如何在UNIX中使用if statment

if condition
then
    command1
    command2
    ...
else
    command3
    command4
    ...
fi

if [ -e filename ]
then
    echo "文件存在"
else
    echo "文件不存在"
fi

if [ $num1 -gt $num2 ]
then
    echo "$num1 大于 $num2"
else
    echo "$num1 小于等于 $num2"
fi

if [ -z "$str" ]
then
    echo "字符串为空"
else
    echo "字符串不为空"
fi