bash backup script error

bash backup script error

So I'm writing a simple backup script that when called will either back up
a specific file or all the files in my current directory into my backup
directory. This is my script
#!/bin/bash
#verify if $1 is empty, if so, copy all content to backup directory
if [ -z "$1" ] then
$files=ls
#Looping through files
for file in $files
do
cp $file ../backup/
done
#else copy files specified
else
$files=ls $1
#Looping through files
for file in $files
do
cp $file ../backup/
done
fi
and the only error I'm getting is: ./backup: line 7: =ls: command not found
I'm not sure why the script won't recognize ls as a command. Any ideas?