 git-commit-tree `git-write-tree` -p $JC -p $LT 295---------------- 296 297what you would commit is a pure merge between $JC and $LT without 298your work-in-progress changes, and your work tree would be 299updated to the result of the merge. 300 301However, if you have local changes in the working tree that 302would be overwritten by this merge,`git-read-tree` will refuse 303to run to prevent your changes from being lost. 304 305In other words, there is no need to worry about what exists only 306in the working tree. When you have local changes in a part of 307the project that is not involved in the merge, your changes do 308not interfere with the merge, and are kept intact. When they 309*do* interfere, the merge does not even start (`git-read-tree` 310complains loudly and fails without modifying anything). In such 311a case, you can simply continue doing what you were in the 312middle of doing, and when your working tree is ready (i.e. you 313have finished your work-in-progress), attempt the merge again. 314 315 316See Also 317-------- 318gitlink:git-write-tree[1]; gitlink:git-ls-files[1] 319 320 321Author 322------ 323Written by Linus Torvalds <torvalds@osdl.org> 324 325Documentation 326-------------- 327Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 328 329GIT 330--- 331Part of the gitlink:git[7] suite 332