1git-read-tree(1) 2================ 3v0.1, May 2005 4 5NAME 6---- 7git-read-tree - Reads tree information into the directory cache 8 9 10SYNOPSIS 11-------- 12'git-read-tree' (<tree-ish> | [-m [-u|-i]] <tree-ish1> [<tree-ish2> [<tree-ish3>]]) 13 14 15DESCRIPTION 16----------- 17Reads the tree information given by <tree-ish> into the directory cache, 18but does not actually *update* any of the files it "caches". (see: 19git-checkout-index) 20 21Optionally, it can merge a tree into the cache, perform a 22fast-forward (i.e. 2-way) merge, or a 3-way merge, with the -m 23flag. When used with -m, the -u flag causes it to also update 24the files in the work tree with the result of the merge. 25 26Trivial merges are done by "git-read-tree" itself. Only conflicting paths 27will be in unmerged state when "git-read-tree" returns. 28 29OPTIONS 30------- 31-m:: 32 Perform a merge, not just a read. 33 34-u:: 35 After a successful merge, update the files in the work 36 tree with the result of the merge. 37 38-i:: 39 Usually a merge requires the index file as well as the 40 files in the working tree are up to date with the 41 current head commit, in order not to lose local 42 changes. This flag disables the check with the working 43 tree and is meant to be used when creating a merge of 44 trees that are not directly related to the current 45 working tree status into a temporary index file. 46 47 48<tree-ish#>:: 49 The id of the tree object(s) to be read/merged. 50 51 52Merging 53------- 54If '-m' is specified, "git-read-tree" can perform 3 kinds of 55merge, a single tree merge if only 1 tree is given, a 56fast-forward merge with 2 trees, or a 3-way merge if 3 trees are 57provided. 58 59 60Single Tree Merge 61~~~~~~~~~~~~~~~~~ 62If only 1 tree is specified, git-read-tree operates as if the user did not 63specify '-m', except that if the original cache has an entry for a 64given pathname, and the contents of the path matches with the tree 65being read, the stat info from the cache is used. (In other words, the 66cache's stat()s take precedence over the merged tree's). 67 68That means that if you do a "git-read-tree -m <newtree>" followed by a 69"git-checkout-index -f -u -a", the "git-checkout-index" only checks out 70the stuff that really changed. 71 72This is used to avoid unnecessary false hits when "git-diff-files" is 73run after git-read-tree. 74 75 76Two Tree Merge 77~~~~~~~~~~~~~~ 78 79Typically, this is invoked as "git-read-tree -m $H $M", where $H 80is the head commit of the current repository, and $M is the head 81of a foreign tree, which is simply ahead of $H (i.e. we are in a 82fast forward situation). 83 84When two trees are specified, the user is telling git-read-tree 85the following: 86 87 1. The current index and work tree is derived from $H, but 88 the user may have local changes in them since $H; 89 90 2. The user wants to fast-forward to $M. 91 92In this case, the "git-read-tree -m $H $M" command makes sure 93that no local change is lost as the result of this "merge". 94Here are the "carry forward" rules: 95 96 I (index) H M Result 97 ------------------------------------------------------- 98 0 nothing nothing nothing (does not happen) 99 1 nothing nothing exists use M 100 2 nothing exists nothing remove path from cache 101 3 nothing exists exists use M 102 103 clean I==H I==M 104 ------------------ 105 4 yes N/A N/A nothing nothing keep index 106 5 no N/A N/A nothing nothing keep index 107 108 6 yes N/A yes nothing exists keep index 109 7 no N/A yes nothing exists keep index 110 8 yes N/A no nothing exists fail 111 9 no N/A no nothing exists fail 112 113 10 yes yes N/A exists nothing remove path from cache 114 11 no yes N/A exists nothing fail 115 12 yes no N/A exists nothing fail 116 13 no no N/A exists nothing fail 117 118 clean (H=M) 119 ------ 120 14 yes exists exists keep index 121 15 no exists exists keep index 122 123 clean I==H I==M (H!=M) 124 ------------------ 125 16 yes no no exists exists fail 126 17 no no no exists exists fail 127 18 yes no yes exists exists keep index 128 19 no no yes exists exists keep index 129 20 yes yes no exists exists use M 130 21 no yes no exists exists fail 131 132In all "keep index" cases, the cache entry stays as in the 133original index file. If the entry were not up to date, 134git-read-tree keeps the copy in the work tree intact when 135operating under the -u flag. 136 137When this form of git-read-tree returns successfully, you can 138see what "local changes" you made are carried forward by running 139"git-diff-index --cached $M". Note that this does not 140necessarily match "git-diff-index --cached $H" would have 141produced before such a two tree merge. This is because of cases 14218 and 19 --- if you already had the changes in $M (e.g. maybe 143you picked it up via e-mail in a patch form), "git-diff-index 144--cached $H" would have told you about the change before this 145merge, but it would not show in "git-diff-index --cached $M" 146output after two-tree merge. 147 148 1493-Way Merge 150~~~~~~~~~~~ 151Each "index" entry has two bits worth of "stage" state. stage 0 is the 152normal one, and is the only one you'd see in any kind of normal use. 153 154However, when you do "git-read-tree" with three trees, the "stage" 155starts out at 1. 156 157This means that you can do 158 159 git-read-tree -m <tree1> <tree2> <tree3> 160 161and you will end up with an index with all of the <tree1> entries in 162"stage1", all of the <tree2> entries in "stage2" and all of the 163<tree3> entries in "stage3". 164 165Furthermore, "git-read-tree" has special-case logic that says: if you see 166a file that matches in all respects in the following states, it 167"collapses" back to "stage0": 168 169 - stage 2 and 3 are the same; take one or the other (it makes no 170 difference - the same work has been done on stage 2 and 3) 171 172 - stage 1 and stage 2 are the same and stage 3 is different; take 173 stage 3 (some work has been done on stage 3) 174 175 - stage 1 and stage 3 are the same and stage 2 is different take 176 stage 2 (some work has been done on stage 2) 177 178The "git-write-tree" command refuses to write a nonsensical tree, and it 179will complain about unmerged entries if it sees a single entry that is not 180stage 0. 181 182Ok, this all sounds like a collection of totally nonsensical rules, 183but it's actually exactly what you want in order to do a fast 184merge. The different stages represent the "result tree" (stage 0, aka 185"merged"), the original tree (stage 1, aka "orig"), and the two trees 186you are trying to merge (stage 2 and 3 respectively). 187 188The order of stages 1, 2 and 3 (hence the order of three 189<tree-ish> command line arguments) are significant when you 190start a 3-way merge with an index file that is already 191populated. Here is an outline of how the algorithm works: 192 193- if a file exists in identical format in all three trees, it will 194 automatically collapse to "merged" state by git-read-tree. 195 196- a file that has _any_ difference what-so-ever in the three trees 197 will stay as separate entries in the index. It's up to "porcelain 198 policy" to determine how to remove the non-0 stages, and insert a 199 merged version. 200 201- the index file saves and restores with all this information, so you 202 can merge things incrementally, but as long as it has entries in 203 stages 1/2/3 (ie "unmerged entries") you can't write the result. So 204 now the merge algorithm ends up being really simple: 205 206 * you walk the index in order, and ignore all entries of stage 0, 207 since they've already been done. 208 209 * if you find a "stage1", but no matching "stage2" or "stage3", you 210 know it's been removed from both trees (it only existed in the 211 original tree), and you remove that entry. 212 213 * if you find a matching "stage2" and "stage3" tree, you remove one 214 of them, and turn the other into a "stage0" entry. Remove any 215 matching "stage1" entry if it exists too. .. all the normal 216 trivial rules .. 217 218You would normally use "git-merge-index" with supplied 219"git-merge-one-file" to do this last step. The script 220does not touch the files in the work tree, and the entire merge 221happens in the index file. In other words, there is no need to 222worry about what is in the working directory, since it is never 223shown and never used. 224 225When you start a 3-way merge with an index file that is already 226populated, it is assumed that it represents the state of the 227files in your work tree, and you can even have files with 228changes unrecorded in the index file. It is further assumed 229that this state is "derived" from the stage 2 tree. The 3-way 230merge refuses to run if it finds an entry in the original index 231file that does not match stage 2. 232 233This is done to prevent you from losing your work-in-progress 234changes. To illustrate, suppose you start from what has been 235commited last to your repository: 236 237 $ JC=`cat .git/HEAD` 238 $ git-checkout-index -f -u -a $JC 239 240You do random edits, without running git-update-index. And then 241you notice that the tip of your "upstream" tree has advanced 242since you pulled from him: 243 244 $ git-fetch rsync://.... linus 245 $ LT=`cat .git/MERGE_HEAD` 246 247Your work tree is still based on your HEAD ($JC), but you have 248some edits since. Three-way merge makes sure that you have not 249added or modified cache entries since $JC, and if you haven't, 250then does the right thing. So with the following sequence: 251 252 $ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT 253 $ git-merge-index git-merge-one-file -a 254 $ echo "Merge with Linus" | \ 255 git-commit-tree `git-write-tree` -p $JC -p $LT 256 257what you would commit is a pure merge between $JC and LT without 258your work-in-progress changes, and your work tree would be 259updated to the result of the merge. 260 261 262See Also 263-------- 264gitlink:git-write-tree[1]; gitlink:git-ls-files[1] 265 266 267Author 268------ 269Written by Linus Torvalds <torvalds@osdl.org> 270 271Documentation 272-------------- 273Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 274 275GIT 276--- 277Part of the gitlink:git[7] suite 278