$ git clone --recursive https://github.com/caffe2/caffe2
Cloning into 'caffe2'...
remote: Counting objects: 36771, done.
remote: Compressing objects: 100% (12/12), done.
remote: Total 36771 (delta 6), reused 7 (delta 5), pack-reused 36754
Receiving objects: 100% (36771/36771), 149.13 MiB | 2.76 MiB/s, done.
Resolving deltas: 100% (26926/26926), done.

 
(这里只以子模块protobuf为例，其他子模块略)
Submodule 'third_party/protobuf' (https://github.com/google/protobuf.git) registered for path 'third_party/protobuf'
（略）
Cloning into 'third_party/protobuf'...
remote: Counting objects: 47717, done.
remote: Compressing objects: 100% (49/49), done.
remote: Total 47717 (delta 23), reused 23 (delta 5), pack-reused 47657
Receiving objects: 100% (47717/47717), 40.97 MiB | 59.00 KiB/s, done.
Resolving deltas: 100% (32097/32097), done.
Checking connectivity... done.
Submodule path 'third_party/protobuf': checked out 'a428e42072765993ff674fda72863c9f1aa2d268'
（略） 

git submodule update --init --recursive1

--recursive, --recurse-submodules
 After the clone is created, initialize all submodules within, using their default settings. This is equivalent to running git submodule
 update --init --recursive immediately after the clone is finished. This option is ignored if the cloned repository does not have a
 worktree/checkout (i.e. if any of --no-checkout/-n, --bare, or --mirror is given) 

--recurse-submodules[=<pathspec]
 After the clone is created, initialize and clone submodules within based on the provided pathspec. If
 no pathspec is provided, all submodules are initialized and cloned. Submodules are initialized and
 cloned using their default settings. The resulting clone has submodule.active set to the provided
 pathspec, or "." (meaning all submodules) if no pathspec is provided. This is equivalent to running git
 submodule update --init --recursive immediately after the clone is finished. This option is ignored if
 the cloned repository does not have a worktree/checkout (i.e. if any of --no-checkout/-n, --bare, or
 --mirror is given)