git submodule git submodule management related operations

Some command notes related to Git submodule operation:

# When using git cloned project with submodule, the content of submodule will not be downloaded automatically at the initial time, you need to execute the following command; 
git submodule update --init --recursive (note that it will download recursively at this time Third-party module) 
# When there is a submodule in the project that is cloned using git, the following command will not download the third-party module recursively ; 
git submodule update --init

 git add submodule:

# git submodule add <url> <path>
git submodule add https://github.com/google/googletest.git third-party/gtest

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