1git-read-tree(1) 2================ 3 4NAME 5---- 6git-read-tree - Reads tree information into the index 7 8 9SYNOPSIS 10-------- 11[verse] 12'git read-tree' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] 13 [-u [--exclude-per-directory=<gitignore>] | -i]] 14 [--index-output=<file>] [--no-sparse-checkout] 15 (--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]]) 16 17 18DESCRIPTION 19----------- 20Reads the tree information given by <tree-ish> into the index, 21but does not actually *update* any of the files it "caches". (see: 22linkgit:git-checkout-index[1]) 23 24Optionally, it can merge a tree into the index, perform a 25fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m` 26flag. When used with `-m`, the `-u` flag causes it to also update 27the files in the work tree with the result of the merge. 28 29Trivial merges are done by 'git read-tree' itself. Only conflicting paths 30will be in unmerged state when 'git read-tree' returns. 31 32OPTIONS 33------- 34-m:: 35 Perform a merge, not just a read. The command will 36 refuse to run if your index file has unmerged entries, 37 indicating that you have not finished previous merge you 38 started. 39 40--reset:: 41 Same as -m, except that unmerged entries are discarded 42 instead of failing. 43 44-u:: 45 After a successful merge, update the files in the work 46 tree with the result of the merge. 47 48-i:: 49 Usually a merge requires the index file as well as the 50 files in the working tree to be up to date with the 51 current head commit, in order not to lose local 52 changes. This flag disables the check with the working 53 tree and is meant to be used when creating a merge of 54 trees that are not directly related to the current 55 working tree status into a temporary index file. 56 57-n:: 58--dry-run:: 59 Check if the command would error out, without updating the index 60 or the files in the working tree for real. 61 62-v:: 63 Show the progress of checking files out. 64 65--trivial:: 66 Restrict three-way merge by 'git read-tree' to happen 67 only if there is no file-level merging required, instead 68 of resolving merge for trivial cases and leaving 69 conflicting files unresolved in the index. 70 71--aggressive:: 72 Usually a three-way merge by 'git read-tree' resolves 73 the merge for really trivial cases and leaves other 74 cases unresolved in the index, so that porcelains can 75 implement different merge policies. This flag makes the 76 command resolve a few more cases internally: 77+ 78* when one side removes a path and the other side leaves the path 79 unmodified. The resolution is to remove that path. 80* when both sides remove a path. The resolution is to remove that path. 81* when both sides add a path identically. The resolution 82 is to add that path. 83 84--prefix=<prefix>:: 85 Keep the current index contents, and read the contents 86 of the named tree-ish under the directory at `<prefix>`. 87 The command will refuse to overwrite entries that already 88 existed in the original index file. 89 90--exclude-per-directory=<gitignore>:: 91 When running the command with `-u` and `-m` options, the 92 merge result may need to overwrite paths that are not 93 tracked in the current branch. The command usually 94 refuses to proceed with the merge to avoid losing such a 95 path. However this safety valve sometimes gets in the 96 way. For example, it often happens that the other 97 branch added a file that used to be a generated file in 98 your branch, and the safety valve triggers when you try 99 to switch to that branch after you ran `make` but before 100 running `make clean` to remove the generated file. This 101 option tells the command to read per-directory exclude 102 file (usually '.gitignore') and allows such an untracked 103 but explicitly ignored file to be overwritten. 104 105--index-output=<file>:: 106 Instead of writing the results out to `$GIT_INDEX_FILE`, 107 write the resulting index in the named file. While the 108 command is operating, the original index file is locked 109 with the same mechanism as usual. The file must allow 110 to be rename(2)ed into from a temporary file that is 111 created next to the usual index file; typically this 112 means it needs to be on the same filesystem as the index 113 file itself, and you need write permission to the 114 directories the index file and index output file are 115 located in. 116 117--[no-]recurse-submodules:: 118 Using --recurse-submodules will update the content of all initialized 119 submodules according to the commit recorded in the superproject by 120 calling read-tree recursively, also setting the submodules HEAD to be 121 detached at that commit. 122 123--no-sparse-checkout:: 124 Disable sparse checkout support even if `core.sparseCheckout` 125 is true. 126 127--empty:: 128 Instead of reading tree object(s) into the index, just empty 129 it. 130 131<tree-ish#>:: 132 The id of the tree object(s) to be read/merged. 133 134 135MERGING 136------- 137If `-m` is specified, 'git read-tree' can perform 3 kinds of 138merge, a single tree merge if only 1 tree is given, a 139fast-forward merge with 2 trees, or a 3-way merge if 3 or more trees are 140provided. 141 142 143Single Tree Merge 144~~~~~~~~~~~~~~~~~ 145If only 1 tree is specified, 'git read-tree' operates as if the user did not 146specify `-m`, except that if the original index has an entry for a 147given pathname, and the contents of the path match with the tree 148being read, the stat info from the index is used. (In other words, the 149index's stat()s take precedence over the merged tree's). 150 151That means that if you do a `git read-tree -m <newtree>` followed by a 152`git checkout-index -f -u -a`, the 'git checkout-index' only checks out 153the stuff that really changed. 154 155This is used to avoid unnecessary false hits when 'git diff-files' is 156run after 'git read-tree'. 157 158 159Two Tree Merge 160~~~~~~~~~~~~~~ 161 162Typically, this is invoked as `git read-tree -m $H $M`, where $H 163is the head commit of the current repository, and $M is the head 164of a foreign tree, which is simply ahead of $H (i.e. we are in a 165fast-forward situation). 166 167When two trees are specified, the user is telling 'git read-tree' 168the following: 169 170 1. The current index and work tree is derived from $H, but 171 the user may have local changes in them since $H. 172 173 2. The user wants to fast-forward to $M. 174 175In this case, the `git read-tree -m $H $M` command makes sure 176that no local change is lost as the result of this "merge". 177Here are the "carry forward" rules, where "I" denotes the index, 178"clean" means that index and work tree coincide, and "exists"/"nothing" 179refer to the presence of a path in the specified commit: 180 181.... 182 I H M Result 183 ------------------------------------------------------- 184 0 nothing nothing nothing (does not happen) 185 1 nothing nothing exists use M 186 2 nothing exists nothing remove path from index 187 3 nothing exists exists, use M if "initial checkout", 188 H == M keep index otherwise 189 exists, fail 190 H != M 191 192 clean I==H I==M 193 ------------------ 194 4 yes N/A N/A nothing nothing keep index 195 5 no N/A N/A nothing nothing keep index 196 197 6 yes N/A yes nothing exists keep index 198 7 no N/A yes nothing exists keep index 199 8 yes N/A no nothing exists fail 200 9 no N/A no nothing exists fail 201 202 10 yes yes N/A exists nothing remove path from index 203 11 no yes N/A exists nothing fail 204 12 yes no N/A exists nothing fail 205 13 no no N/A exists nothing fail 206 207 clean (H==M) 208 ------ 209 14 yes exists exists keep index 210 15 no exists exists keep index 211 212 clean I==H I==M (H!=M) 213 ------------------ 214 16 yes no no exists exists fail 215 17 no no no exists exists fail 216 18 yes no yes exists exists keep index 217 19 no no yes exists exists keep index 218 20 yes yes no exists exists use M 219 21 no yes no exists exists fail 220.... 221 222In all "keep index" cases, the index entry stays as in the 223original index file. If the entry is not up to date, 224'git read-tree' keeps the copy in the work tree intact when 225operating under the -u flag. 226 227When this form of 'git read-tree' returns successfully, you can 228see which of the "local changes" that you made were carried forward by running 229`git diff-index --cached $M`. Note that this does not 230necessarily match what `git diff-index --cached $H` would have 231produced before such a two tree merge. This is because of cases 23218 and 19 --- if you already had the changes in $M (e.g. maybe 233you picked it up via e-mail in a patch form), `git diff-index 234--cached $H` would have told you about the change before this 235merge, but it would not show in `git diff-index --cached $M` 236output after the two-tree merge. 237 238Case 3 is slightly tricky and needs explanation. The result from this 239rule logically should be to remove the path if the user staged the removal 240of the path and then switching to a new branch. That however will prevent 241the initial checkout from happening, so the rule is modified to use M (new 242tree) only when the content of the index is empty. Otherwise the removal 243of the path is kept as long as $H and $M are the same. 244 2453-Way Merge 246~~~~~~~~~~~ 247Each "index" entry has two bits worth of "stage" state. stage 0 is the 248normal one, and is the only one you'd see in any kind of normal use. 249 250However, when you do 'git read-tree' with three trees, the "stage" 251starts out at 1. 252 253This means that you can do 254 255---------------- 256$ git read-tree -m <tree1> <tree2> <tree3> 257---------------- 258 259and you will end up with an index with all of the <tree1> entries in 260"stage1", all of the <tree2> entries in "stage2" and all of the 261<tree3> entries in "stage3". When performing a merge of another 262branch into the current branch, we use the common ancestor tree 263as <tree1>, the current branch head as <tree2>, and the other 264branch head as <tree3>. 265 266Furthermore, 'git read-tree' has special-case logic that says: if you see 267a file that matches in all respects in the following states, it 268"collapses" back to "stage0": 269 270 - stage 2 and 3 are the same; take one or the other (it makes no 271 difference - the same work has been done on our branch in 272 stage 2 and their branch in stage 3) 273 274 - stage 1 and stage 2 are the same and stage 3 is different; take 275 stage 3 (our branch in stage 2 did not do anything since the 276 ancestor in stage 1 while their branch in stage 3 worked on 277 it) 278 279 - stage 1 and stage 3 are the same and stage 2 is different take 280 stage 2 (we did something while they did nothing) 281 282The 'git write-tree' command refuses to write a nonsensical tree, and it 283will complain about unmerged entries if it sees a single entry that is not 284stage 0. 285 286OK, this all sounds like a collection of totally nonsensical rules, 287but it's actually exactly what you want in order to do a fast 288merge. The different stages represent the "result tree" (stage 0, aka 289"merged"), the original tree (stage 1, aka "orig"), and the two trees 290you are trying to merge (stage 2 and 3 respectively). 291 292The order of stages 1, 2 and 3 (hence the order of three 293<tree-ish> command-line arguments) are significant when you 294start a 3-way merge with an index file that is already 295populated. Here is an outline of how the algorithm works: 296 297- if a file exists in identical format in all three trees, it will 298 automatically collapse to "merged" state by 'git read-tree'. 299 300- a file that has _any_ difference what-so-ever in the three trees 301 will stay as separate entries in the index. It's up to "porcelain 302 policy" to determine how to remove the non-0 stages, and insert a 303 merged version. 304 305- the index file saves and restores with all this information, so you 306 can merge things incrementally, but as long as it has entries in 307 stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So 308 now the merge algorithm ends up being really simple: 309 310 * you walk the index in order, and ignore all entries of stage 0, 311 since they've already been done. 312 313 * if you find a "stage1", but no matching "stage2" or "stage3", you 314 know it's been removed from both trees (it only existed in the 315 original tree), and you remove that entry. 316 317 * if you find a matching "stage2" and "stage3" tree, you remove one 318 of them, and turn the other into a "stage0" entry. Remove any 319 matching "stage1" entry if it exists too. .. all the normal 320 trivial rules .. 321 322You would normally use 'git merge-index' with supplied 323'git merge-one-file' to do this last step. The script updates 324the files in the working tree as it merges each path and at the 325end of a successful merge. 326 327When you start a 3-way merge with an index file that is already 328populated, it is assumed that it represents the state of the 329files in your work tree, and you can even have files with 330changes unrecorded in the index file. It is further assumed 331that this state is "derived" from the stage 2 tree. The 3-way 332merge refuses to run if it finds an entry in the original index 333file that does not match stage 2. 334 335This is done to prevent you from losing your work-in-progress 336changes, and mixing your random changes in an unrelated merge 337commit. To illustrate, suppose you start from what has been 338committed last to your repository: 339 340---------------- 341$ JC=`git rev-parse --verify "HEAD^0"` 342$ git checkout-index -f -u -a $JC 343---------------- 344 345You do random edits, without running 'git update-index'. And then 346you notice that the tip of your "upstream" tree has advanced 347since you pulled from him: 348 349---------------- 350$ git fetch git://.... linus 351$ LT=`git rev-parse FETCH_HEAD` 352---------------- 353 354Your work tree is still based on your HEAD ($JC), but you have 355some edits since. Three-way merge makes sure that you have not 356added or modified index entries since $JC, and if you haven't, 357then does the right thing. So with the following sequence: 358 359---------------- 360$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT 361$ git merge-index git-merge-one-file -a 362$ echo "Merge with Linus" | \ 363 git commit-tree `git write-tree` -p $JC -p $LT 364---------------- 365 366what you would commit is a pure merge between $JC and $LT without 367your work-in-progress changes, and your work tree would be 368updated to the result of the merge. 369 370However, if you have local changes in the working tree that 371would be overwritten by this merge, 'git read-tree' will refuse 372to run to prevent your changes from being lost. 373 374In other words, there is no need to worry about what exists only 375in the working tree. When you have local changes in a part of 376the project that is not involved in the merge, your changes do 377not interfere with the merge, and are kept intact. When they 378*do* interfere, the merge does not even start ('git read-tree' 379complains loudly and fails without modifying anything). In such 380a case, you can simply continue doing what you were in the 381middle of doing, and when your working tree is ready (i.e. you 382have finished your work-in-progress), attempt the merge again. 383 384 385SPARSE CHECKOUT 386--------------- 387 388"Sparse checkout" allows populating the working directory sparsely. 389It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell 390Git whether a file in the working directory is worth looking at. 391 392'git read-tree' and other merge-based commands ('git merge', 'git 393checkout'...) can help maintaining the skip-worktree bitmap and working 394directory update. `$GIT_DIR/info/sparse-checkout` is used to 395define the skip-worktree reference bitmap. When 'git read-tree' needs 396to update the working directory, it resets the skip-worktree bit in the index 397based on this file, which uses the same syntax as .gitignore files. 398If an entry matches a pattern in this file, skip-worktree will not be 399set on that entry. Otherwise, skip-worktree will be set. 400 401Then it compares the new skip-worktree value with the previous one. If 402skip-worktree turns from set to unset, it will add the corresponding 403file back. If it turns from unset to set, that file will be removed. 404 405While `$GIT_DIR/info/sparse-checkout` is usually used to specify what 406files are in, you can also specify what files are _not_ in, using 407negate patterns. For example, to remove the file `unwanted`: 408 409---------------- 410/* 411!unwanted 412---------------- 413 414Another tricky thing is fully repopulating the working directory when you 415no longer want sparse checkout. You cannot just disable "sparse 416checkout" because skip-worktree bits are still in the index and your working 417directory is still sparsely populated. You should re-populate the working 418directory with the `$GIT_DIR/info/sparse-checkout` file content as 419follows: 420 421---------------- 422/* 423---------------- 424 425Then you can disable sparse checkout. Sparse checkout support in 'git 426read-tree' and similar commands is disabled by default. You need to 427turn `core.sparseCheckout` on in order to have sparse checkout 428support. 429 430 431SEE ALSO 432-------- 433linkgit:git-write-tree[1]; linkgit:git-ls-files[1]; 434linkgit:gitignore[5] 435 436GIT 437--- 438Part of the linkgit:git[1] suite