What is a best practice to represent a boolean value in a shell script?

I know, that there are boolean values in bash, but I don't ever see them used anywhere. I want to write a wrapper for some often looked up information on my machine, for example, is this particular USB drive inserted/mounted. What would be the best practice to achieve that? * A string? drive_xyz_available=true * A number (0 for true, ≠0 for false)? drive_xyz_available=0 # evaluates to true * A function? drive_xyz_available() { if available_magic; then return 0 else return 1 fi } I mostly wonder about, what would be expected by other people who would want to use the wrapper. Would they expect a boolean value, a command like variable or a function to call? From a security standpoint I would think the second option is the safest, but I would love to hear your experiences.