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-v:: 54 Show the progress of checking files out. 55 56--trivial:: 57 Restrict three-way merge by `git-read-tree` to happen 58 only if there is no file-level merging required, instead 59 of resolving merge for trivial cases and leaving 60 conflicting files unresolved in the index. 61 62--aggressive:: 63 Usually a three-way merge by `git-read-tree` resolves 64 the merge for really trivial cases and leaves other 65 cases unresolved in the index, so that Porcelains can 66 implement different merge policies. This flag makes the 67 command to resolve a few more cases internally: 68+ 69* when one side removes a path and the other side leaves the path 70 unmodified. The resolution is to remove that path. 71* when both sides remove a path. The resolution is to remove that path. 72* when both sides adds a path identically. The resolution 73 is to add that path. 74 75--prefix=<prefix>/:: 76 Keep the current index contents, and read the contents 77 of named tree-ish under directory at `<prefix>`. The 78 original index file cannot have anything at the path 79 `<prefix>` itself, and have nothing in `<prefix>/` 80 directory. Note that the `<prefix>/` value must end 81 with a slash. 82 83--exclude-per-directory=<gitignore>:: 84 When running the command with `-u` and `-m` options, the 85 merge result may need to overwrite paths that are not 86 tracked in the current branch. The command usually 87 refuses to proceed with the merge to avoid losing such a 88 path. However this safety valve sometimes gets in the 89 way. For example, it often happens that the other 90 branch added a file that used to be a generated file in 91 your branch, and the safety valve triggers when you try 92 to switch to that branch after you ran `make` but before 93 running `make clean` to remove the generated file. This 94 option tells the command to read per-directory exclude 95 file (usually '.gitignore') and allows such an untracked 96 but explicitly ignored file to be overwritten. 97 98--index-output=<file>:: 99 Instead of writing the results out to `$GIT_INDEX_FILE`, 100 write the resulting index in the named file. While the 101 command is operating, the original index file is locked 102 with the same mechanism as usual. The file must allow 103 to be rename(2)ed into from a temporary file that is 104 created next to the usual index file; typically this 105 means it needs to be on the same filesystem as the index 106 file itself, and you need write permission to the 107 directories the index file and index output file are 108 located in. 109 110<tree-ish#>:: 111 The id of the tree object(s) to be read/merged. 112 113 114Merging 115------- 116If `-m` is specified, `git-read-tree` can perform 3 kinds of 117merge, a single tree merge if only 1 tree is given, a 118fast-forward merge with 2 trees, or a 3-way merge if 3 trees are 119provided. 120 121 122Single Tree Merge 123~~~~~~~~~~~~~~~~~ 124If only 1 tree is specified, git-read-tree operates as if the user did not 125specify `-m`, except that if the original index has an entry for a 126given pathname, and the contents of the path matches with the tree 127being read, the stat info from the index is used. (In other words, the 128index's stat()s take precedence over the merged tree's). 129 130That means that if you do a `git-read-tree -m <newtree>` followed by a 131`git-checkout-index -f -u -a`, the `git-checkout-index` only checks out 132the stuff that really changed. 133 134This is used to avoid unnecessary false hits when `git-diff-files` is 135run after `git-read-tree`. 136 137 138Two Tree Merge 139~~~~~~~~~~~~~~ 140 141Typically, this is invoked as `git-read-tree -m $H $M`, where $H 142is the head commit of the current repository, and $M is the head 143of a foreign tree, which is simply ahead of $H (i.e. we are in a 144fast forward situation). 145 146When two trees are specified, the user is telling git-read-tree 147the following: 148 149 1. The current index and work tree is derived from $H, but 150 the user may have local changes in them since $H; 151 152 2. The user wants to fast-forward to $M. 153 154In this case, the `git-read-tree -m $H $M` command makes sure 155that no local change is lost as the result of this "merge". 156Here are the "carry forward" rules: 157 158 I (index) H M Result 159 ------------------------------------------------------- 160 0 nothing nothing nothing (does not happen) 161 1 nothing nothing exists use M 162 2 nothing exists nothing remove path from index 163 3 nothing exists exists use M 164 165 clean I==H I==M 166 ------------------ 167 4 yes N/A N/A nothing nothing keep index 168 5 no N/A N/A nothing nothing keep index 169 170 6 yes N/A yes nothing exists keep index 171 7 no N/A yes nothing exists keep index 172 8 yes N/A no nothing exists fail 173 9 no N/A no nothing exists fail 174 175 10 yes yes N/A exists nothing remove path from index 176 11 no yes N/A exists nothing fail 177 12 yes no N/A exists nothing fail 178 13 no no N/A exists nothing fail 179 180 clean (H=M) 181 ------ 182 14 yes exists exists keep index 183 15 no exists exists keep index 184 185 clean I==H I==M (H!=M) 186 ------------------ 187 16 yes no no exists exists fail 188 17 no no no exists exists fail 189 18 yes no yes exists exists keep index 190 19 no no yes exists exists keep index 191 20 yes yes no exists exists use M 192 21 no yes no exists exists fail 193 194In all "keep index" cases, the index entry stays as in the 195original index file. If the entry were not up to date, 196git-read-tree keeps the copy in the work tree intact when 197operating under the -u flag. 198 199When this form of git-read-tree returns successfully, you can 200see what "local changes" you made are carried forward by running 201`git-diff-index --cached $M`. Note that this does not 202necessarily match `git-diff-index --cached $H` would have 203produced before such a two tree merge. This is because of cases 20418 and 19 --- if you already had the changes in $M (e.g. maybe 205you picked it up via e-mail in a patch form), `git-diff-index 206--cached $H` would have told you about the change before this 207merge, but it would not show in `git-diff-index --cached $M` 208output after two-tree merge. 209 210 2113-Way Merge 212~~~~~~~~~~~ 213Each "index" entry has two bits worth of "stage" state. stage 0 is the 214normal one, and is the only one you'd see in any kind of normal use. 215 216However, when you do `git-read-tree` with three trees, the "stage" 217starts out at 1. 218 219This means that you can do 220 221---------------- 222$ git-read-tree -m <tree1> <tree2> <tree3> 223---------------- 224 225and you will end up with an index with all of the <tree1> entries in 226"stage1", all of the <tree2> entries in "stage2" and all of the 227<tree3> entries in "stage3". When performing a merge of another 228branch into the current branch, we use the common ancestor tree 229as <tree1>, the current branch head as <tree2>, and the other 230branch head as <tree3>. 231 232Furthermore, `git-read-tree` has special-case logic that says: if you see 233a file that matches in all respects in the following states, it 234"collapses" back to "stage0": 235 236 - stage 2 and 3 are the same; take one or the other (it makes no 237 difference - the same work has been done on our branch in 238 stage 2 and their branch in stage 3) 239 240 - stage 1 and stage 2 are the same and stage 3 is different; take 241 stage 3 (our branch in stage 2 did not do anything since the 242 ancestor in stage 1 while their branch in stage 3 worked on 243 it) 244 245 - stage 1 and stage 3 are the same and stage 2 is different take 246 stage 2 (we did something while they did nothing) 247 248The `git-write-tree` command refuses to write a nonsensical tree, and it 249will complain about unmerged entries if it sees a single entry that is not 250stage 0. 251 252OK, this all sounds like a collection of totally nonsensical rules, 253but it's actually exactly what you want in order to do a fast 254merge. The different stages represent the "result tree" (stage 0, aka 255"merged"), the original tree (stage 1, aka "orig"), and the two trees 256you are trying to merge (stage 2 and 3 respectively). 257 258The order of stages 1, 2 and 3 (hence the order of three 259<tree-ish> command line arguments) are significant when you 260start a 3-way merge with an index file that is already 261populated. Here is an outline of how the algorithm works: 262 263- if a file exists in identical format in all three trees, it will 264 automatically collapse to "merged" state by git-read-tree. 265 266- a file that has _any_ difference what-so-ever in the three trees 267 will stay as separate entries in the index. It's up to "porcelain 268 policy" to determine how to remove the non-0 stages, and insert a 269 merged version. 270 271- the index file saves and restores with all this information, so you 272 can merge things incrementally, but as long as it has entries in 273 stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So 274 now the merge algorithm ends up being really simple: 275 276 * you walk the index in order, and ignore all entries of stage 0, 277 since they've already been done. 278 279 * if you find a "stage1", but no matching "stage2" or "stage3", you 280 know it's been removed from both trees (it only existed in the 281 original tree), and you remove that entry. 282 283 * if you find a matching "stage2" and "stage3" tree, you remove one 284 of them, and turn the other into a "stage0" entry. Remove any 285 matching "stage1" entry if it exists too. .. all the normal 286 trivial rules .. 287 288You would normally use `git-merge-index` with supplied 289`git-merge-one-file` to do this last step. The script updates 290the files in the working tree as it merges each path and at the 291end of a successful merge. 292 293When you start a 3-way merge with an index file that is already 294populated, it is assumed that it represents the state of the 295files in your work tree, and you can even have files with 296changes unrecorded in the index file. It is further assumed 297that this state is "derived" from the stage 2 tree. The 3-way 298merge refuses to run if it finds an entry in the original index 299file that does not match stage 2. 300 301This is done to prevent you from losing your work-in-progress 302changes, and mixing your random changes in an unrelated merge 303commit. To illustrate, suppose you start from what has been 304committed last to your repository: 305 306---------------- 307$ JC=`git-rev-parse --verify "HEAD^0"` 308$ git-checkout-index -f -u -a $JC 309---------------- 310 311You do random edits, without running git-update-index. And then 312you notice that the tip of your "upstream" tree has advanced 313since you pulled from him: 314 315---------------- 316$ git-fetch git://.... linus 317$ LT=`cat .git/FETCH_HEAD` 318---------------- 319 320Your work tree is still based on your HEAD ($JC), but you have 321some edits since. Three-way merge makes sure that you have not 322added or modified index entries since $JC, and if you haven't, 323then does the right thing. So with the following sequence: 324 325---------------- 326$ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT 327$ git-merge-index git-merge-one-file -a 328$ echo "Merge with Linus" | \ 329 git-commit-tree `git-write-tree` -p $JC -p $LT 330---------------- 331 332what you would commit is a pure merge between $JC and $LT without 333your work-in-progress changes, and your work tree would be 334updated to the result of the merge. 335 336However, if you have local changes in the working tree that 337would be overwritten by this merge,`git-read-tree` will refuse 338to run to prevent your changes from being lost. 339 340In other words, there is no need to worry about what exists only 341in the working tree. When you have local changes in a part of 342the project that is not involved in the merge, your changes do 343not interfere with the merge, and are kept intact. When they 344*do* interfere, the merge does not even start (`git-read-tree` 345complains loudly and fails without modifying anything). In such 346a case, you can simply continue doing what you were in the 347middle of doing, and when your working tree is ready (i.e. you 348have finished your work-in-progress), attempt the merge again. 349 350 351SEE ALSO 352-------- 353linkgit:git-write-tree[1]; linkgit:git-ls-files[1]; 354linkgit:gitignore[5] 355 356 357Author 358------ 359Written by Linus Torvalds <torvalds@osdl.org> 360 361Documentation 362-------------- 363Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 364 365GIT 366--- 367Part of the linkgit:git[1] suite