What does “$#” mean in bash? [duplicate]

Answer 1

The pound sign counts things.

1. If it's just $#, it's the number of positional parameters, like $1, $2, $3. (Not counting $0, mind you.)
2. If it's ${#var}, it's the number of characters in the expansion of the parameter. (String length)
3. If it's ${#var[@]}, it's the number of elements in the array. Since bash arrays are sparse, this can be different from the index of the last element plus one.

Answer 2

It's the number of arguments passed.

You can read it here, search for "Detecting command line arguments"