Why submodules manage nested repos in Git - Why It Works

Introduction

Sometimes you want to include one project inside another but keep them separate. Git submodules help by linking a nested repository inside a main repository, so you can work on both without mixing their files or histories.

When you want to include a shared library project inside your main project without copying its files.

When your project depends on another project that is developed separately and updated independently.

When you want to keep the history of the nested project separate but still track its version inside your main project.

When you want to clone a big project but only update parts of it selectively.

When you want to contribute to both the main project and the nested project but keep their codebases cleanly separated.

Commands

This command adds the shared-library repository as a submodule inside the libs/shared-library folder of your main project. It links the nested repo without copying all files directly.

Terminal

git submodule add https://github.com/example/shared-library.git libs/shared-library

Expected Output

Cloning into 'libs/shared-library'...

remote: Enumerating objects: 10, done.

remote: Counting objects: 100% (10/10), done.

remote: Compressing objects: 100% (8/8), done.

remote: Total 10 (delta 1), reused 10 (delta 1), pack-reused 0

Receiving objects: 100% (10/10), done.

Resolving deltas: 100% (1/1), done.

This commits the change to your main project, recording the submodule link and the specific commit of the nested repo it points to.

Terminal

git commit -m "Add shared-library submodule"

Expected Output

[main abc1234] Add shared-library submodule

 1 file changed, 1 insertion(+)

 create mode 160000 libs/shared-library

When cloning a project with submodules, this command clones the main project and automatically fetches all submodules so you get the full nested repos ready to use.

Terminal

git clone --recurse-submodules https://github.com/example/main-project.git

Expected Output

Cloning into 'main-project'...

remote: Enumerating objects: 50, done.

remote: Counting objects: 100% (50/50), done.

remote: Compressing objects: 100% (30/30), done.

remote: Total 50 (delta 10), reused 40 (delta 5), pack-reused 0

Receiving objects: 100% (50/50), done.

Resolving deltas: 100% (10/10), done.

Submodule 'libs/shared-library' (https://github.com/example/shared-library.git) registered for path 'libs/shared-library'

Cloning into 'libs/shared-library'...

remote: Enumerating objects: 10, done.

remote: Counting objects: 100% (10/10), done.

remote: Compressing objects: 100% (8/8), done.

remote: Total 10 (delta 1), reused 10 (delta 1), pack-reused 0

Receiving objects: 100% (10/10), done.

Resolving deltas: 100% (1/1), done.

→

--recurse-submodules - Fetch all submodules automatically when cloning

This command updates the submodules to the latest commit from their remote repositories, so you can keep nested repos current without mixing their code into the main project.

Terminal

git submodule update --remote

Expected Output

Submodule path 'libs/shared-library': checked out 'def5678'

→

--remote - Fetch latest commits from the submodule's remote repository

Key Concept

Git submodules let you include and track separate projects inside a main project without mixing their files or histories.

Common Mistakes

Cloning a repo with submodules without using --recurse-submodules

The nested repositories will not be cloned automatically, leaving empty folders or missing code.

Always clone with git clone --recurse-submodules to get all nested repos.

Editing files inside a submodule folder and committing only in the main repo

Changes inside the submodule are not tracked by the main repo and can be lost or cause confusion.

Commit changes inside the submodule repo separately, then update the main repo to point to the new submodule commit.

Deleting submodule folders manually without removing the submodule config

Git still tracks the submodule and can cause errors or confusion in the main repo.

Use git submodule deinit and remove the submodule entry properly before deleting files.

Summary

Use git submodule add to link a nested repository inside your main project without copying files.

Commit the submodule link in your main repo to track the specific version of the nested repo.

Clone projects with submodules using --recurse-submodules to get all nested repos automatically.

Update submodules separately to keep nested projects current without mixing their code.

Practice

1. Why do Git submodules help when you want to include another repository inside your project?

easy

A. They merge the nested repository's history into the main project.

B. They keep the nested repository separate with its own history.

C. They copy all files from the nested repository permanently.

D. They delete the nested repository after adding it.

Solution

1. Step 1: Understand submodule purpose

   Git submodules allow you to include another repository inside your project but keep it separate.

2. Step 2: Recognize history separation

   This means the nested repo keeps its own commit history and does not merge with the main project.

3. Final Answer:

   They keep the nested repository separate with its own history. -> Option B

4. Quick Check:

   Submodules keep histories separate = A [OK]

Hint: Submodules keep nested repos separate, not merged [OK]

Common Mistakes:

• Thinking submodules merge histories
• Assuming submodules copy files permanently
• Believing submodules delete nested repos

2. Which command correctly adds a Git submodule to your project?

easy

A. git submodule add https://github.com/example/repo.git

B. git add submodule https://github.com/example/repo.git

C. git clone submodule https://github.com/example/repo.git

D. git submodule clone https://github.com/example/repo.git

Solution

1. Step 1: Recall submodule add syntax

   The correct command to add a submodule is git submodule add <repo-url>.

2. Step 2: Check options for correct syntax

   Only git submodule add https://github.com/example/repo.git matches this syntax exactly.

3. Final Answer:

   git submodule add https://github.com/example/repo.git -> Option A

4. Quick Check:

   Use 'git submodule add' to add submodules = C [OK]

Hint: Add submodules with 'git submodule add' command [OK]

Common Mistakes:

• Using 'git add submodule' which is invalid
• Trying 'git clone submodule' which is not a command
• Confusing 'git submodule clone' which doesn't exist

3. What is the output of running git submodule update --init after cloning a repository with submodules?

medium

A. It lists all submodules without downloading them.

B. It deletes all submodule directories from the project.

C. It fetches and checks out the submodule repositories at the recorded commit.

D. It merges submodule commits into the main repository.

Solution

1. Step 1: Understand 'git submodule update --init'

   This command downloads the submodule repositories and checks out the commit recorded in the main repo.

2. Step 2: Compare options with command behavior

   Only It fetches and checks out the submodule repositories at the recorded commit. correctly describes this behavior; others describe incorrect actions.

3. Final Answer:

   It fetches and checks out the submodule repositories at the recorded commit. -> Option C

4. Quick Check:

   'git submodule update --init' fetches submodules = D [OK]

Hint: Update and initialize submodules with 'git submodule update --init' [OK]

Common Mistakes:

• Thinking it deletes submodules
• Assuming it merges submodule commits
• Believing it only lists submodules

4. You cloned a repository with submodules but forgot to run git submodule update --init. What problem will you face?

medium

A. The submodule folders will be empty or missing files.

B. The main repository will not clone properly.

C. The submodules will automatically update without any command.

D. The repository will merge submodule commits automatically.

Solution

1. Step 1: Understand cloning with submodules

   When cloning, submodule folders exist but are empty until you initialize and update them.

2. Step 2: Identify the effect of missing update

   Without running git submodule update --init, submodule content is not fetched, so folders are empty or incomplete.

3. Final Answer:

   The submodule folders will be empty or missing files. -> Option A

4. Quick Check:

   Missing 'git submodule update --init' causes empty submodule folders = B [OK]

Hint: Always run 'git submodule update --init' after cloning repos with submodules [OK]

Common Mistakes:

• Expecting submodules to auto-update
• Thinking main repo cloning fails
• Assuming submodules merge automatically

5. You want to update a submodule to a newer commit and push this change to the main repository. Which sequence of commands is correct?

hard

A. cd submodule_dir; git pull; git merge main; git push

B. git submodule update --init; git commit -m 'Update submodule'; git push

C. git clone submodule_dir; git push; git commit -m 'Update submodule'

D. cd submodule_dir; git checkout new_commit; cd ..; git add submodule_dir; git commit -m 'Update submodule'; git push

Solution

1. Step 1: Update submodule commit

   Navigate into the submodule directory and checkout the desired new commit.

2. Step 2: Stage and commit submodule change in main repo

   Return to main repo, add the submodule directory change, commit with a message, then push.

3. Final Answer:

   cd submodule_dir; git checkout new_commit; cd ..; git add submodule_dir; git commit -m 'Update submodule'; git push -> Option D

4. Quick Check:

   Update submodule commit then commit in main repo = A [OK]

Hint: Update submodule inside folder, then commit main repo change [OK]

Common Mistakes:

• Trying to update submodule without committing main repo
• Running 'git submodule update --init' instead of checkout
• Confusing clone with update commands