Mastering git, Part 9, git Submodule

Set up: This allows the repository to be local a file directory:

ROOT=/tmp
echo $ROOT
git config --global protocol.file.allow always

ROOT=/tmp

echo $ROOT

git config --global protocol.file.allow always

Create remote:

cd $ROOT
mkdir remote
git init --bare remote

cd $ROOT

mkdir remote

git init --bare remote

Creating repos1

mkdir repos1
cd $ROOT/repos1
git init
touch main.cpp
git add .
git commit -m 'main.cpp added'

git remote add origin ../remote/
git push --set-upstream origin master
git push origin master

mkdir repos1

cd $ROOT/repos1

git init

touch main.cpp

git add .

git commit -m 'main.cpp added'

git remote add origin ../remote/

git push --set-upstream origin master

git push origin master

Creating lib1

cd $ROOT
mkdir lib1
cd $ROOT/lib1
git init .
touch vision.hpp
git add .
git commit -m 'vision.hpp added' 

# this alows to push into this repository, as it is a non-bare repository 
git config --local receive.denyCurrentBranch updateInstead

cd $ROOT

mkdir lib1

cd $ROOT/lib1

git init .

touch vision.hpp

git add .

git commit -m 'vision.hpp added'

# this alows to push into this repository, as it is a non-bare repository

git config --local receive.denyCurrentBranch updateInstead

Adding lib1 as submodule to repos1

cd $ROOT/repos1
git submodule add ../lib1/
git diff --cached lib1/

# This command lists the submodules recorded in the .gitmodules file along with their current checked out commit.
git submodule

# Gives you the commit from repose lib1 that has been added 
git ls-tree master

git add .
git commit -m 'lib1 as submodule added'
git push origin master

cd $ROOT/repos1

git submodule add ../lib1/

git diff --cached lib1/

# This command lists the submodules recorded in the .gitmodules file along with their current checked out commit.

git submodule

# Gives you the commit from repose lib1 that has been added

git ls-tree master

git add .

git commit -m 'lib1 as submodule added'

git push origin master

Cloning a Project with Submodules to repos2

cd $ROOT
mkdir $ROOT/repos2
cd $ROOT/repos2
git clone $ROOT/remote/ .
git submodule init
git submodule update

# Simpler way:
# git clone --recurse-submodules ../remote/ .

#If you already cloned the project and forgot --recurse-submodules, you can combine the git submodule init and git submodule update

# We are in Detached HEAD State
cd $ROOT/repos2/lib1/
git checkout master
git branch -vv

cd $ROOT

mkdir $ROOT/repos2

cd $ROOT/repos2

git clone $ROOT/remote/ .

git submodule init

git submodule update

# Simpler way:

# git clone --recurse-submodules ../remote/ .

#If you already cloned the project and forgot --recurse-submodules, you can combine the git submodule init and git submodule update

# We are in Detached HEAD State

cd $ROOT/repos2/lib1/

git checkout master

git branch -vv

Working on Submodule lib1 in repos2:

cd $ROOT/repos2/lib1/
touch reader.hpp
git add .
git commit -m 'reader added from repos2'
cd  ../
git add .
git commit -m 'submodule lib1 updated'
git push

cd $ROOT/repos2/lib1/

touch reader.hpp

git add .

git commit -m 'reader added from repos2'

cd ../

git add .

git commit -m 'submodule lib1 updated'

git push

Pulling in Upstream Changes from the Submodule Remote in repos1:

cd $ROOT/repos1/lib1
git fetch
git merge
now 
cd ../
git diff --submodule

# There is an easier way to do this 
git submodule update --remote

cd $ROOT/repos1/lib1

git fetch

git merge

now

cd ../

git diff --submodule

# There is an easier way to do this

git submodule update --remote

Working on Submodule lib1 in repos1

cd $ROOT/repos1/lib1
touch codec.hpp
git add .
git commit -m "codec added from repose1"
git push

cd ../
git add .
git commit -m "changes regarding lib1 from repose1"
git push

cd $ROOT/repos1/lib1

touch codec.hpp

git add .

git commit -m "codec added from repose1"

git push

cd ../

git add .

git commit -m "changes regarding lib1 from repose1"

git push

Cloning repos3

cd $ROOT/
mkdir $ROOT/repos3
cd repos3
git clone --recurse-submodules ../remote/ .

cd $ROOT/

mkdir $ROOT/repos3

cd repos3

git clone --recurse-submodules ../remote/ .

Let’s break down these Git commands related to submodules and their effects:

git submodule update:
- Effect: This command is used to update the submodules to the commit specified in the superproject. When you have a project with submodules, these submodules are usually fixed to specific commits. Running git submodule update will ensure your submodules are set to those specific commits.
- Example: Suppose your main project (superproject) has a submodule at commit abc123. If someone else updates the submodule and you pull the latest changes in the superproject, git submodule update will set the submodule to commit abc123.
- Difference: Unlike git submodule update --remote, it does not update the submodule to the latest commit in its own branch; it updates to the commit specified in the superproject.
git submodule sync:
- Effect: This command is used to update the URL of the submodule. If the submodule’s repository URL changes in the superproject configuration, running git submodule sync will update your local submodule configuration to the new URL.
- Example: If the URL of a submodule repository was changed in the .gitmodules file, git submodule sync will update your local configuration to match this new URL.
- Difference: This command does not change any submodule content, unlike git submodule update, which updates the submodule to a specific commit.
git submodule update --remote:
- Effect: This updates the submodules to the latest commit available on their respective branches as specified in .gitmodules, rather than the commit specified in the superproject.
- Example: If a submodule is set to track the master branch, git submodule update --remote will update the submodule to the latest commit on the master branch.
- Difference: Unlike the basic git submodule update, this command does not adhere to the commit specified in the superproject but fetches the latest from the tracked branch.
git pull --recurse-submodules:
- Effect: This command updates both the superproject and the submodules. It fetches and merges changes from the remote for the superproject and the submodules.
- Example: When you have changes in both your superproject and submodules in the remote repository, git pull --recurse-submodules will update all of them to their latest commits.
- Difference: It not only updates the superproject (like a regular git pull) but also ensures that all submodules are updated.
git push --recurse-submodules=check:
- Effect: This command, when pushing, checks if any commits in submodules need to be pushed to their respective remotes. If there are such commits, the push is aborted.
- Example: Before pushing changes in your superproject, this command ensures that all submodule changes are also pushed. If not, it prevents the push to the superproject.
- Difference: It provides a safety check before pushing, unlike a regular git push, which does not consider submodule statuses.
git push --recurse-submodules=on-demand:
- Effect: Similar to the above, but instead of just checking, it attempts to push the changes of the submodules to their respective remotes automatically if needed.
- Example: If submodule changes haven’t been pushed, using this command will push both the superproject and the necessary submodule changes.
- Difference: It actively pushes submodule changes if needed, as opposed to git push --recurse-submodules=check, which only checks and aborts if needed.

To list all the submodules:

git submodule

1	git submodule

If you want more detailed information about each submodule

git submodule status

1	git submodule status

Removing a Submodule

In modern git:

git rm <path-to-submodule>

1	git rm <path-to-submodule>

Refs: [1]

In older git versions:

Delete the Submodule Reference:
Remove the submodule entry from the index and delete the relevant section from .gitmodules and .git/config:

git submodule deinit -f path-to-submodule
git rm --cached path-to-submodule

1 2	git submodule deinit -f path-to-submodule git rm --cached path-to-submodule

Remove Submodule Files:
Manually delete the submodule’s files from your working directory:

rm -rf path-to-submodule
rm -rf .git/modules/path-to-submodule

1 2	rm -rf path-to-submodule rm -rf .git/modules/path-to-submodule

Commit the Changes:
Commit these changes to the repository:

git commit -m "Removed submodule SomeLibrary"

1	git commit -m "Removed submodule SomeLibrary"

Adding a Submodule

Add the Submodule:
To add a new submodule, you use the git submodule add command followed by the repository URL and the path where you want to place it in your project:

git submodule add repository-url path-to-submodule

1	git submodule add repository-url path-to-submodule

For example:

git submodule add https://github.com/example/SomeLibrary.git lib/SomeLibrary

1	git submodule add https://github.com/example/SomeLibrary.git lib/SomeLibrary

Initialize and Fetch the Submodule Contents:
If the submodule has nested submodules, initialize and update them:

git submodule update --init --recursive

1	git submodule update --init --recursive

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