1. if...elif...else

#!/bin/bash
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 "what a fuck"
fi 

2. case...esac

echo 'Input a number between 1 to 4'
echo 'Your number is:'
read aNum
case $aNum in
    1)  echo 'You select 1'
    ;;
    2)  echo 'You select 2'
    ;;
    3)  echo 'You select 3'
    ;;
    4)  echo 'You select 4'
    ;;
    *)  echo 'You do not select a number between 1 to 4'
    ;;
esac

[root@localhost shell_study]# ./test6.sh 
Input a number between 1 to 4
Your number is:
2
You select 2

表达方式：

1|2|3|4|5) echo "Your number is $aNum!"
;;

3. for循环

for value in 1 2
do
    echo "The value is: $value"
done

输出：
The value is: 1
The value is: 2

for str in 'This is a string'
do
    echo $str
done

输出：
This
is
a
string

for FILE in $HOME/.bash*
do
    echo $FILE
done

遍历当前用户HOME目录下以.bash结尾的所有文件

4. while循环

#!/bin/bash
i=1
while ((i<10))
do
   echo "$i"
   i=`expr $i + 1`
done

5. 跳出循环

#!/bin/bash
for var1 in 1 2 3
do
   for var2 in 4 5
   do
      if ((var1 == 2 && var2 == 5))
      then
         break 2
      else
         echo "内层循环：$var1 $var2"
      fi
   done
   echo "外层循环：$var1"
done

[root@localhost shell_study]# ./test.sh 
内层循环：1 4
内层循环：1 5
外层循环：1
内层循环：2 4

注意：

1. if elif后面必须有then
2. []与变量之间必须有空格
   
3. 当变量是数字时,可用表达式：
   
   if [ $a == $b ]
   
   if [ a == b ]
   
   if ((a == b))
   
   if [ $a -gt $b ]
   
   if [ $a -lt $b ]
   
   if [ a > b ]
   
   if ((a < b))
   
   if ((a == b && a > 0))
   
   if ((a == b || a > 0))
   
4. 当变量是字符串时,可用表达式：
   
   if [ $a == $b ]
   
   if [ a == b ]
   
   if [ a == b -a a == "love" ]
   
   if [ a == b -o a == "love" ]
   
   if ((a == b && a == "love"))
   
   if ((a == b || a == "love"))