What does the “$#” special parameter mean in bash?

Question

Ive come across the code

if [ $# -eq 1  ]; then
   echo "usage: Phar ~/flashmem ~/archive"
   exit
fi

Ive never come across [ $# -eq 1 ]; before and I cant seem to find a meaning

Answer 1

The "$#" return the number of parameters passed as argument

#!/bin/bash
echo $#

Now

./testess.sh test1 test2 test3

this return 3

./testess.sh test1 test2 test3 test4 test5

this return 5

So in your code if "$#" equal the number one (just one argument passed), execute the echo command

Answer 2

It Expands to the number of positional parameters in decimal.

(copied from man bash, under Special Parameters).