| git-read-tree(1) |
| ================ |
| |
| NAME |
| ---- |
| git-read-tree - Reads tree information into the index |
| |
| |
| SYNOPSIS |
| -------- |
| [verse] |
| 'git read-tree' [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] |
| [-u [--exclude-per-directory=<gitignore>] | -i]] |
| [--index-output=<file>] [--no-sparse-checkout] |
| (--empty | <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 to be 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. |
| |
| -n:: |
| --dry-run:: |
| Check if the command would error out, without updating the index |
| or the files in the working tree for real. |
| |
| -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 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 add a path identically. The resolution |
| is to add that path. |
| |
| --prefix=<prefix>:: |
| Keep the current index contents, and read the contents |
| of the named tree-ish under the directory at `<prefix>`. |
| The command will refuse to overwrite entries that already |
| existed in the original index file. |
| |
| --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. |
| |
| --[no-]recurse-submodules:: |
| Using --recurse-submodules will update the content of all initialized |
| submodules according to the commit recorded in the superproject by |
| calling read-tree recursively, also setting the submodules HEAD to be |
| detached at that commit. |
| |
| --no-sparse-checkout:: |
| Disable sparse checkout support even if `core.sparseCheckout` |
| is true. |
| |
| --empty:: |
| Instead of reading tree object(s) into the index, just empty |
| it. |
| |
| <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 or more 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 match 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, where "I" denotes the index, |
| "clean" means that index and work tree coincide, and "exists"/"nothing" |
| refer to the presence of a path in the specified commit: |
| |
| .... |
| I 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 is 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 which of the "local changes" that you made were carried forward by running |
| `git diff-index --cached $M`. Note that this does not |
| necessarily match what `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 the 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 switching 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 content 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=`git rev-parse 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 $LT without |
| your work-in-progress changes, and your work tree would be |
| updated to the result of the merge. |
| |
| However, if you have local changes in the working tree that |
| would be overwritten by this merge, 'git read-tree' will refuse |
| to run to prevent your changes from being lost. |
| |
| In other words, there is no need to worry about what exists only |
| in the working tree. When you have local changes in a part of |
| the project that is not involved in the merge, your changes do |
| not interfere with the merge, and are kept intact. When they |
| *do* interfere, the merge does not even start ('git read-tree' |
| complains loudly and fails without modifying anything). In such |
| a case, you can simply continue doing what you were in the |
| middle of doing, and when your working tree is ready (i.e. you |
| have finished your work-in-progress), attempt the merge again. |
| |
| |
| SPARSE CHECKOUT |
| --------------- |
| |
| "Sparse checkout" allows populating the working directory sparsely. |
| It uses the skip-worktree bit (see linkgit:git-update-index[1]) to tell |
| Git whether a file in the working directory is worth looking at. |
| |
| 'git read-tree' and other merge-based commands ('git merge', 'git |
| checkout'...) can help maintaining the skip-worktree bitmap and working |
| directory update. `$GIT_DIR/info/sparse-checkout` is used to |
| define the skip-worktree reference bitmap. When 'git read-tree' needs |
| to update the working directory, it resets the skip-worktree bit in the index |
| based on this file, which uses the same syntax as .gitignore files. |
| If an entry matches a pattern in this file, skip-worktree will not be |
| set on that entry. Otherwise, skip-worktree will be set. |
| |
| Then it compares the new skip-worktree value with the previous one. If |
| skip-worktree turns from set to unset, it will add the corresponding |
| file back. If it turns from unset to set, that file will be removed. |
| |
| While `$GIT_DIR/info/sparse-checkout` is usually used to specify what |
| files are in, you can also specify what files are _not_ in, using |
| negate patterns. For example, to remove the file `unwanted`: |
| |
| ---------------- |
| /* |
| !unwanted |
| ---------------- |
| |
| Another tricky thing is fully repopulating the working directory when you |
| no longer want sparse checkout. You cannot just disable "sparse |
| checkout" because skip-worktree bits are still in the index and your working |
| directory is still sparsely populated. You should re-populate the working |
| directory with the `$GIT_DIR/info/sparse-checkout` file content as |
| follows: |
| |
| ---------------- |
| /* |
| ---------------- |
| |
| Then you can disable sparse checkout. Sparse checkout support in 'git |
| read-tree' and similar commands is disabled by default. You need to |
| turn `core.sparseCheckout` on in order to have sparse checkout |
| support. |
| |
| |
| SEE ALSO |
| -------- |
| linkgit:git-write-tree[1]; linkgit:git-ls-files[1]; |
| linkgit:gitignore[5] |
| |
| GIT |
| --- |
| Part of the linkgit:git[1] suite |
