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. Note that the `<prefix>/` 89 value must end with a slash. 90 91--exclude-per-directory=<gitignore>:: 92 When running the command with `-u` and `-m` options, the 93 merge result may need to overwrite paths that are not 94 tracked in the current branch. The command usually 95 refuses to proceed with the merge to avoid losing such a 96 path. However this safety valve sometimes gets in the 97 way. For example, it often happens that the other 98 branch added a file that used to be a generated file in 99 your branch, and the safety valve triggers when you try 100 to switch to that branch after you ran `make` but before 101 running `make clean` to remove the generated file. This 102 option tells the command to read per-directory exclude 103 file (usually '.gitignore') and allows such an untracked 104 but explicitly ignored file to be overwritten. 105 106--index-output=<file>:: 107 Instead of writing the results out to `$GIT_INDEX_FILE`, 108 write the resulting index in the named file. While the 109 command is operating, the original index file is locked 110 with the same mechanism as usual. The file must allow 111 to be rename(2)ed into from a temporary file that is 112 created next to the usual index file; typically this 113 means it needs to be on the same filesystem as the index 114 file itself, and you need write permission to the 115 directories the index file and index output file are 116 located in. 117 118--[no-]recurse-submodules:: 119 Using --recurse-submodules will update the content of all initialized 120 submodules according to the commit recorded in the superproject by 121 calling read-tree recursively, also setting the submodules HEAD to be 122 detached at that commit. 123 124--no-sparse-checkout:: 125 Disable sparse checkout support even if `core.sparseCheckout` 126 is true. 127 128--empty:: 129 Instead of reading tree object(s) into the index, just empty 130 it. 131 132<tree-ish#>:: 133 The id of the tree object(s) to be read/merged. 134 135 136Merging 137------- 138If `-m` is specified, 'git read-tree' can perform 3 kinds of 139merge, a single tree merge if only 1 tree is given, a 140fast-forward merge with 2 trees, or a 3-way merge if 3 or more trees are 141provided. 142 143 144Single Tree Merge 145~~~~~~~~~~~~~~~~~ 146If only 1 tree is specified, 'git read-tree' operates as if the user did not 147specify `-m`, except that if the original index has an entry for a 148given pathname, and the contents of the path match with the tree 149being read, the stat info from the index is used. (In other words, the 150index's stat()s take precedence over the merged tree's). 151 152That means that if you do a `git read-tree -m <newtree>` followed by a 153`git checkout-index -f -u -a`, the 'git checkout-index' only checks out 154the stuff that really changed. 155 156This is used to avoid unnecessary false hits when 'git diff-files' is 157run after 'git read-tree'. 158 159 160Two Tree Merge 161~~~~~~~~~~~~~~ 162 163Typically, this is invoked as `git read-tree -m $H $M`, where $H 164is the head commit of the current repository, and $M is the head 165of a foreign tree, which is simply ahead of $H (i.e. we are in a 166fast-forward situation). 167 168When two trees are specified, the user is telling 'git read-tree' 169the following: 170 171 1. The current index and work tree is derived from $H, but 172 the user may have local changes in them since $H. 173 174 2. The user wants to fast-forward to $M. 175 176In this case, the `git read-tree -m $H $M` command makes sure 177that no local change is lost as the result of this "merge". 178Here are the "carry forward" rules, where "I" denotes the index, 179"clean" means that index and work tree coincide, and "exists"/"nothing" 180refer to the presence of a path in the specified commit: 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 221In all "keep index" cases, the index entry stays as in the 222original index file. If the entry is not up to date, 223'git read-tree' keeps the copy in the work tree intact when 224operating under the -u flag. 225 226When this form of 'git read-tree' returns successfully, you can 227see which of the "local changes" that you made were carried forward by running 228`git diff-index --cached $M`. Note that this does not 229necessarily match what `git diff-index --cached $H` would have 230produced before such a two tree merge. This is because of cases 23118 and 19 --- if you already had the changes in $M (e.g. maybe 232you picked it up via e-mail in a patch form), `git diff-index 233--cached $H` would have told you about the change before this 234merge, but it would not show in `git diff-index --cached $M` 235output after the two-tree merge. 236 237Case 3 is slightly tricky and needs explanation. The result from this 238rule logically should be to remove the path if the user staged the removal 239of the path and then switching to a new branch. That however will prevent 240the initial checkout from happening, so the rule is modified to use M (new 241tree) only when the content of the index is empty. Otherwise the removal 242of the path is kept as long as $H and $M are the same. 243 2443-Way Merge 245~~~~~~~~~~~ 246Each "index" entry has two bits worth of "stage" state. stage 0 is the 247normal one, and is the only one you'd see in any kind of normal use. 248 249However, when you do 'git read-tree' with three trees, the "stage" 250starts out at 1. 251 252This means that you can do 253 254---------------- 255$ git read-tree -m <tree1> <tree2> <tree3> 256---------------- 257 258and you will end up with an index with all of the <tree1> entries in 259"stage1", all of the <tree2> entries in "stage2" and all of the 260<tree3> entries in "stage3". When performing a merge of another 261branch into the current branch, we use the common ancestor tree 262as <tree1>, the current branch head as <tree2>, and the other 263branch head as <tree3>. 264 265Furthermore, 'git read-tree' has special-case logic that says: if you see 266a file that matches in all respects in the following states, it 267"collapses" back to "stage0": 268 269 - stage 2 and 3 are the same; take one or the other (it makes no 270 difference - the same work has been done on our branch in 271 stage 2 and their branch in stage 3) 272 273 - stage 1 and stage 2 are the same and stage 3 is different; take 274 stage 3 (our branch in stage 2 did not do anything since the 275 ancestor in stage 1 while their branch in stage 3 worked on 276 it) 277 278 - stage 1 and stage 3 are the same and stage 2 is different take 279 stage 2 (we did something while they did nothing) 280 281The 'git write-tree' command refuses to write a nonsensical tree, and it 282will complain about unmerged entries if it sees a single entry that is not 283stage 0. 284 285OK, this all sounds like a collection of totally nonsensical rules, 286but it's actually exactly what you want in order to do a fast 287merge. The different stages represent the "result tree" (stage 0, aka 288"merged"), the original tree (stage 1, aka "orig"), and the two trees 289you are trying to merge (stage 2 and 3 respectively). 290 291The order of stages 1, 2 and 3 (hence the order of three 292<tree-ish> command-line arguments) are significant when you 293start a 3-way merge with an index file that is already 294populated. Here is an outline of how the algorithm works: 295 296- if a file exists in identical format in all three trees, it will 297 automatically collapse to "merged" state by 'git read-tree'. 298 299- a file that has _any_ difference what-so-ever in the three trees 300 will stay as separate entries in the index. It's up to "porcelain 301 policy" to determine how to remove the non-0 stages, and insert a 302 merged version. 303 304- the index file saves and restores with all this information, so you 305 can merge things incrementally, but as long as it has entries in 306 stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So 307 now the merge algorithm ends up being really simple: 308 309 * you walk the index in order, and ignore all entries of stage 0, 310 since they've already been done. 311 312 * if you find a "stage1", but no matching "stage2" or "stage3", you 313 know it's been removed from both trees (it only existed in the 314 original tree), and you remove that entry. 315 316 * if you find a matching "stage2" and "stage3" tree, you remove one 317 of them, and turn the other into a "stage0" entry. Remove any 318 matching "stage1" entry if it exists too. .. all the normal 319 trivial rules .. 320 321You would normally use 'git merge-index' with supplied 322'git merge-one-file' to do this last step. The script updates 323the files in the working tree as it merges each path and at the 324end of a successful merge. 325 326When you start a 3-way merge with an index file that is already 327populated, it is assumed that it represents the state of the 328files in your work tree, and you can even have files with 329changes unrecorded in the index file. It is further assumed 330that this state is "derived" from the stage 2 tree. The 3-way 331merge refuses to run if it finds an entry in the original index 332file that does not match stage 2. 333 334This is done to prevent you from losing your work-in-progress 335changes, and mixing your random changes in an unrelated merge 336commit. To illustrate, suppose you start from what has been 337committed last to your repository: 338 339---------------- 340$ JC=`git rev-parse --verify "HEAD^0"` 341$ git checkout-index -f -u -a $JC 342---------------- 343 344You do random edits, without running 'git update-index'. And then 345you notice that the tip of your "upstream" tree has advanced 346since you pulled from him: 347 348---------------- 349$ git fetch git://.... linus 350$ LT=`git rev-parse FETCH_HEAD` 351---------------- 352 353Your work tree is still based on your HEAD ($JC), but you have 354some edits since. Three-way merge makes sure that you have not 355added or modified index entries since $JC, and if you haven't, 356then does the right thing. So with the following sequence: 357 358---------------- 359$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT 360$ git merge-index git-merge-one-file -a 361$ echo "Merge with Linus" | \ 362 git commit-tree `git write-tree` -p $JC -p $LT 363---------------- 364 365what you would commit is a pure merge between $JC and $LT without 366your work-in-progress changes, and your work tree would be 367updated to the result of the merge. 368 369However, if you have local changes in the working tree that 370would be overwritten by this merge, 'git read-tree' will refuse 371to run to prevent your changes from being lost. 372 373In other words, there is no need to worry about what exists only 374in the working tree. When you have local changes in a part of 375the project that is not involved in the merge, your changes do 376not interfere with the merge, and are kept intact. When they 377*do* interfere, the merge does not even start ('git read-tree' 378complains loudly and fails without modifying anything). In such 379a case, you can simply continue doing what you were in the 380middle of doing, and when your working tree is ready (i.e. you 381have finished your work-in-progress), attempt the merge again. 382 383 384Sparse checkout 385--------------- 386 387"Sparse checkout" allows populating the working directory sparsely. 388It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell 389Git whether a file in the working directory is worth looking at. 390 391'git read-tree' and other merge-based commands ('git merge', 'git 392checkout'...) can help maintaining the skip-worktree bitmap and working 393directory update. `$GIT_DIR/info/sparse-checkout` is used to 394define the skip-worktree reference bitmap. When 'git read-tree' needs 395to update the working directory, it resets the skip-worktree bit in the index 396based on this file, which uses the same syntax as .gitignore files. 397If an entry matches a pattern in this file, skip-worktree will not be 398set on that entry. Otherwise, skip-worktree will be set. 399 400Then it compares the new skip-worktree value with the previous one. If 401skip-worktree turns from set to unset, it will add the corresponding 402file back. If it turns from unset to set, that file will be removed. 403 404While `$GIT_DIR/info/sparse-checkout` is usually used to specify what 405files are in, you can also specify what files are _not_ in, using 406negate patterns. For example, to remove the file `unwanted`: 407 408---------------- 409/* 410!unwanted 411---------------- 412 413Another tricky thing is fully repopulating the working directory when you 414no longer want sparse checkout. You cannot just disable "sparse 415checkout" because skip-worktree bits are still in the index and your working 416directory is still sparsely populated. You should re-populate the working 417directory with the `$GIT_DIR/info/sparse-checkout` file content as 418follows: 419 420---------------- 421/* 422---------------- 423 424Then you can disable sparse checkout. Sparse checkout support in 'git 425read-tree' and similar commands is disabled by default. You need to 426turn `core.sparseCheckout` on in order to have sparse checkout 427support. 428 429 430SEE ALSO 431-------- 432linkgit:git-write-tree[1]; linkgit:git-ls-files[1]; 433linkgit:gitignore[5] 434 435GIT 436--- 437Part of the linkgit:git[1] suite