A common construct an bash scripts is an if block like so:

if [ -z "$STRING" ]; then
  echo "Length of string is zero"
fi

The syntax seems similar to what you find in many other languages. For example

if (string.length) {
  console.log('Length of string is zero');
}

But if you've tried using a bash if block, you may have noticed that you'll get an error if you omit the space between the brackets and their content:

if [-z "$STRING" ]; then
  echo "Length of string is zero"
fi
[-z: command not found

This seemed like Bash was just being a bit picky about the formatting here. But today I learned this isn't so.

In fact, [ is essentially just an alias for the test command: it's not just for grouping things. So, when you run

[-z "$STRING" ]

you are actually running

test-z "$STRING"

And of course, test-z is not a command, so it produces an error.