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]] <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-cache) 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<tree-ish#>:: 39 The id of the tree object(s) to be read/merged. 40 41 42Merging 43------- 44If '-m' is specified, "git-read-tree" can perform 3 kinds of 45merge, a single tree merge if only 1 tree is given, a 46fast-forward merge with 2 trees, or a 3-way merge if 3 trees are 47provided. 48 49 50Single Tree Merge 51~~~~~~~~~~~~~~~~~ 52If only 1 tree is specified, git-read-tree operates as if the user did not 53specify '-m', except that if the original cache has an entry for a 54given pathname, and the contents of the path matches with the tree 55being read, the stat info from the cache is used. (In other words, the 56cache's stat()s take precedence over the merged tree's). 57 58That means that if you do a "git-read-tree -m <newtree>" followed by a 59"git-checkout-cache -f -u -a", the "git-checkout-cache" only checks out 60the stuff that really changed. 61 62This is used to avoid unnecessary false hits when "git-diff-files" is 63run after git-read-tree. 64 65 66Two Tree Merge 67~~~~~~~~~~~~~~ 68 69Typically, this is invoked as "git-read-tree -m $H $M", where $H 70is the head commit of the current repository, and $M is the head 71of a foreign tree, which is simply ahead of $H (i.e. we are in a 72fast forward situation). 73 74When two trees are specified, the user is telling git-read-tree 75the following: 76 77 (1) The current index and work tree is derived from $H, but 78 the user may have local changes in them since $H; 79 80 (2) The user wants to fast-forward to $M. 81 82In this case, the "git-read-tree -m $H $M" command makes sure 83that no local change is lost as the result of this "merge". 84Here are the "carry forward" rules: 85 86 I (index) H M Result 87 ------------------------------------------------------- 88 0 nothing nothing nothing (does not happen) 89 1 nothing nothing exists use M 90 2 nothing exists nothing remove path from cache 91 3 nothing exists exists use M 92 93 clean I==H I==M 94 ------------------ 95 4 yes N/A N/A nothing nothing keep index 96 5 no N/A N/A nothing nothing keep index 97 98 6 yes N/A yes nothing exists keep index 99 7 no N/A yes nothing exists keep index 100 8 yes N/A no nothing exists fail 101 9 no N/A no nothing exists fail 102 103 10 yes yes N/A exists nothing remove path from cache 104 11 no yes N/A exists nothing fail 105 12 yes no N/A exists nothing fail 106 13 no no N/A exists nothing fail 107 108 clean (H=M) 109 ------ 110 14 yes exists exists keep index 111 15 no exists exists keep index 112 113 clean I==H I==M (H!=M) 114 ------------------ 115 16 yes no no exists exists fail 116 17 no no no exists exists fail 117 18 yes no yes exists exists keep index 118 19 no no yes exists exists keep index 119 20 yes yes no exists exists use M 120 21 no yes no exists exists fail 121 122In all "keep index" cases, the cache entry stays as in the 123original index file. If the entry were not up to date, 124git-read-tree keeps the copy in the work tree intact when 125operating under the -u flag. 126 127When this form of git-read-tree returns successfully, you can 128see what "local changes" you made are carried forward by running 129"git-diff-cache --cached $M". Note that this does not 130necessarily match "git-diff-cache --cached $H" would have 131produced before such a two tree merge. This is because of cases 13218 and 19 --- if you already had the changes in $M (e.g. maybe 133you picked it up via e-mail in a patch form), "git-diff-cache 134--cached $H" would have told you about the change before this 135merge, but it would not show in "git-diff-cache --cached $M" 136output after two-tree merge. 137 138 1393-Way Merge 140~~~~~~~~~~~ 141Each "index" entry has two bits worth of "stage" state. stage 0 is the 142normal one, and is the only one you'd see in any kind of normal use. 143 144However, when you do "git-read-tree" with three trees, the "stage" 145starts out at 1. 146 147This means that you can do 148 149 git-read-tree -m <tree1> <tree2> <tree3> 150 151and you will end up with an index with all of the <tree1> entries in 152"stage1", all of the <tree2> entries in "stage2" and all of the 153<tree3> entries in "stage3". 154 155Furthermore, "git-read-tree" has special-case logic that says: if you see 156a file that matches in all respects in the following states, it 157"collapses" back to "stage0": 158 159 - stage 2 and 3 are the same; take one or the other (it makes no 160 difference - the same work has been done on stage 2 and 3) 161 162 - stage 1 and stage 2 are the same and stage 3 is different; take 163 stage 3 (some work has been done on stage 3) 164 165 - stage 1 and stage 3 are the same and stage 2 is different take 166 stage 2 (some work has been done on stage 2) 167 168The "git-write-tree" command refuses to write a nonsensical tree, and it 169will complain about unmerged entries if it sees a single entry that is not 170stage 0. 171 172Ok, this all sounds like a collection of totally nonsensical rules, 173but it's actually exactly what you want in order to do a fast 174merge. The different stages represent the "result tree" (stage 0, aka 175"merged"), the original tree (stage 1, aka "orig"), and the two trees 176you are trying to merge (stage 2 and 3 respectively). 177 178The order of stages 1, 2 and 3 (hence the order of three 179<tree-ish> command line arguments) are significant when you 180start a 3-way merge with an index file that is already 181populated. Here is an outline of how the algorithm works: 182 183- if a file exists in identical format in all three trees, it will 184 automatically collapse to "merged" state by git-read-tree. 185 186- a file that has _any_ difference what-so-ever in the three trees 187 will stay as separate entries in the index. It's up to "porcelain 188 policy" to determine how to remove the non-0 stages, and insert a 189 merged version. 190 191- the index file saves and restores with all this information, so you 192 can merge things incrementally, but as long as it has entries in 193 stages 1/2/3 (ie "unmerged entries") you can't write the result. So 194 now the merge algorithm ends up being really simple: 195 196 * you walk the index in order, and ignore all entries of stage 0, 197 since they've already been done. 198 199 * if you find a "stage1", but no matching "stage2" or "stage3", you 200 know it's been removed from both trees (it only existed in the 201 original tree), and you remove that entry. 202 203 * if you find a matching "stage2" and "stage3" tree, you remove one 204 of them, and turn the other into a "stage0" entry. Remove any 205 matching "stage1" entry if it exists too. .. all the normal 206 trivial rules .. 207 208You would normally use "git-merge-cache" with supplied 209"git-merge-one-file-script" to do this last step. The script 210does not touch the files in the work tree, and the entire merge 211happens in the index file. In other words, there is no need to 212worry about what is in the working directory, since it is never 213shown and never used. 214 215When you start a 3-way merge with an index file that is already 216populated, it is assumed that it represents the state of the 217files in your work tree, and you can even have files with 218changes unrecorded in the index file. It is further assumed 219that this state is "derived" from the stage 2 tree. The 3-way 220merge refuses to run if it finds an entry in the original index 221file that does not match stage 2. 222 223This is done to prevent you from losing your work-in-progress 224changes. To illustrate, suppose you start from what has been 225commited last to your repository: 226 227 $ JC=`cat .git/HEAD` 228 $ git-checkout-cache -f -u -a $JC 229 230You do random edits, without running git-update-cache. And then 231you notice that the tip of your "upstream" tree has advanced 232since you pulled from him: 233 234 $ git-fetch-script rsync://.... linus 235 $ LT=`cat .git/MERGE_HEAD` 236 237Your work tree is still based on your HEAD ($JC), but you have 238some edits since. Three-way merge makes sure that you have not 239added or modified cache entries since $JC, and if you haven't, 240then does the right thing. So with the following sequence: 241 242 $ git-read-tree -m -u `git-merge-base $JC $LT` $JC $LT 243 $ git-merge-cache git-merge-one-file-script -a 244 $ echo "Merge with Linus" | \ 245 git-commit-tree `git-write-tree` -p $JC -p $LT 246 247what you would commit is a pure merge between $JC and LT without 248your work-in-progress changes, and your work tree would be 249updated to the result of the merge. 250 251 252See Also 253-------- 254link:git-write-tree.html[git-write-tree]; link:git-ls-files.html[git-ls-files] 255 256 257Author 258------ 259Written by Linus Torvalds <torvalds@osdl.org> 260 261Documentation 262-------------- 263Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 264 265GIT 266--- 267Part of the link:git.html[git] suite 268