shell 条件判断 与 或 非

1. 判断字符串"a.txt"等于变量filename 且 当前目录下不存在文件b.txt

#!/bin/bash 

filename="a.txt"

if [ "a.txt" = $filename -a ! -e "b.txt" ]; then
    echo "yes"
else
    echo "no"
fi