1. GIT Commit Sign

Code 1.1. Install GPG
$ brew install gpg
# Installs GPG on Mac

$ sudo apt install gpg2
# Installs GPG on Linux
Code 1.2. Set environment variable
$ echo 'export GPG_TTY=$(tty)' >> ~/.profile
Code 1.3. Generate GPG key
$ gpg --full-generate-key
* Select RSA and DSA
* Use 4096 bits
* 0 = key does not expire
Code 1.4. Add GPG key to GIT
$ gpg --list-secret-keys --keyid-format LONG
/Users/myuser/.gnupg/secring.gpg
------------------------------------
sec   rsa4096/3AA5C34371567BD2 1970-01-01 [SC]
uid                 [ultimate] Mark Watney <mark.watney@nasa.gov>
ssb   rsa4096/42B317FD4BA89E7A 1970-01-01 [E]

# In this example, the GPG key ID is 3AA5C34371567BD2

$ git config --global user.signingkey 3AA5C34371567BD2
$ git config --global commit.gpgsign true
Code 1.5. Add GPG key to GitHub
$ gpg --armor --export 3AA5C34371567BD2
# Copy output of the GPG key ID, in ASCII armor format
# Copy with -----BEGIN PGP PUBLIC KEY BLOCK----- and -----END PGP PUBLIC KEY BLOCK-----

$ open https://github.com/settings/gpg/new
# Paste GPG key ID, in ASCII armor format (from previous step)
Code 1.6. Commit files using -S. Commits should be signed automatically if git config commit.gpgsign is set to true (no need to use -S each time)
$ git commit -S -am "My Commit Message"

Note

Some GPG installations on Linux may require you to use gpg2 --list-keys --keyid-format LONG to view a list of your existing keys instead. In this case you will also need to configure Git to use gpg2 by running git config --global gpg.program gpg2.