if 判断

语法：

bash

if [ 测试条件成立 ] 或 命令返回值是否为 0
then
	执行命令
else
	执行命令
fi

使用：

bash

#!/bin/bash

if [ -e foo.txt ]; then
    echo "foo.txt exists"
else
    echo "foo.txt does not exist"
fi

if-elif-else

bash

#!/bin/bash

if [ -d foo ]; then
    echo "foo is a directory"
elif [ -f foo ]; then
    echo "foo is a file"
else
    echo "foo does not exist"
fi

[]与[[]]