You have a shell script that takes arguments supplied on the command line. You’d like to provide default values so that the most common value(s) can be used without needing to type them every time.

Use the ${:-} syntax when referring to the parameter, and use it to supply a default value:

FILEDIR=${1:-/tmp}

There are a series of special operators available when referencing a shell variable. This one, the : -operator, says that if $1 is not set or is null then it will use what follows, /tmp in our example, as the value. Otherwise it will use the value that is already set in $1. It can be used on any shell variable, not just the positional parameters (1, 2, 3, etc.), but they are probably the most common use.

Of course you could do this the long way by constructing an if statement and checking to see if the variable is null or unset (we leave that as an exercise to the reader), but this sort of thing is so common in shell scripts that this syntax has been welcomed as a convenient shorthand.