linux 几个控制流语句的格式例子：
if 语句例子；
#!/bin/sh
a=10
b=20
if [ $a == $b ]
then
echo "a is equal to b"
elif [ $a -gt $b ]
then
echo "a is greater than b"
elif [ $a -lt $b ]
then
echo "a is less than b"
else
echo "None of the condition met"
fi
if ... else 语句也可以写成一行，以命令的方式来运行
if test $[2*3] -eq $[1+5]; then echo 'The two numbers are equal!'; fi;
if ... else 语句也经常与 test 命令结合使用
num1=$[2*3]
num2=$[1+5]
if test $[num1] -eq $[num2]
then
echo 'The two numbers are equal!'
else
echo 'The two numbers are not equal!'
fi