The linux shell has its own set of flow control statements, including conditional statements (if), loop statements (for, while), and selection statements (case). Below I will introduce the use of each statement through an example.
1. Shell conditional statement (if usage)
if statement structure [if/then/elif/else/fi]
if conditional test statement
then
action
[elif condition
action
else
action
]
be
If you are not very clear about the conditional test statement, you can refer to: Linux shell logical operators, logical expressions detailed
Shell commands can be separated by semicolons or by newlines. If you want to write multiple commands in one line, you can separate them by "';".
Such as:
[chengmo@centos5 ~]$ a=5;if [[ a -gt 4 ]] ;then echo 'ok';fi;
ok
Example: (test.sh)
#!/bin/sh
scores=40;
if [[ $scores -gt 90 ]]; then
echo "very good!";
elif [[ $scores -gt 80 ]]; then
echo "good!";
elif [[ $scores -gt 60 ]]; then
echo "pass!";
else
echo "no pass!";
fi;
Conditional tests include: [[]], [], test. Note: [[]] and variables are separated by spaces.
Second, the loop statement (for, while, until usage):
- for循环使用方法(for/do/done)
语法结构：
1.for … in 语句
for 变量 in seq字符串
do
action
done
说明：seq字符串 只要用空格字符分割，每次for…in 读取时候，就会按顺序将读到值，给前面的变量。
实例(testfor.sh)：
#!/bin/sh
for i in $(seq 10); do
echo $i;
done;
seq 10 产生 1 2 3 。。。。10空格分隔字符串。
2.for((赋值；条件；运算语句))
for((赋值；条件；运算语句))
do
action
done;
实例(testfor2.sh)：
#!/bin/sh
for((i=1;i<=10;i++));do
echo $i;
done;
- while循环使用（while/do/done)
while语句结构
while 条件语句
do
action
done;
实例1:
#!/bin/sh
i=10;
while [[ $i -gt 5 ]];do
echo $i;
((i--));
done;运行结果：========================
sh testwhile1.sh
10
9
8
7
6实例2：(循环读取文件内容：)
#!/bin/sh
while read line;do
echo $line;
done < /etc/hosts;
运行结果：===================
sh testwhile2.sh
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 centos5 localhost.localdomain localhost
- until循环语句
语法结构：
until 条件
do
action
done
意思是：直到满足条件，就退出。否则执行action.
实例(testuntil.sh)：
#!/bin/sh
a=10;
until [[ $a -lt 0 ]];do
echo $a;
((a—));
done;
结果：
sh testuntil.sh
10
9
8
7
6
5
4
3
2
1
0
三、shell选择语句（case、select用法）
- case选择语句使用（case/esac)
语法结构
case $arg in
pattern | sample) # arg in pattern or sample
;;
pattern1) # arg in pattern1
;;
*) #default
;;
esac说明：pattern1 是正则表达式,可以用下面字符：
* 任意字串
? 任意字元
[abc] a, b, 或c三字元其中之一
[a-n] 从a到n的任一字元
| 多重选择
实例：
#!/bin/sh
case $1 in
start | begin)
echo "start something"
;;
stop | end)
echo "stop something"
;;
*)
echo "Ignorant"
;;
esac运行结果：======================
testcase.sh start
start something
- select语句使用方法（产生菜单选择）
语法：
select 变量name in seq变量
do
action
done
实例：
#!/bin/sh
select ch in "begin" "end" "exit"
do
case $ch in
"begin")
echo "start something"
;;
"end")
echo "stop something"
;;
"exit")
echo "exit"
break;
;;
*)
echo "Ignorant"
;;
esac
done;运行结果：
说明：select是循环选择，一般与case语句使用。
