| git-read-tree(1) | |
| ================ | |
| NAME | |
| ---- | |
| git-read-tree - Reads tree information into the index | |
| SYNOPSIS | |
| -------- | |
| 'git read-tree' (<tree-ish> | [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] [-u | -i]] [--exclude-per-directory=<gitignore>] [--index-output=<file>] <tree-ish1> [<tree-ish2> [<tree-ish3>]]) | |
| DESCRIPTION | |
| ----------- | |
| Reads the tree information given by <tree-ish> into the index, | |
| but does not actually *update* any of the files it "caches". (see: | |
| linkgit:git-checkout-index[1]) | |
| Optionally, it can merge a tree into the index, perform a | |
| fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m` | |
| flag. When used with `-m`, the `-u` flag causes it to also update | |
| the files in the work tree with the result of the merge. | |
| Trivial merges are done by 'git-read-tree' itself. Only conflicting paths | |
| will be in unmerged state when 'git-read-tree' returns. | |
| OPTIONS | |
| ------- | |
| -m:: | |
| Perform a merge, not just a read. The command will | |
| refuse to run if your index file has unmerged entries, | |
| indicating that you have not finished previous merge you | |
| started. | |
| --reset:: | |
| Same as -m, except that unmerged entries are discarded | |
| instead of failing. | |
| -u:: | |
| After a successful merge, update the files in the work | |
| tree with the result of the merge. | |
| -i:: | |
| Usually a merge requires the index file as well as the | |
| files in the working tree are up to date with the | |
| current head commit, in order not to lose local | |
| changes. This flag disables the check with the working | |
| tree and is meant to be used when creating a merge of | |
| trees that are not directly related to the current | |
| working tree status into a temporary index file. | |
| -v:: | |
| Show the progress of checking files out. | |
| --trivial:: | |
| Restrict three-way merge by 'git-read-tree' to happen | |
| only if there is no file-level merging required, instead | |
| of resolving merge for trivial cases and leaving | |
| conflicting files unresolved in the index. | |
| --aggressive:: | |
| Usually a three-way merge by 'git-read-tree' resolves | |
| the merge for really trivial cases and leaves other | |
| cases unresolved in the index, so that Porcelains can | |
| implement different merge policies. This flag makes the | |
| command to resolve a few more cases internally: | |
| + | |
| * when one side removes a path and the other side leaves the path | |
| unmodified. The resolution is to remove that path. | |
| * when both sides remove a path. The resolution is to remove that path. | |
| * when both sides adds a path identically. The resolution | |
| is to add that path. | |
| --prefix=<prefix>/:: | |
| Keep the current index contents, and read the contents | |
| of named tree-ish under directory at `<prefix>`. The | |
| original index file cannot have anything at the path | |
| `<prefix>` itself, and have nothing in `<prefix>/` | |
| directory. Note that the `<prefix>/` value must end | |
| with a slash. | |
| --exclude-per-directory=<gitignore>:: | |
| When running the command with `-u` and `-m` options, the | |
| merge result may need to overwrite paths that are not | |
| tracked in the current branch. The command usually | |
| refuses to proceed with the merge to avoid losing such a | |
| path. However this safety valve sometimes gets in the | |
| way. For example, it often happens that the other | |
| branch added a file that used to be a generated file in | |
| your branch, and the safety valve triggers when you try | |
| to switch to that branch after you ran `make` but before | |
| running `make clean` to remove the generated file. This | |
| option tells the command to read per-directory exclude | |
| file (usually '.gitignore') and allows such an untracked | |
| but explicitly ignored file to be overwritten. | |
| --index-output=<file>:: | |
| Instead of writing the results out to `$GIT_INDEX_FILE`, | |
| write the resulting index in the named file. While the | |
| command is operating, the original index file is locked | |
| with the same mechanism as usual. The file must allow | |
| to be rename(2)ed into from a temporary file that is | |
| created next to the usual index file; typically this | |
| means it needs to be on the same filesystem as the index | |
| file itself, and you need write permission to the | |
| directories the index file and index output file are | |
| located in. | |
| <tree-ish#>:: | |
| The id of the tree object(s) to be read/merged. | |
| Merging | |
| ------- | |
| If `-m` is specified, 'git-read-tree' can perform 3 kinds of | |
| merge, a single tree merge if only 1 tree is given, a | |
| fast-forward merge with 2 trees, or a 3-way merge if 3 trees are | |
| provided. | |
| Single Tree Merge | |
| ~~~~~~~~~~~~~~~~~ | |
| If only 1 tree is specified, 'git-read-tree' operates as if the user did not | |
| specify `-m`, except that if the original index has an entry for a | |
| given pathname, and the contents of the path matches with the tree | |
| being read, the stat info from the index is used. (In other words, the | |
| index's stat()s take precedence over the merged tree's). | |
| That means that if you do a `git read-tree -m <newtree>` followed by a | |
| `git checkout-index -f -u -a`, the 'git-checkout-index' only checks out | |
| the stuff that really changed. | |
| This is used to avoid unnecessary false hits when 'git-diff-files' is | |
| run after 'git-read-tree'. | |
| Two Tree Merge | |
| ~~~~~~~~~~~~~~ | |
| Typically, this is invoked as `git read-tree -m $H $M`, where $H | |
| is the head commit of the current repository, and $M is the head | |
| of a foreign tree, which is simply ahead of $H (i.e. we are in a | |
| fast forward situation). | |
| When two trees are specified, the user is telling 'git-read-tree' | |
| the following: | |
| 1. The current index and work tree is derived from $H, but | |
| the user may have local changes in them since $H; | |
| 2. The user wants to fast-forward to $M. | |
| In this case, the `git read-tree -m $H $M` command makes sure | |
| that no local change is lost as the result of this "merge". | |
| Here are the "carry forward" rules: | |
| I (index) H M Result | |
| ------------------------------------------------------- | |
| 0 nothing nothing nothing (does not happen) | |
| 1 nothing nothing exists use M | |
| 2 nothing exists nothing remove path from index | |
| 3 nothing exists exists, use M if "initial checkout" | |
| H == M keep index otherwise | |
| exists fail | |
| H != M | |
| clean I==H I==M | |
| ------------------ | |
| 4 yes N/A N/A nothing nothing keep index | |
| 5 no N/A N/A nothing nothing keep index | |
| 6 yes N/A yes nothing exists keep index | |
| 7 no N/A yes nothing exists keep index | |
| 8 yes N/A no nothing exists fail | |
| 9 no N/A no nothing exists fail | |
| 10 yes yes N/A exists nothing remove path from index | |
| 11 no yes N/A exists nothing fail | |
| 12 yes no N/A exists nothing fail | |
| 13 no no N/A exists nothing fail | |
| clean (H=M) | |
| ------ | |
| 14 yes exists exists keep index | |
| 15 no exists exists keep index | |
| clean I==H I==M (H!=M) | |
| ------------------ | |
| 16 yes no no exists exists fail | |
| 17 no no no exists exists fail | |
| 18 yes no yes exists exists keep index | |
| 19 no no yes exists exists keep index | |
| 20 yes yes no exists exists use M | |
| 21 no yes no exists exists fail | |
| In all "keep index" cases, the index entry stays as in the | |
| original index file. If the entry were not up to date, | |
| 'git-read-tree' keeps the copy in the work tree intact when | |
| operating under the -u flag. | |
| When this form of 'git-read-tree' returns successfully, you can | |
| see what "local changes" you made are carried forward by running | |
| `git diff-index --cached $M`. Note that this does not | |
| necessarily match `git diff-index --cached $H` would have | |
| produced before such a two tree merge. This is because of cases | |
| 18 and 19 --- if you already had the changes in $M (e.g. maybe | |
| you picked it up via e-mail in a patch form), `git diff-index | |
| --cached $H` would have told you about the change before this | |
| merge, but it would not show in `git diff-index --cached $M` | |
| output after two-tree merge. | |
| Case #3 is slightly tricky and needs explanation. The result from this | |
| rule logically should be to remove the path if the user staged the removal | |
| of the path and then swiching to a new branch. That however will prevent | |
| the initial checkout from happening, so the rule is modified to use M (new | |
| tree) only when the contents of the index is empty. Otherwise the removal | |
| of the path is kept as long as $H and $M are the same. | |
| 3-Way Merge | |
| ~~~~~~~~~~~ | |
| Each "index" entry has two bits worth of "stage" state. stage 0 is the | |
| normal one, and is the only one you'd see in any kind of normal use. | |
| However, when you do 'git-read-tree' with three trees, the "stage" | |
| starts out at 1. | |
| This means that you can do | |
| ---------------- | |
| $ git read-tree -m <tree1> <tree2> <tree3> | |
| ---------------- | |
| and you will end up with an index with all of the <tree1> entries in | |
| "stage1", all of the <tree2> entries in "stage2" and all of the | |
| <tree3> entries in "stage3". When performing a merge of another | |
| branch into the current branch, we use the common ancestor tree | |
| as <tree1>, the current branch head as <tree2>, and the other | |
| branch head as <tree3>. | |
| Furthermore, 'git-read-tree' has special-case logic that says: if you see | |
| a file that matches in all respects in the following states, it | |
| "collapses" back to "stage0": | |
| - stage 2 and 3 are the same; take one or the other (it makes no | |
| difference - the same work has been done on our branch in | |
| stage 2 and their branch in stage 3) | |
| - stage 1 and stage 2 are the same and stage 3 is different; take | |
| stage 3 (our branch in stage 2 did not do anything since the | |
| ancestor in stage 1 while their branch in stage 3 worked on | |
| it) | |
| - stage 1 and stage 3 are the same and stage 2 is different take | |
| stage 2 (we did something while they did nothing) | |
| The 'git-write-tree' command refuses to write a nonsensical tree, and it | |
| will complain about unmerged entries if it sees a single entry that is not | |
| stage 0. | |
| OK, this all sounds like a collection of totally nonsensical rules, | |
| but it's actually exactly what you want in order to do a fast | |
| merge. The different stages represent the "result tree" (stage 0, aka | |
| "merged"), the original tree (stage 1, aka "orig"), and the two trees | |
| you are trying to merge (stage 2 and 3 respectively). | |
| The order of stages 1, 2 and 3 (hence the order of three | |
| <tree-ish> command line arguments) are significant when you | |
| start a 3-way merge with an index file that is already | |
| populated. Here is an outline of how the algorithm works: | |
| - if a file exists in identical format in all three trees, it will | |
| automatically collapse to "merged" state by 'git-read-tree'. | |
| - a file that has _any_ difference what-so-ever in the three trees | |
| will stay as separate entries in the index. It's up to "porcelain | |
| policy" to determine how to remove the non-0 stages, and insert a | |
| merged version. | |
| - the index file saves and restores with all this information, so you | |
| can merge things incrementally, but as long as it has entries in | |
| stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So | |
| now the merge algorithm ends up being really simple: | |
| * you walk the index in order, and ignore all entries of stage 0, | |
| since they've already been done. | |
| * if you find a "stage1", but no matching "stage2" or "stage3", you | |
| know it's been removed from both trees (it only existed in the | |
| original tree), and you remove that entry. | |
| * if you find a matching "stage2" and "stage3" tree, you remove one | |
| of them, and turn the other into a "stage0" entry. Remove any | |
| matching "stage1" entry if it exists too. .. all the normal | |
| trivial rules .. | |
| You would normally use 'git-merge-index' with supplied | |
| 'git-merge-one-file' to do this last step. The script updates | |
| the files in the working tree as it merges each path and at the | |
| end of a successful merge. | |
| When you start a 3-way merge with an index file that is already | |
| populated, it is assumed that it represents the state of the | |
| files in your work tree, and you can even have files with | |
| changes unrecorded in the index file. It is further assumed | |
| that this state is "derived" from the stage 2 tree. The 3-way | |
| merge refuses to run if it finds an entry in the original index | |
| file that does not match stage 2. | |
| This is done to prevent you from losing your work-in-progress | |
| changes, and mixing your random changes in an unrelated merge | |
| commit. To illustrate, suppose you start from what has been | |
| committed last to your repository: | |
| ---------------- | |
| $ JC=`git rev-parse --verify "HEAD^0"` | |
| $ git checkout-index -f -u -a $JC | |
| ---------------- | |
| You do random edits, without running 'git-update-index'. And then | |
| you notice that the tip of your "upstream" tree has advanced | |
| since you pulled from him: | |
| ---------------- | |
| $ git fetch git://.... linus | |
| $ LT=`cat .git/FETCH_HEAD` | |
| ---------------- | |
| Your work tree is still based on your HEAD ($JC), but you have | |
| some edits since. Three-way merge makes sure that you have not | |
| added or modified index entries since $JC, and if you haven't, | |
| then does the right thing. So with the following sequence: | |
| ---------------- | |
| $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT | |
| $ git merge-index git-merge-one-file -a | |
| $ echo "Merge with Linus" | \ | |
| git commit-tree `git write-tree` -p $JC -p $LT | |
| ---------------- | |
| what you would commit is a pure merge between $JC and $LT without | |
| your work-in-progress changes, and your work tree would be | |
| updated to the result of the merge. | |
| However, if you have local changes in the working tree that | |
| would be overwritten by this merge, 'git-read-tree' will refuse | |
| to run to prevent your changes from being lost. | |
| In other words, there is no need to worry about what exists only | |
| in the working tree. When you have local changes in a part of | |
| the project that is not involved in the merge, your changes do | |
| not interfere with the merge, and are kept intact. When they | |
| *do* interfere, the merge does not even start ('git-read-tree' | |
| complains loudly and fails without modifying anything). In such | |
| a case, you can simply continue doing what you were in the | |
| middle of doing, and when your working tree is ready (i.e. you | |
| have finished your work-in-progress), attempt the merge again. | |
| SEE ALSO | |
| -------- | |
| linkgit:git-write-tree[1]; linkgit:git-ls-files[1]; | |
| linkgit:gitignore[5] | |
| Author | |
| ------ | |
| Written by Linus Torvalds <torvalds@osdl.org> | |
| Documentation | |
| -------------- | |
| Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. | |
| GIT | |
| --- | |
| Part of the linkgit:git[1] suite |