... | ... | @@ -79,36 +79,50 @@ Do this before you commit them! |
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When working with GHC, there are a lot of ways you can use Git to make your life easier. Below are some of them:
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- **Selectively add changes to commit, darcs-style.** Do you miss Darcs? Do you hate it when a file contains a bugfix \*and\* a new feature, and you want to commit both separately? That's OK! Just run:
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### Selectively record changes to commit
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Do you miss Darcs? Do you hate it when a file contains a bugfix \*and\* a new feature, and you want to commit both separately? That's OK! Just run:
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```wiki
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$ git add -p
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```
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This opens the **interactive diff selector**, which behaves a lot like `darcs record`. It will go through every change you have made in the working tree, asking if you want to `git add` it to the index, so you can commit it afterwords.
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> **Nota bene**: this only adds files *to the index*, it does not commit them. Afterwords, you may commit the result using `git commit`. **Do not use `git commit -a`**, or you will just add all the changes to the commit!
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**Nota bene**: this only adds files *to the index*, it does not commit them. Afterwords, you may commit the result using `git commit`. **Do not use `git commit -a`**, or you will just add all the changes to the commit!
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### Selectively cherry-pick a commit from a branch
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- **Selectively merge a singular change, darcs-style.** You still miss Darcs. One thing that would be great is if you could just 'pluck' one commit from a branch into your tree, but not the others. Sounds good - `git cherry-pick` to the rescue!
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You still miss Darcs. One thing that would be great is if you could just 'pluck' one commit from a branch into your tree, but not the others. Sounds good - `git cherry-pick` to the rescue!
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```wiki
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$ git checkout master
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$ git cherry-pick <sha1 id>
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```
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this will checkout to master, and pull in *only' the commit you refer to. **It does not create a merge**, it's as if the commit had existed on this branch all along. This is wonderfully useful for selectively plucking changes from someone's Git tree, or branch.
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*
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- **Merge a branch in a super commit.** Let's say you have a branch `foo` you would like to merge into master, but you have 10 small commits on `foo`, and you only want to make 1 Big Commit on master. Many times, we land features in a single 'big commit' to keep the history clean. This is easily doable with:
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this will checkout to master, and pull in *only* the commit you refer to. **It does not create a merge**, it's as if the commit had existed on this branch all along. This is wonderfully useful for selectively plucking changes from someone's Git tree, or branch.
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### Merge a branch into a Super Big Commit
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Let's say you have a branch `foo` you would like to merge into master, but you have 10 small commits on `foo`, and you only want to make 1 Big Commit on master. Many times, we land features in a single 'big commit' to keep the history clean. This is easily doable with:
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```wiki
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$ git checkout master
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$ git merge --squash foo
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```
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and then you can commit your new, unstaged changes into a big commit after fixing any conflicts. `--squash` basically tells git *to merge the changes, but not merge the commits*. This is exactly what you want.
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- **Rebase a branch.** What if you have a branch that's slightly out of date called `foo`, and you want to bring it up to date with master?
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### Basic rebases
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What if you have a branch that's slightly out of date called `foo`, and you want to bring it up to date with master?
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```wiki
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$ git checkout master
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... | ... | @@ -116,12 +130,14 @@ When working with GHC, there are a lot of ways you can use Git to make your life |
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$ git rebase master foo
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```
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This will:
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- Checkout to master.
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- Update master to the latest upstream version.
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- Rebase `foo` onto `master`.
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Where *rebasing* includes:
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- Checkout to the branch `foo`.
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... | ... | @@ -129,49 +145,50 @@ When working with GHC, there are a lot of ways you can use Git to make your life |
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- Bring `foo` up to date with `master` (by *fast-forwarding* the tree)
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- Replay all your previous commits from `foo` onto the New-And-Improved `foo` branch
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>
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> This, in effect, will bring `foo` up to date with master, while preserving your commits.
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>
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> Q: **But there was a conflict**! A: That's OK. If `git rebase` encounters a conflict while replaying your work, **it will stop and tell you so**. It will ask you to **fix the conflict, and `git add` the conflicting files**. Then you can continue using `git rebase --continue`.
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This, in effect, will bring `foo` up to date with master, while preserving your commits.
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>
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> Q: **I started to rebase, but I confused myself and don't know how to get out! Help**! A: You can always run `git rebase --abort`, which will abort the current rebase operation, and return you to your working tree.
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- **Using the reflog.** Eventually when working in the repository, you'll invariably do something on accident that will delete work. If you have never committed the changes, then you're out of luck (commit often, commit early - even locally!) But have you ever done something like:
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Q: **But there was a conflict**! A: That's OK. If `git rebase` encounters a conflict while replaying your work, **it will stop and tell you so**. It will ask you to **fix the conflict, and `git add` the conflicting files**. Then you can continue using `git rebase --continue`.
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Q: **I started to rebase, but I confused myself and don't know how to get out! Help**! A: You can always run `git rebase --abort`, which will abort the current rebase operation, and return you to your working tree.
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### Using the reflog
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Eventually when working in the repository, you'll invariably do something on accident that will delete work. If you have never committed the changes, then you're out of luck (commit often, commit early - even locally!) But have you ever done something like:
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- Accidentally lost a commit, by deleting a branch?
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- Accidentally lost a commit through rebasing?
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- Amended a commit (`git commit --amend`), only to find out you broke it, and you want to *undo* the amendment?
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- Accidentally overwrote a branch with dangerous operation, like `git push --force`?
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> **The reflog can save you from all of these, and more**. In short, the reflog *is a log that records every modification which Git tracks*. To understand that, first understand this: despite its appearance, the Git data model has a core tenant: *it is immutable - data is never deleted, only new copies can be made* (the only exception is when garbage collection deletes nodes which have no outstanding references - much like our own GC!) Not even a rebase - which can rewrite the history - can actually delete old data.
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**The reflog can save you from all of these, and more**. In short, the reflog *is a log that records every modification which Git tracks*. To understand that, first understand this: despite its appearance, the Git data model has a core tenant: *it is immutable - data is never deleted, only new copies can be made* (the only exception is when garbage collection deletes nodes which have no outstanding references - much like our own GC!) Not even a rebase - which can rewrite the history - can actually delete old data.
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>
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> Second, we need to understand an **important part of `git checkout`**: the purpose of `checkout` is *not* to switch branches. Checkout, roughly speaking, **allows you to check out your tree to any state, revision, or copy in the history**. You don't have to checkout to a branch: you can checkout to a commit from 3 weeks ago, a commit that *does not exist on a branch*, or a completely empty branch with nothing in common. You can checkout the entire tree, or you could checkout an individual file, or a single directory. The point being: **checkout takes you to a state in the history.**
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Second, we need to understand an **important part of `git checkout`**: the purpose of `checkout` is *not* to switch branches. Checkout, roughly speaking, **allows you to check out your tree to any state, revision, or copy in the history**. You don't have to checkout to a branch: you can checkout to a commit from 3 weeks ago, a commit that *does not exist on a branch*, or a completely empty branch with nothing in common. You can checkout the entire tree, or you could checkout an individual file, or a single directory. The point being: **checkout takes you to a state in the history.**
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>
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> So with that in mind, think of `reflog` like the audit log you can use to see what operations were performed on the immutable git history. *Every* operation is tracked. Let's look at an example, from Austin's *validation tree* he uses to push commits:
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>
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> ```wiki
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> $ git reflog --date=relative # this will open an interactive pager
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> ad15c2b HEAD@{5 hours ago}: pull -tu origin master: Fast-forward
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> 75a9664 HEAD@{27 hours ago}: merge amp: Fast-forward
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> 1ef941a HEAD@{27 hours ago}: checkout: moving from amp to master
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> 75a9664 HEAD@{27 hours ago}: commit (amend): Implement the AMP warning (#8004)
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> daa9a30 HEAD@{28 hours ago}: rebase -i (finish): returning to refs/heads/amp
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> daa9a30 HEAD@{28 hours ago}: rebase -i (pick): Implement the AMP warning (#8004)
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> b20cf4e HEAD@{28 hours ago}: rebase -i (pick): Fix AMP warnings.
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> 1ef941a HEAD@{28 hours ago}: checkout: moving from amp to 1ef941a82eafb8f22c19e2643685679d2454c24a
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> 3e8c33e HEAD@{28 hours ago}: commit: Fix AMP warnings.
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> 70406bc HEAD@{28 hours ago}: reset: moving to HEAD~
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> d2afc83 HEAD@{28 hours ago}: cherry-pick: Fix most AMP warnings.
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> 70406bc HEAD@{28 hours ago}: commit (amend): Implement the AMP warning (#8004)
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> 697f9da HEAD@{28 hours ago}: cherry-pick: Implement the AMP warning (#8004)
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> 1ef941a HEAD@{28 hours ago}: checkout: moving from master to amp
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> ```
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>
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> **The most recent operations are first, and older operations appear chronologically**. Let's note a few things:
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So with that in mind, think of `reflog` like the audit log you can use to see what operations were performed on the immutable git history. *Every* operation is tracked. Let's look at an example, from Austin's *validation tree* he uses to push commits:
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```wiki
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$ git reflog --date=relative # this will open an interactive pager
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ad15c2b HEAD@{5 hours ago}: pull -tu origin master: Fast-forward
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75a9664 HEAD@{27 hours ago}: merge amp: Fast-forward
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1ef941a HEAD@{27 hours ago}: checkout: moving from amp to master
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75a9664 HEAD@{27 hours ago}: commit (amend): Implement the AMP warning (#8004)
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daa9a30 HEAD@{28 hours ago}: rebase -i (finish): returning to refs/heads/amp
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daa9a30 HEAD@{28 hours ago}: rebase -i (pick): Implement the AMP warning (#8004)
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b20cf4e HEAD@{28 hours ago}: rebase -i (pick): Fix AMP warnings.
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1ef941a HEAD@{28 hours ago}: checkout: moving from amp to 1ef941a82eafb8f22c19e2643685679d2454c24a
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3e8c33e HEAD@{28 hours ago}: commit: Fix AMP warnings.
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70406bc HEAD@{28 hours ago}: reset: moving to HEAD~
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d2afc83 HEAD@{28 hours ago}: cherry-pick: Fix most AMP warnings.
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70406bc HEAD@{28 hours ago}: commit (amend): Implement the AMP warning (#8004)
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697f9da HEAD@{28 hours ago}: cherry-pick: Implement the AMP warning (#8004)
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1ef941a HEAD@{28 hours ago}: checkout: moving from master to amp
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```
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**The most recent operations are first, and older operations appear chronologically**. Let's note a few things:
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- The *work you previously had still exists, and has a commit ID*. It is on the far left.
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- The reflog tells you what operation resulted in the commit: in my history, we can see I did:
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- Later on, I pulled my tree and I got some updates from upstream.
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- The reflog tells you what was modified; in this case it shows you the commits I changed.
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>
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> With this information, **I can now restore my tree to any of those partial states**. For example, let's say I `git commit --amend` the AMP patch in `75a9664`, and did some more stuff. But then it turns out I didn't want any of that, **and I didn't want the amendment either**. I can easily do:
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>
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> ```wiki
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> $ git checkout -b temp daa9a30
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> ```
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>
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> Now, I am on the `temp` branch, **and my HEAD commit points to the patch, without any amendments**. I've essentially checked out to a point in the tree without any of those changes - because `git` never modifies the original data, this old copy still exists. Now that I am on the `temp` branch, I can do any number of things. Perhaps I can just delete the old `amp` branch, and merge the `temp` branch instead now.
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With this information, **I can now restore my tree to any of those partial states**. For example, let's say I `git commit --amend` the AMP patch in `75a9664`, and did some more stuff. But then it turns out I didn't want any of that, **and I didn't want the amendment either**. I can easily do:
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>
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> As you can see, the `reflog` saved me here: I undid some nasty work in my personal tree, which otherwise might have been much more error prone or difficult to perform.
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```wiki
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$ git checkout -b temp daa9a30
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```
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> **The reflog is not needed often, but it is often indispensable when you need it.**
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Now, I am on the `temp` branch, **and my HEAD commit points to the patch, without any amendments**. I've essentially checked out to a point in the tree without any of those changes - because `git` never modifies the original data, this old copy still exists. Now that I am on the `temp` branch, I can do any number of things. Perhaps I can just delete the old `amp` branch, and merge the `temp` branch instead now.
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As you can see, the `reflog` saved me here: I undid some nasty work in my personal tree, which otherwise might have been much more error prone or difficult to perform.
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**The reflog is not needed often, but it is often indispensable when you need it.**
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## Advanced Git tricks
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Finally, there are some **advanced tips**, not for the faint of heart:
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- **Rebasing interactively.** At a certain point of git usage, you'll want to rewrite history by *rebasing interactively*. This can be done by running:
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### Interactive rebases
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At a certain point of git usage, you'll want to rewrite history by *rebasing interactively*. This can be done by running:
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```wiki
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$ git rebase -i <commit range>
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```
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For example:
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>
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> For example:
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>
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> ```wiki
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> $ git rebase -i HEAD~10
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> ```
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```wiki
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$ git rebase -i HEAD~10
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```
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will allow you to interactively rebase the last 10 commits on your branch. This power allows you to:
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