循环

循环执行

将某代码重复运行多次

重复运行多少次：

循环次数事先已知

循环次数事先未知

有进入条件和退出条件

循环执行语句：

for，while，until

for

for

for 变量名 in 列表；do

循环体

done

执行机制：

依次将列表中的元素赋值给“变量名”；每次赋值后即执行一次循环体；直到列表中的元素耗尽，循环结束

列表生成方式：

(1)直接给出列表

(2)整数列表：

(a){start..end}

(b)$(seq [start [step]] end)

(3)返回列表的命令

$(COMMAND)

(4)使用glob，如：*.sh

(5)变量引用：

$@,$*

例子：

编写一个脚本，ping一个范围内的主机，显示ping结果，并统计ping通的主机有多少台，没ping通的主机有多少台

######方法一：

[root@centos SC]#vim scanip.sh

#!/bin/bash

net=172.17.252

up=0

down=0

for i in {1..10}

do

{ ping -c1 -W1 $net.$i &> /dev/null; } && { echo $net.$i is up; let ++up; } || { ech

o $net.$i is down;let ++down; }

done

echo The up host is $up

echo The down host is $down

~

~

~

~

▽

~

~

~

~

~

~

"scanip.sh" 10L, 235C written

######测试结果：

[root@centos SC]#./scanip.sh

172.17.252.1 is down

172.17.252.2 is down

172.17.252.3 is down

172.17.252.4 is down

172.17.252.5 is down

172.17.252.6 is down

172.17.252.7 is down

172.17.252.8 is down

172.17.252.9 is down

172.17.252.10 is down

The up host is 0

The down host is 10

######方法二：

[root@centos SC]#vim scanip1.sh

#!/bin/bash

net=192.168.227

up=0

down=0

for i in {128..138}

do

if ping -c1 -W1 $net.$i &> /dev/null; then

echo $net.$i is up

let up++ ;

else

echo $net.$i is down

let down++ ;

fi

done

echo The up host is $up

echo The down host is $down

~

~

~

~

~

~

~

"scanip1.sh" 16L, 248C written

######测试结果：

[root@centos SC]#./scanip1.sh

192.168.227.128 is down

192.168.227.129 is down

192.168.227.130 is up

192.168.227.131 is down

192.168.227.132 is up

192.168.227.133 is down

192.168.227.134 is down

192.168.227.135 is down

192.168.227.136 is down

192.168.227.137 is down

192.168.227.138 is down

The up host is 2

The down host is 9

特殊用法：

双小括号方法，即((...))格式，也可以用于算术运算

双小括号方法也可以使bash shell实现C语言风格的变量操作

\#I=10

\#((I++))

for循环的特殊格式：

for((控制变量初始化；条件判断表达式；控制变量的修正表达式))

do

循环体

done

控制变量初始化：仅在运行到循环代码段时执行一次

控制变量的修正表达式：每轮循环结束会先进行控制变量修正运算，而后再做条件判断

[root@centos SC]#sum=0;i=1;while [ "$i" -le 100 ];do let sum+=i i+=2;done;echo sum=$sum

sum=2500

[root@centos SC]#sum=0;for i in {1..100..2};do let sum+=i;done;echo sum=$sum

sum=2500

[root@centos SC]#for((sum=0,i=1;i<=100;i+=2));do let sum+=i;done;echo sum=$sum

sum=2500

while

while

while CONDITION ; do

循环体

done

CONDITION:循环控制条件，进入循环之前，先做一次判断；每一次循环之后会再次做判断；条件为“true”，则执行一次循环；指定条件测试状态为“false”终止循环，因此，CONDITION一般应有循环控制变量；而此变量的值会在循环体不断的被修正

进入条件：CONDITION为true

退出条件：CONDITION为false

示例：编写一个脚本，创建十个用户，并给用户设定八位的随机口令

[root@centos SC]#vim useradd.sh

#!/bin/bash

i=1

while [ "$i" -le 10 ];do

useradd user$i

echo "user$i is created"

######生成随机口令

password=`cat /dev/urandom |tr -dc 'a-zA-Z0-9'|head -c 8`

######给新建用户设定随机口令

echo $password | passwd --stdin user$i &> /dev/nmll

let i++

done

~

~

~

~

~

~

~

"useradd.sh" [New] 9L, 209C written

[root@centos SC]#chmod +x useradd.sh

######测试结果

[root@centos SC]#./useradd.sh

user1 is created

user2 is created

user3 is created

user4 is created

user5 is created

user6 is created

user7 is created

user8 is created

user9 is created

user10 is created

[root@centos SC]#cat /etc/shadow | grep "^user*"

user1:$6$1AhalPjO$W92BskUXlQweC.WT.2jYs90SKm7QbWgBOKbaZ4fWO/n687Kv1t3pRALIYB5nFf9yacJssRGaihMxKhd57U3ag.:17351:0:99999:7:::

user2:$6$fjDqy8Cy$KhwVkLCSWfA.kFDMjQBlOlvfLOVM/C/l/FMyorQtSzadFHv.M1EZl/0atWVkFRx1mkGA6dCf2JfNsMKlndblv/:17351:0:99999:7:::

user3:$6$bW.zRP2R$yBZ7EpHe2dZyUlnRaSjbp69ZoRptdL7QlbHAfvkKEO2FAA3TItf.l5gyzG8Q9PPWNm.MnP7mc.ub/ryr65Zc41:17351:0:99999:7:::

user4:$6$BeboRIh7$XGzKa0FEZ9le5ZoF01yHEGc7McwwHG5fagrfHunloOmcHpv4PYRg9LjwEDd2CmG9vgPecCocf5Akx5aDIY5XY0:17351:0:99999:7:::

user5:$6$QBO0TMwF$kBHG5U7XMd5HhKeEFBtyA9UhCIwUkP7JTVmm2prwjckwIWkyfbt2fPUbcZT7glQR5uzTL3LCC5U1Edb8w9N1O1:17351:0:99999:7:::

user6:$6$9ex4OXkS$2/xeqayIyvq.ByakgGpcwTqhNpSqrPrqZSRrsFpAnVTMwgow4TW7m6XXIchakBYVhVeRVOLiN23jwInGcfbv41:17351:0:99999:7:::

user7:$6$510jI2iP$UKo7QGg2fI3aDPN0k6C7U8Ueoz3SsDsSzlA8uENaApdEuxrsDXwue6LQAlrh8gBUNk.rdtBKnib4PRW0we8xS1:17351:0:99999:7:::

user8:$6$DwD1a33y$mKun3ZQkgf49NGSGIjnD5Ccp1Wx15ar7YBlJwfryVxFIJmLm5HCAaZVhK96KGZdh5v8C4U2WmuIOmA8kAVQjQ0:17351:0:99999:7:::

user9:$6$5mZ3OW1V$/2QlZ6uKbp3AnZxctd7Yxp8SzYTKH8IHSactDJfUwO0vLCtzKIOIv.Td4xFam7mprQG0KhxYy4GWZqEq0trjj0:17351:0:99999:7:::

user10:$6$E8tDCBtU$qVWCI0VsAUxBH1GgsK0E710FzIeX1023050AI//kTNF0Xtpx1N4ENiJnbvXPul7kue9j/hRVPdsG/K2YrrKPB0:17351:0:99999:7:::

while循环的特殊用法(遍历文件的每一行)：

while read line; do

循环体

done < /PATH/FROM/SOMEFILE

依次读取/PATH/FROM/SOMEFILE文件中的每一行，且将 行赋值给变量line

示例：编写一个脚本，读取/etc/passwd文件中的每一行，判断哪些用户是系统用户，哪些用户是普通用户并显示

[root@centos SC]#vim readline.sh

#!/bin/bash

while read passwdline

do

uid=`echo $passwdline |cut -d: -f3`

user=`echo $passwdline |cut -d: -f1`

[ "$uid" -ge 1000 ] && echo "$user is common user" || echo "$user is system user"

done < /etc/passwd

unset passwdline user uid

~

~

~

~

~

~

~

"readline.sh" [New] 8L, 240C written

[root@centos SC]#chmod +x readline.sh

[root@centos SC]#./readline.sh

root is system user

bin is system user

daemon is system user

adm is system user

lp is system user

sync is system user

shutdown is system user

halt is system user

mail is system user

operator is system user

games is system user

ftp is system user

nobody is system user

systemd-bus-proxy is system user

systemd-network is system user

dbus is system user

polkitd is system user

abrt is system user

unbound is system user

tss is system user

libstoragemgmt is system user

rpc is system user

colord is system user

usbmuxd is system user

saslauth is system user

geoclue is system user

rtkit is system user

rpcuser is system user

nfsnobody is common user

radvd is system user

qemu is system user

ntp is system user

chrony is system user

setroubleshoot is system user

sssd is system user

pulse is system user

gdm is system user

gnome-initial-setup is system user

sshd is system user

avahi is system user

postfix is system user

tcpdump is system user

apache is system user

xmj is common user

huahua is common user

until

until

until CONDITION; do

循环体

done

进入条件：CONDITION为false

退出条件：CONDITION为true

示例：

[root@centos SC]#vim h.sh

#!/bin/bash

i=1

######输出显示i=1-4当i=5时跳出脚本

until false;do

[ $i -eq 5 ]&&break

echo i=$i

sleep 0.3

let i++

done

~

~

~

~

~

~

~

######测试结果

[root@centos SC]#./h.sh

i=1

i=2

i=3

i=4

[root@centos SC]#bash -x h.sh

+ i=1

+ false

+ '[' 1 -eq 5 ']'

+ echo i=1

i=1

+ sleep 0.3

+ let i++

+ false

+ '[' 2 -eq 5 ']'

+ echo i=2

i=2

+ sleep 0.3

+ let i++

+ false

+ '[' 3 -eq 5 ']'

+ echo i=3

i=3

+ sleep 0.3

+ let i++

+ false

+ '[' 4 -eq 5 ']'

+ echo i=4

i=4

+ sleep 0.3

+ let i++

+ false

+ '[' 5 -eq 5 ']'

+ break

创建无限循环

while true; do

循环体

done

...

until false； do

循环体

done