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' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] 12 [-u [--exclude-per-directory=<gitignore>] | -i]] 13 [--index-output=<file>] [--no-sparse-checkout] 14 <tree-ish1> [<tree-ish2> [<tree-ish3>]] 15 16 17DESCRIPTION 18----------- 19Reads the tree information given by <tree-ish> into the index, 20but does not actually *update* any of the files it "caches". (see: 21linkgit:git-checkout-index[1]) 22 23Optionally, it can merge a tree into the index, perform a 24fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m` 25flag. When used with `-m`, the `-u` flag causes it to also update 26the files in the work tree with the result of the merge. 27 28Trivial merges are done by 'git read-tree' itself. Only conflicting paths 29will be in unmerged state when 'git read-tree' returns. 30 31OPTIONS 32------- 33-m:: 34 Perform a merge, not just a read. The command will 35 refuse to run if your index file has unmerged entries, 36 indicating that you have not finished previous merge you 37 started. 38 39--reset:: 40 Same as -m, except that unmerged entries are discarded 41 instead of failing. 42 43-u:: 44 After a successful merge, update the files in the work 45 tree with the result of the merge. 46 47-i:: 48 Usually a merge requires the index file as well as the 49 files in the working tree are up to date with the 50 current head commit, in order not to lose local 51 changes. This flag disables the check with the working 52 tree and is meant to be used when creating a merge of 53 trees that are not directly related to the current 54 working tree status into a temporary index file. 55 56-v:: 57 Show the progress of checking files out. 58 59--trivial:: 60 Restrict three-way merge by 'git read-tree' to happen 61 only if there is no file-level merging required, instead 62 of resolving merge for trivial cases and leaving 63 conflicting files unresolved in the index. 64 65--aggressive:: 66 Usually a three-way merge by 'git read-tree' resolves 67 the merge for really trivial cases and leaves other 68 cases unresolved in the index, so that Porcelains can 69 implement different merge policies. This flag makes the 70 command to resolve a few more cases internally: 71+ 72* when one side removes a path and the other side leaves the path 73 unmodified. The resolution is to remove that path. 74* when both sides remove a path. The resolution is to remove that path. 75* when both sides adds a path identically. The resolution 76 is to add that path. 77 78--prefix=<prefix>/:: 79 Keep the current index contents, and read the contents 80 of named tree-ish under directory at `<prefix>`. The 81 original index file cannot have anything at the path 82 `<prefix>` itself, and have nothing in `<prefix>/` 83 directory. Note that the `<prefix>/` value must end 84 with a slash. 85 86--exclude-per-directory=<gitignore>:: 87 When running the command with `-u` and `-m` options, the 88 merge result may need to overwrite paths that are not 89 tracked in the current branch. The command usually 90 refuses to proceed with the merge to avoid losing such a 91 path. However this safety valve sometimes gets in the 92 way. For example, it often happens that the other 93 branch added a file that used to be a generated file in 94 your branch, and the safety valve triggers when you try 95 to switch to that branch after you ran `make` but before 96 running `make clean` to remove the generated file. This 97 option tells the command to read per-directory exclude 98 file (usually '.gitignore') and allows such an untracked 99 but explicitly ignored file to be overwritten. 100 101--index-output=<file>:: 102 Instead of writing the results out to `$GIT_INDEX_FILE`, 103 write the resulting index in the named file. While the 104 command is operating, the original index file is locked 105 with the same mechanism as usual. The file must allow 106 to be rename(2)ed into from a temporary file that is 107 created next to the usual index file; typically this 108 means it needs to be on the same filesystem as the index 109 file itself, and you need write permission to the 110 directories the index file and index output file are 111 located in. 112 113--no-sparse-checkout:: 114 Disable sparse checkout support even if `core.sparseCheckout` 115 is true. 116 117<tree-ish#>:: 118 The id of the tree object(s) to be read/merged. 119 120 121Merging 122------- 123If `-m` is specified, 'git read-tree' can perform 3 kinds of 124merge, a single tree merge if only 1 tree is given, a 125fast-forward merge with 2 trees, or a 3-way merge if 3 trees are 126provided. 127 128 129Single Tree Merge 130~~~~~~~~~~~~~~~~~ 131If only 1 tree is specified, 'git read-tree' operates as if the user did not 132specify `-m`, except that if the original index has an entry for a 133given pathname, and the contents of the path match with the tree 134being read, the stat info from the index is used. (In other words, the 135index's stat()s take precedence over the merged tree's). 136 137That means that if you do a `git read-tree -m <newtree>` followed by a 138`git checkout-index -f -u -a`, the 'git checkout-index' only checks out 139the stuff that really changed. 140 141This is used to avoid unnecessary false hits when 'git diff-files' is 142run after 'git read-tree'. 143 144 145Two Tree Merge 146~~~~~~~~~~~~~~ 147 148Typically, this is invoked as `git read-tree -m $H $M`, where $H 149is the head commit of the current repository, and $M is the head 150of a foreign tree, which is simply ahead of $H (i.e. we are in a 151fast-forward situation). 152 153When two trees are specified, the user is telling 'git read-tree' 154the following: 155 156 1. The current index and work tree is derived from $H, but 157 the user may have local changes in them since $H. 158 159 2. The user wants to fast-forward to $M. 160 161In this case, the `git read-tree -m $H $M` command makes sure 162that no local change is lost as the result of this "merge". 163Here are the "carry forward" rules, where "I" denotes the index, 164"clean" means that index and work tree coincide, and "exists"/"nothing" 165refer to the presence of a path in the specified commit: 166 167 I H M Result 168 ------------------------------------------------------- 169 0 nothing nothing nothing (does not happen) 170 1 nothing nothing exists use M 171 2 nothing exists nothing remove path from index 172 3 nothing exists exists, use M if "initial checkout", 173 H == M keep index otherwise 174 exists, fail 175 H != M 176 177 clean I==H I==M 178 ------------------ 179 4 yes N/A N/A nothing nothing keep index 180 5 no N/A N/A nothing nothing keep index 181 182 6 yes N/A yes nothing exists keep index 183 7 no N/A yes nothing exists keep index 184 8 yes N/A no nothing exists fail 185 9 no N/A no nothing exists fail 186 187 10 yes yes N/A exists nothing remove path from index 188 11 no yes N/A exists nothing fail 189 12 yes no N/A exists nothing fail 190 13 no no N/A exists nothing fail 191 192 clean (H==M) 193 ------ 194 14 yes exists exists keep index 195 15 no exists exists keep index 196 197 clean I==H I==M (H!=M) 198 ------------------ 199 16 yes no no exists exists fail 200 17 no no no exists exists fail 201 18 yes no yes exists exists keep index 202 19 no no yes exists exists keep index 203 20 yes yes no exists exists use M 204 21 no yes no exists exists fail 205 206In all "keep index" cases, the index entry stays as in the 207original index file. If the entry is not up to date, 208'git read-tree' keeps the copy in the work tree intact when 209operating under the -u flag. 210 211When this form of 'git read-tree' returns successfully, you can 212see which of the "local changes" that you made were carried forward by running 213`git diff-index --cached $M`. Note that this does not 214necessarily match what `git diff-index --cached $H` would have 215produced before such a two tree merge. This is because of cases 21618 and 19 --- if you already had the changes in $M (e.g. maybe 217you picked it up via e-mail in a patch form), `git diff-index 218--cached $H` would have told you about the change before this 219merge, but it would not show in `git diff-index --cached $M` 220output after the two-tree merge. 221 222Case 3 is slightly tricky and needs explanation. The result from this 223rule logically should be to remove the path if the user staged the removal 224of the path and then switching to a new branch. That however will prevent 225the initial checkout from happening, so the rule is modified to use M (new 226tree) only when the content of the index is empty. Otherwise the removal 227of the path is kept as long as $H and $M are the same. 228 2293-Way Merge 230~~~~~~~~~~~ 231Each "index" entry has two bits worth of "stage" state. stage 0 is the 232normal one, and is the only one you'd see in any kind of normal use. 233 234However, when you do 'git read-tree' with three trees, the "stage" 235starts out at 1. 236 237This means that you can do 238 239---------------- 240$ git read-tree -m <tree1> <tree2> <tree3> 241---------------- 242 243and you will end up with an index with all of the <tree1> entries in 244"stage1", all of the <tree2> entries in "stage2" and all of the 245<tree3> entries in "stage3". When performing a merge of another 246branch into the current branch, we use the common ancestor tree 247as <tree1>, the current branch head as <tree2>, and the other 248branch head as <tree3>. 249 250Furthermore, 'git read-tree' has special-case logic that says: if you see 251a file that matches in all respects in the following states, it 252"collapses" back to "stage0": 253 254 - stage 2 and 3 are the same; take one or the other (it makes no 255 difference - the same work has been done on our branch in 256 stage 2 and their branch in stage 3) 257 258 - stage 1 and stage 2 are the same and stage 3 is different; take 259 stage 3 (our branch in stage 2 did not do anything since the 260 ancestor in stage 1 while their branch in stage 3 worked on 261 it) 262 263 - stage 1 and stage 3 are the same and stage 2 is different take 264 stage 2 (we did something while they did nothing) 265 266The 'git write-tree' command refuses to write a nonsensical tree, and it 267will complain about unmerged entries if it sees a single entry that is not 268stage 0. 269 270OK, this all sounds like a collection of totally nonsensical rules, 271but it's actually exactly what you want in order to do a fast 272merge. The different stages represent the "result tree" (stage 0, aka 273"merged"), the original tree (stage 1, aka "orig"), and the two trees 274you are trying to merge (stage 2 and 3 respectively). 275 276The order of stages 1, 2 and 3 (hence the order of three 277<tree-ish> command line arguments) are significant when you 278start a 3-way merge with an index file that is already 279populated. Here is an outline of how the algorithm works: 280 281- if a file exists in identical format in all three trees, it will 282 automatically collapse to "merged" state by 'git read-tree'. 283 284- a file that has _any_ difference what-so-ever in the three trees 285 will stay as separate entries in the index. It's up to "porcelain 286 policy" to determine how to remove the non-0 stages, and insert a 287 merged version. 288 289- the index file saves and restores with all this information, so you 290 can merge things incrementally, but as long as it has entries in 291 stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So 292 now the merge algorithm ends up being really simple: 293 294 * you walk the index in order, and ignore all entries of stage 0, 295 since they've already been done. 296 297 * if you find a "stage1", but no matching "stage2" or "stage3", you 298 know it's been removed from both trees (it only existed in the 299 original tree), and you remove that entry. 300 301 * if you find a matching "stage2" and "stage3" tree, you remove one 302 of them, and turn the other into a "stage0" entry. Remove any 303 matching "stage1" entry if it exists too. .. all the normal 304 trivial rules .. 305 306You would normally use 'git merge-index' with supplied 307'git merge-one-file' to do this last step. The script updates 308the files in the working tree as it merges each path and at the 309end of a successful merge. 310 311When you start a 3-way merge with an index file that is already 312populated, it is assumed that it represents the state of the 313files in your work tree, and you can even have files with 314changes unrecorded in the index file. It is further assumed 315that this state is "derived" from the stage 2 tree. The 3-way 316merge refuses to run if it finds an entry in the original index 317file that does not match stage 2. 318 319This is done to prevent you from losing your work-in-progress 320changes, and mixing your random changes in an unrelated merge 321commit. To illustrate, suppose you start from what has been 322committed last to your repository: 323 324---------------- 325$ JC=`git rev-parse --verify "HEAD^0"` 326$ git checkout-index -f -u -a $JC 327---------------- 328 329You do random edits, without running 'git update-index'. And then 330you notice that the tip of your "upstream" tree has advanced 331since you pulled from him: 332 333---------------- 334$ git fetch git://.... linus 335$ LT=`cat .git/FETCH_HEAD` 336---------------- 337 338Your work tree is still based on your HEAD ($JC), but you have 339some edits since. Three-way merge makes sure that you have not 340added or modified index entries since $JC, and if you haven't, 341then does the right thing. So with the following sequence: 342 343---------------- 344$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT 345$ git merge-index git-merge-one-file -a 346$ echo "Merge with Linus" | \ 347 git commit-tree `git write-tree` -p $JC -p $LT 348---------------- 349 350what you would commit is a pure merge between $JC and $LT without 351your work-in-progress changes, and your work tree would be 352updated to the result of the merge. 353 354However, if you have local changes in the working tree that 355would be overwritten by this merge, 'git read-tree' will refuse 356to run to prevent your changes from being lost. 357 358In other words, there is no need to worry about what exists only 359in the working tree. When you have local changes in a part of 360the project that is not involved in the merge, your changes do 361not interfere with the merge, and are kept intact. When they 362*do* interfere, the merge does not even start ('git read-tree' 363complains loudly and fails without modifying anything). In such 364a case, you can simply continue doing what you were in the 365middle of doing, and when your working tree is ready (i.e. you 366have finished your work-in-progress), attempt the merge again. 367 368 369Sparse checkout 370--------------- 371 372"Sparse checkout" allows to sparsely populate working directory. 373It uses skip-worktree bit (see linkgit:git-update-index[1]) to tell 374Git whether a file on working directory is worth looking at. 375 376"git read-tree" and other merge-based commands ("git merge", "git 377checkout"...) can help maintaining skip-worktree bitmap and working 378directory update. `$GIT_DIR/info/sparse-checkout` is used to 379define the skip-worktree reference bitmap. When "git read-tree" needs 380to update working directory, it will reset skip-worktree bit in index 381based on this file, which uses the same syntax as .gitignore files. 382If an entry matches a pattern in this file, skip-worktree will be 383set on that entry. Otherwise, skip-worktree will be unset. 384 385Then it compares the new skip-worktree value with the previous one. If 386skip-worktree turns from unset to set, it will add the corresponding 387file back. If it turns from set to unset, that file will be removed. 388 389While `$GIT_DIR/info/sparse-checkout` is usually used to specify what 390files are in. You can also specify what files are _not_ in, using 391negate patterns. For example, to remove file "unwanted": 392 393---------------- 394* 395!unwanted 396---------------- 397 398Another tricky thing is fully repopulating working directory when you 399no longer want sparse checkout. You cannot just disable "sparse 400checkout" because skip-worktree are still in the index and you working 401directory is still sparsely populated. You should re-populate working 402directory with the `$GIT_DIR/info/sparse-checkout` file content as 403follows: 404 405---------------- 406* 407---------------- 408 409Then you can disable sparse checkout. Sparse checkout support in "git 410read-tree" and similar commands is disabled by default. You need to 411turn `core.sparseCheckout` on in order to have sparse checkout 412support. 413 414 415SEE ALSO 416-------- 417linkgit:git-write-tree[1]; linkgit:git-ls-files[1]; 418linkgit:gitignore[5] 419 420 421Author 422------ 423Written by Linus Torvalds <torvalds@osdl.org> 424 425Documentation 426-------------- 427Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 428 429GIT 430--- 431Part of the linkgit:git[1] suite