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 [--trivial] [--aggressive] | --reset | --prefix=<prefix>] [-u | -i]] [--exclude-per-directory=<gitignore>] [--index-output=<file>] <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: 18linkgit: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--trivial:: 54 Restrict three-way merge by `git-read-tree` to happen 55 only if there is no file-level merging required, instead 56 of resolving merge for trivial cases and leaving 57 conflicting files unresolved in the index. 58 59--aggressive:: 60 Usually a three-way merge by `git-read-tree` resolves 61 the merge for really trivial cases and leaves other 62 cases unresolved in the index, so that Porcelains can 63 implement different merge policies. This flag makes the 64 command to resolve a few more cases internally: 65+ 66* when one side removes a path and the other side leaves the path 67 unmodified. The resolution is to remove that path. 68* when both sides remove a path. The resolution is to remove that path. 69* when both sides adds a path identically. The resolution 70 is to add that path. 71 72--prefix=<prefix>/:: 73 Keep the current index contents, and read the contents 74 of named tree-ish under directory at `<prefix>`. The 75 original index file cannot have anything at the path 76 `<prefix>` itself, and have nothing in `<prefix>/` 77 directory. Note that the `<prefix>/` value must end 78 with a slash. 79 80--exclude-per-directory=<gitignore>:: 81 When running the command with `-u` and `-m` options, the 82 merge result may need to overwrite paths that are not 83 tracked in the current branch. The command usually 84 refuses to proceed with the merge to avoid losing such a 85 path. However this safety valve sometimes gets in the 86 way. For example, it often happens that the other 87 branch added a file that used to be a generated file in 88 your branch, and the safety valve triggers when you try 89 to switch to that branch after you ran `make` but before 90 running `make clean` to remove the generated file. This 91 option tells the command to read per-directory exclude 92 file (usually '.gitignore') and allows such an untracked 93 but explicitly ignored file to be overwritten. 94 95--index-output=<file>:: 96 Instead of writing the results out to `$GIT_INDEX_FILE`, 97 write the resulting index in the named file. While the 98 command is operating, the original index file is locked 99 with the same mechanism as usual. The file must allow 100 to be rename(2)ed into from a temporary file that is 101 created next to the usual index file; typically this 102 means it needs to be on the same filesystem as the index 103 file itself, and you need write permission to the 104 directories the index file and index output file are 105 located in. 106 107<tree-ish#>:: 108 The id of the tree object(s) to be read/merged. 109 110 111Merging 112------- 113If `-m` is specified, `git-read-tree` can perform 3 kinds of 114merge, a single tree merge if only 1 tree is given, a 115fast-forward merge with 2 trees, or a 3-way merge if 3 trees are 116provided. 117 118 119Single Tree Merge 120~~~~~~~~~~~~~~~~~ 121If only 1 tree is specified, git-read-tree operates as if the user did not 122specify `-m`, except that if the original index has an entry for a 123given pathname, and the contents of the path matches with the tree 124being read, the stat info from the index is used. (In other words, the 125index's stat()s take precedence over the merged tree's). 126 127That means that if you do a `git-read-tree -m <newtree>` followed by a 128`git-checkout-index -f -u -a`, the `git-checkout-index` only checks out 129the stuff that really changed. 130 131This is used to avoid unnecessary false hits when `git-diff-files` is 132run after `git-read-tree`. 133 134 135Two Tree Merge 136~~~~~~~~~~~~~~ 137 138Typically, this is invoked as `git-read-tree -m $H $M`, where $H 139is the head commit of the current repository, and $M is the head 140of a foreign tree, which is simply ahead of $H (i.e. we are in a 141fast forward situation). 142 143When two trees are specified, the user is telling git-read-tree 144the following: 145 146 1. The current index and work tree is derived from $H, but 147 the user may have local changes in them since $H; 148 149 2. The user wants to fast-forward to $M. 150 151In this case, the `git-read-tree -m $H $M` command makes sure 152that no local change is lost as the result of this "merge". 153Here are the "carry forward" rules: 154 155 I (index) H M Result 156 ------------------------------------------------------- 157 0 nothing nothing nothing (does not happen) 158 1 nothing nothing exists use M 159 2 nothing exists nothing remove path from index 160 3 nothing exists exists use M 161 162 clean I==H I==M 163 ------------------ 164 4 yes N/A N/A nothing nothing keep index 165 5 no N/A N/A nothing nothing keep index 166 167 6 yes N/A yes nothing exists keep index 168 7 no N/A yes nothing exists keep index 169 8 yes N/A no nothing exists fail 170 9 no N/A no nothing exists fail 171 172 10 yes yes N/A exists nothing remove path from index 173 11 no yes N/A exists nothing fail 174 12 yes no N/A exists nothing fail 175 13 no no N/A exists nothing fail 176 177 clean (H=M) 178 ------ 179 14 yes exists exists keep index 180 15 no exists exists keep index 181 182 clean I==H I==M (H!=M) 183 ------------------ 184 16 yes no no exists exists fail 185 17 no no no exists exists fail 186 18 yes no yes exists exists keep index 187 19 no no yes exists exists keep index 188 20 yes yes no exists exists use M 189 21 no yes no exists exists fail 190 191In all "keep index" cases, the index entry stays as in the 192original index file. If the entry were not up to date, 193git-read-tree keeps the copy in the work tree intact when 194operating under the -u flag. 195 196When this form of git-read-tree returns successfully, you can 197see what "local changes" you made are carried forward by running 198`git-diff-index --cached $M`. Note that this does not 199necessarily match `git-diff-index --cached $H` would have 200produced before such a two tree merge. This is because of cases 20118 and 19 --- if you already had the changes in $M (e.g. maybe 202you picked it up via e-mail in a patch form), `git-diff-index 203--cached $H` would have told you about the change before this 204merge, but it would not show in `git-diff-index --cached $M` 205output after two-tree merge. 206 207 2083-Way Merge 209~~~~~~~~~~~ 210Each "index" entry has two bits worth of "stage" state. stage 0 is the 211normal one, and is the only one you'd see in any kind of normal use. 212 213However, when you do `git-read-tree` with three trees, the "stage" 214starts out at 1. 215 216This means that you can do 217 218---------------- 219$ git-read-tree -m <tree1> <tree2> <tree3> 220---------------- 221 222and you will end up with an index with all of the <tree1> entries in 223"stage1", all of the <tree2> entries in "stage2" and all of the 224<tree3> entries in "stage3". When performing a merge of another 225branch into the current branch, we use the common ancestor tree 226as <tree1>, the current branch head as <tree2>, and the other 227branch head as <tree3>. 228 229Furthermore, `git-read-tree` has special-case logic that says: if you see 230a file that matches in all respects in the following states, it 231"collapses" back to "stage0": 232 233 - stage 2 and 3 are the same; take one or the other (it makes no 234 difference - the same work has been done on our branch in 235 stage 2 and their branch in stage 3) 236 237 - stage 1 and stage 2 are the same and stage 3 is different; take 238 stage 3 (our branch in stage 2 did not do anything since the 239 ancestor in stage 1 while their branch in stage 3 worked on 240 it) 241 242 - stage 1 and stage 3 are the same and stage 2 is different take 243 stage 2 (we did something while they did nothing) 244 245The `git-write-tree` command refuses to write a nonsensical tree, and it 246will complain about unmerged entries if it sees a single entry that is not 247stage 0. 248 249OK, this all sounds like a collection of totally nonsensical rules, 250but it's actually exactly what you want in order to do a fast 251merge. The different stages represent the "result tree" (stage 0, aka 252"merged"), the original tree (stage 1, aka "orig"), and the two trees 253you are trying to merge (stage 2 and 3 respectively). 254 255The order of stages 1, 2 and 3 (hence the order of three 256<tree-ish> command line arguments) are significant when you 257start a 3-way merge with an index file that is already 258populated. Here is an outline of how the algorithm works: 259 260- if a file exists in identical format in all three trees, it will 261 automatically collapse to "merged" state by git-read-tree. 262 263- a file that has _any_ difference what-so-ever in the three trees 264 will stay as separate entries in the index. It's up to "porcelain 265 policy" to determine how to remove the non-0 stages, and insert a 266 merged version. 267 268- the index file saves and restores with all this information, so you 269 can merge things incrementally, but as long as it has entries in 270 stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So 271 now the merge algorithm ends up being really simple: 272 273 * you walk the index in order, and ignore all entries of stage 0, 274 since they've already been done. 275 276 * if you find a "stage1", but no matching "stage2" or "stage3", you 277 know it's been removed from both trees (it only existed in the 278 original tree), and you remove that entry. 279 280 * if you find a matching "stage2" and "stage3" tree, you remove one 281 of them, and turn the other into a "stage0" entry. Remove any 282 matching "stage1" entry if it exists too. .. all the normal 283 trivial rules .. 284 285You would normally use `git-merge-index` with supplied 286`git-merge-one-file` to do this last step. The script updates 287the files in the working tree as it merges each path and at the 288end of a successful merge. 289 290When you start a 3-way merge with an index file that is already 291populated, it is assumed that it represents the state of the 292files in your work tree, and you can even have files with 293changes unrecorded in the index file. It is further assumed 294that this state is "derived" from the stage 2 tree. The 3-way 295merge refuses to run if it finds an entry in the original index 296file that does not match stage 2. 297 298This is done to prevent you from losing your work-in-progress 299changes, and mixing your random changes in an unrelated merge 300commit. To illustrate, suppose you start from what has been 301committed last to your repository: 302 303---------------- 304$ JC=`git-rev-parse --verify "HEAD^0"` 305$ git-checkout-index -f -u -a $JC 306---------------- 307 308You do random edits, without running git-update-index. And then 309you notice that the tip of your "upstream" tree has advanced 310since you pulled from him: 311 312---------------- 313$ git-fetch git://.... linus 314$ LT=`cat .git/FETCH_HEAD` 315---------------- 316 317Your work tree is still based on your HEAD ($JC), but you have 318some edits since. Three-way merge makes sure that you have not 319added or modified index entries since $JC, and if you haven't, 320then does the right thing. So with the following sequence: 321 322---------------- 323$ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT 324$ git-merge-index git-merge-one-file -a 325$ echo "Merge with Linus" | \ 326 git-commit-tree `git-write-tree` -p $JC -p $LT 327---------------- 328 329what you would commit is a pure merge between $JC and $LT without 330your work-in-progress changes, and your work tree would be 331updated to the result of the merge. 332 333However, if you have local changes in the working tree that 334would be overwritten by this merge,`git-read-tree` will refuse 335to run to prevent your changes from being lost. 336 337In other words, there is no need to worry about what exists only 338in the working tree. When you have local changes in a part of 339the project that is not involved in the merge, your changes do 340not interfere with the merge, and are kept intact. When they 341*do* interfere, the merge does not even start (`git-read-tree` 342complains loudly and fails without modifying anything). In such 343a case, you can simply continue doing what you were in the 344middle of doing, and when your working tree is ready (i.e. you 345have finished your work-in-progress), attempt the merge again. 346 347 348See Also 349-------- 350linkgit:git-write-tree[1]; linkgit:git-ls-files[1]; 351linkgit:gitignore[5] 352 353 354Author 355------ 356Written by Linus Torvalds <torvalds@osdl.org> 357 358Documentation 359-------------- 360Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 361 362GIT 363--- 364Part of the linkgit:git[7] suite