gpg.txt

$ echo hello,gpg > samle.txt

$ cat sample.txt
hello,gpg

$ gpg --sign sample.txt
gpg: no default secret key: secret key not available
gpg: signing failed: secret key not available

$ man gpg

$ gpg --gen-key
# NOTE:
# RSA and RSA
# 2048 bit
# key does not expire at all
# ...

$ gpg --sign sample.txt

$ ls sample.txt*
sample.txt
sample.txt.gpg