新人求助，shell脚本怎么执行啊？

代码:

#! /bin/bash
opt="z"
until ["$opt"="x"];
do
        echo "a. List all files in the present working directory"
        echo "b. Display today's date and time, and your Linux version"
        echo "c. Invoke the shell script of Problem 2"
        echo "d. Display whether a file is a \"simple\" file or a \"directory\""
        echo "e. Create a backup for a file"
        echo "f. Start an ftp session to our course site"
        echo "x. Exit"
        echo "Please enter an option:"
        read opt
done
        echo "Bye-Bye"

代码:

./work1: 行 4: [z=x]: 未找到命令
a. List all files in the present working directory
b. Display today's date and time, and your Linux version
c. Invoke the shell script of Problem 2
d. Display whether a file is a "simple" file or a "directory"
e. Create a backup for a file
f. Start an ftp session to our course site
x. Exit
Please enter an option:

代码:

./work1: 行 4: [z=x]: 未找到命令