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2.2-history.qmd
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---
title: "Intro to `git`"
subtitle: "Block 2.2: History"
---
# Traveling Through Time: `git log` {background-color="black" background-opacity="0.5" background-image="images/backgrounds/back-to-the-future-time.gif"}
::: attribution
Background: https://giphy.com/gifs/back-to-the-future-dgEIhYAo3lZiE
:::
## Seeing History
We can see the history of our committed changes, called `commits` using `git log`.
```bash
git log
# commit ed226e022f60d9f578265c2c246367f5f07756de (HEAD -> main)
# Author: Jan Simson <[email protected]>
# Date: Fri Sep 16 17:00:03 2022 +0200
# Edit hello.txt
# commit 795780d123f6eeedaa09734005c08d1ad89c1976
# Author: Jan Simson <[email protected]>
# Date: Fri Sep 16 16:59:45 2022 +0200
# Adding hello.txt
# lines 1-9
```
You can move up and down with the arrow keys and leave the log view by pressing `q`.
## Practical: Seeing History {background-color="black"}
1. Go back to your Terminal
2. Open the `git log` of your respository
3. Navigate up and down in the history
4. Copy the commit hash of the 2nd commit you made in the repository
5. Also explore the git history in a GUI
# *Properly* Traveling Through Time: <br> `git checkout` {background-color="black" background-opacity="0.5" background-image="images/backgrounds/back-to-the-future-travel.gif"}
::: attribution
Background: https://giphy.com/gifs/back-to-the-future-xsF1FSDbjguis
:::
## `git checkout`
You can go back to any previous commit (and many other things!) with `git checkout`.
```bash
git log
# commit ed226e022f60d9f578265c2c246367f5f07756de (HEAD -> main)
# Author: Jan Simson <[email protected]>
# Date: Fri Sep 16 17:00:03 2022 +0200
# Edit hello.txt
# commit 795780d123f6eeedaa09734005c08d1ad89c1976
# Author: Jan Simson <[email protected]>
# Date: Fri Sep 16 16:59:45 2022 +0200
# Adding hello.txt
# lines 1-9
```
## `git checkout`
You can go back to any previous commit (and many other things!) with `git checkout`.
```bash
git checkout 795780d123f6eeedaa09734005c08d1ad89c1976
```
## `git checkout`
You can go back to any previous commit (and many other things!) with `git checkout`.
```bash
git checkout 795780d
```
:::{.callout-tip}
You don't need to pass the whole commit hash, just the first few characters are usually enough.
:::
## `git checkout`
You can go back to any previous commit (and many other things!) with `git checkout`.
```bash
git checkout 795780d
# Note: switching to '795780d123f6eeedaa09734005c08d1ad89c1976'.
#
# You are in 'detached HEAD' state. You can look around, make experimental
# changes and commit them, and you can discard any commits you make in this
# state without impacting any branches by switching back to a branch.
#
# If you want to create a new branch to retain commits you create, you may
# do so (now or later) by using -c with the switch command. Example:
#
# git switch -c <new-branch-name>
#
# Or undo this operation with:
#
# git switch -
#
# Turn off this advice by setting config variable advice.detachedHead to false
#
# HEAD is now at 795780d Adding hello.txt
```
## Detached `HEAD`, everything OK? 🤕
- When traveling back to a commit, we got a scary warning about a `detached HEAD`
- Normally we are always on a branch in git
- The `HEAD` is always the currently checked out state of your repository
- If the `HEAD` is detached, it is not associated with a branch
- If we make commits with a detached `HEAD` our changes will be lost
- We can go back to `main` with `git checkout main`
## Referencing Commits Relatively
- You will also see `HEAD` in e.g. the `git log` or Sourcetree view
- `HEAD` can be used to reference commits *relatively*
- `HEAD` the current commit
- `HEAD~1` the previous commit
- `HEAD~2` two commits ago
## Tracking changes: Using a GUI
# Questions?
## Practical: Time-Travel ⏱️ {background-color="black"}
1. Go back to your Terminal
2. Open the `git log` of your respository
3. *Checkout* the previous commit using `HEAD~1`
4. Go back to your `main` branch
5. Checkout your very first commit through its hash
## *End of Section* 🎉 {background-color="black"}
:::{.r-fit-text}
Any Questions?
:::
[[🏡 Back to Overview]](./index.html)
[[⏩️ Next Section]](./2.3-branches_merging.html)