In February 2015, I wrote an article on an encryption program that runs from the Linux terminal -bcrypt. I am retracting this endorsement, as bcrypt has been found to be an insecure means of encryption due to vulnerabilities. In lieu of bcrypt, I would recommend scrypt, which also runs from the terminal as well and is secure. For more information on scrypt, read the scrypt man page.
For further reading about bcrypt’s vulnerabilities, you can read an article from the Hacker News, here.