Mark Wilkinson continues his Bash for the Powershell developer series:
An expression is a statement that evaluates to either true or false. In our case we are just checking if the
$hour_of_dayvariable is less than 12 and greater than or equal to 5. When comparing numeric values you must use the same operators you are familiar with from PowerShell:-lt-gt-le-ge-ne. When comparing string values you have to use operators you might be more used to from other languages:==!=and both values must be enclosed in double quotes[ "string1" == "string2" ].
With just the things Mark has shown so far, you can begin to build helpful scripts.