Mailing the story:

The [ command, often called a "test," is a command from the GNU Core Utils package, and initiates a conditional statement in Bash. Its function is exactly the same as the test command. When you want to execute a command only when something is either true or false, use the [ or the test command. However, there's a significant difference between [ or test and [[, and there's a technical difference between those commands and your shell's versions of them...