Version control

Question 1

What is version control?

Answer 1

A system for managing changes to documents, programs, web pages

Question 2

Why do we need version control?

Answer 2

Keep track of a revision history/versions of a document, enable users to collaborate on a collection of documents

Question 3

Other version control systems other than Git?

Answer 3

rcs, cvs, subversion, mercurial

Question 4

What are the 3 types of version control?

Answer 4

Local, centralized, distributed
They differ in how the version database is stored.

Question 5

Local vs centralized?

Answer 5

For local, there is no server involved. The version database is stored locally. Only the local computer can checkout the file.

For centralized, there is a central server and each computer checks out the file from this server to get a local copy of the file on their own computer.

Question 6

Centralized vs distributed?

Answer 6

For centralized, there is one server that saves the version database, no users have the database on their computer.

For distributed, there is a server with the version database and each user has their own version database on their computer.

Question 7

Which VCS stores differences?

Answer 7

mercurial

Question 8

Which VCS stores snapshots?

Answer 8

git

Question 9

Differences vs snapshots?

Answer 9

Differences is when a file is changed, the VCS only saves the changes as a new version (the difference between the previous version to current). If one file among all files is not changed in a version, the file is not updated. For example, if file A is changed, version 1 is only the changes in file A.

Snapshots is when a file is changed, all files are saved as a new version the way they are. For example, if file A is changed, version 1 becomes file A1, B, C.

Question 10

Why is using differences inefficient?

Answer 10

Because every time we want the latest version of a file, we need to start from the initial file and apply all the changes along the versions.

Question 11

Describe the common git workflow.

Answer 11

git init/clone, git add, git commit, git push, git pull (git fetch + git merge)

Question 12

Does git use a three-stage architecture?

Answer 12

Yes

Question 13

Describe the three-stage architecture of Git.

Answer 13

The 3 stages are: working directory, staging area, local repository (.git repository)

Working directory is where the actual project files are located. Any changes made to files here are untracked. To get to staging area, we use “git add”.

Staging area is an intermediate step between the working directory and .git repo. We basically store everything we want to commit in the next commit here. To get to the .git repo, use “git commit”.

.git repo is like a container for Git’s VCS. It stores committed snapshots, the project’s history, and facilitate with Git operation. To move the files to the remote repo, use “git push”.

Commands to interact with the remote repo: git pull, git push, git clone, git fetch

Question 14

Relationship between blobs, trees, and commits?

Answer 14

Each commit has a unique commit id and a tree id of the tree it belongs to. A tree stores its own id and blobs ids. A blob is just an object type used to store file contents and each of them has an id. Can think of each blob as a file in our repo.

Question 15

Is a commit a snapshot of the state of the project?

Answer 15

Yes

Question 16

When does the blob id change?

Answer 16

When the file contents change (even by 1 byte). Each blob id refers to a specific version of a file.

Question 17

What algorithm is used to make commit ids?

Answer 17

SHA-1

Question 18

How many percent of a hash will change when you change a file?

Answer 18

50%

Question 19

How many digits of the 40 digits hash does git use?

Answer 19

First 8 digits

Question 20

What is git tag?

Answer 20

“git tag NAME” is a command we can use to give a name to a certain commit (kind of like a pointer)

Question 21

Where does HEAD always point to?

Answer 21

HEAD always points to the latest commit of the current branch. e.g. HEAD points to master, which is a branch, and master points to the latest commit id.

When there is a branch, we can imagine this branch has a pointer with its name and it points to the latest commit. Then HEAD points to the branch pointer.

Question 22

Should we touch the master branch? What should it be used for?

Answer 22

No. Only well-tested, stable commits. Usually only when we need to release to customers.

Question 23

Other than the master branch, what other branches do we usually use?

Answer 23

1) Development branch
2) Feature branches to test out things before merging into development
3) Release branches to store versions ready to be released
4) Hotfixes include bugs fixed. When a bug is detected and fixed, the development commits do not have this fix, so remember to add it to the dev branch. Then release a new version to customers via the master branch. Next time, when a version is added to the release branch, this bug fixed will be included.

Question 24

In a commit message, should the subject line be attached to the body?

Answer 24

No, there should be a blank line

Question 25

What is the word limit on the subject line in a commit message?

Answer 25

50 characters

Question 26

Should there be a period at the end of the subject line?

Answer 26

No

Question 27

What is the word limit on the body?

Answer 27

72 characters

Question 28

What is the body used to explain?

Answer 28

What and why, not how

Question 29

If we have the terminal, shell, OS, hardware, which part(s) of this is UNIX?

Answer 29

Shell and OS

Question 30

What is the terminal?

Answer 30

An interface to type commands to interact with the shell

Question 31

What does the shell do?

Answer 31

Interact with the OS using human readable commands

Question 32

cat file | head -n 20

Answer 32

It lists the first 20 lines of a file

Question 33

cat file | sort | uniq -c

Answer 33

Find all unique words in the file and their occurrences

Question 34

ls > output.txt

Answer 34

Save the output of the command into a file

Question 35

ls 2>&1 | tee output.txt

Answer 35

2>&1 redirects the output of stderr to stdout (combining with the existing stdout) and pipe the combined output. tee is used to write/display the output to both stdout and a file.

Question 36

cat file | grep mystring

Answer 36

Search for mystring in the file

Question 37

find [folder] -name “filename”

Answer 37

Search for a file in a folder