Version Control
Version control
Records changes to code (or any file)
Why use version control?
- Large, complicated code base
- Many developers working together
- Tracking causes of bugs
- Rewind history to prior versions
- Log who made code changes
Bank account balance
| Description | Amount |
|---|---|
| TOTAL | $31 |
Says nothing about how you got there. Why do I only have $31?
- Did I spend too much this week?
- Did I not get paid?
Bank account transactions
| Description | Amount |
|---|---|
| Income | $42 |
| Gas | -$15 |
| Food | -$6 |
| Reimbursement | $10 |
| TOTAL | $31 |
Transactions are (generally) append-only. You only add transactions.
You can always reconstruct the total from your transactions.
Version Control Systems
- Record each version to a log
- Document developer descriptions of change
diff
diff hello.c new_hello.cDisplays only - Lines added - Lines removed
A diff is like a bank transaction. It only contains what has changed in the new files (lines added or removed).
Examples:
diff oldhello.c newhello.cgit diff ...
patch
diff hello.c new_hello.c > change.patch
patch hello.c change.patch -o patched_hello.c
cat patched_hello.cpatch is a companion tool that takes diff files and
applies them to the oldfile to get the new file.
To reverse a change: patch -R file.c change.patch
Version history
Sequence of diffs
Diagram
Conceptually, can think of version history as a sequence of diffs, with metadata, like a description, author, date, etc. Additionally, the diff points to the previous version.
Just like a bank balance equals a sequence of transactions, the sequence of diffs applied in order equals the current version.
git
Sections of a git
project
| Section | Description | State of a Change |
|---|---|---|
| Working directory | A copy of one version | Modified |
| Staging area | Collects changes | Staged |
| Local repository | Stores all versions | Committed |
| Remote repository | Stores all versions | Pushed |
(Diagram)
- This is where you edit source files
- This the
.gitsubdirectory in your working directory
Sections of a git
project
https://git-scm.com/book/en/v2/Getting-Started-What-is-Git%3F
- Committed means the changes are logged in the repository
- Staged means specific changes have been marked for commit
- Modified means the files in your working directory (where you edit your files) differ compared to the last version in the repository
Distributed version control
https://git-scm.com/book/en/v2/Getting-Started-About-Version-Control
Full repository copies on each machine
Changing state
| Operation | Description | Where |
|---|---|---|
vim or emacs |
Modify a file | Working directory |
git add |
Stage a file | Staging area |
git commit |
Commit a change | Local repository |
git push |
Synchronize changes | Remote repository |
How operations change state
