简单if结构
简单的if结构是：
if expression
then
command
command
…
fi
在使用这种简单if结构时，要特别注意测试条件后如果没有“；”，则then语句要换行，否则会产生不必要的错误。如果if和then可以处于同一行，则必须用“;”

if [ 1 -eq 1 ];then             -→ 不需要换行
echo Yes
fiif [ 1 -eq 1 ]
then
echo Yes
fi

+++++++++++++++++++++++++

vim if_exam1.sh 
# if_exam1.sh:inputs a integer then compare with 15
#!/bin/bash
#if_exam1
echo "Please input a integer:"
read integer1
if [ "$integer1" -lt 15 ]
then echo "The integer which you input is lower than 15."
fi

+++++++++++++++++++++++++

vim if_exam2.sh 
#if_exam2.sh: first creates a file,and  test the file is existence,
# then test the file's authority.
#!/bin/bash
touch if_file1
if [ -e if_file1 ]
then echo "The file if_file1 is created successfully!"
fi
if [ -r if_file1 ]
then echo "The file can read."
fi
if [ -w if_file1 ]
then echo "The file can write."
fi
if [ -x if_file1 ]
then echo "The file can execute."
fi

+++++++++++++++++++++++++

# vim love.sh
#!/bin/bash
read -p "请输入你是否爱我，我爱你｜我不爱你：" loveif [  我爱你 == "$love" ]
thenecho "我也爱你！"
fi

# chmod +x love.sh
# ./love.sh