第十三章、学习 Shell Scripts 循环 (loop)


while do done, until do done (不定循环)
while [ condition ]  <==中括号内的状态就是判断式
do <==do 是回圈的开始!
程序段落
done <==done 是回圈的结束
until [ condition ]
do
程序段落
done
[root@www scripts]# vi sh13.sh
#!/bin/bash
# Program:
# Repeat question until user input correct answer.
# History:
# 2005/08/29 VBird First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH while [ "$yn" != "yes" -a "$yn" != "YES" ]
do
read -p "Please input yes/YES to stop this program: " yn
done
echo "OK! you input the correct answer."
[root@www scripts]# vi sh13-2.sh
#!/bin/bash
# Program:
# Repeat question until user input correct answer.
# History:
# 2005/08/29 VBird First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH until [ "$yn" == "yes" -o "$yn" == "YES" ]
do
read -p "Please input yes/YES to stop this program: " yn
done
echo "OK! you input the correct answer."
[root@www scripts]# vi sh14.sh
#!/bin/bash
# Program:
# Use loop to calculate "1+2+3+...+100" result.
# History:
# 2005/08/29 VBird First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH s=0 # 这是加总的数值变量
i=0 # 这是累计的数值,亦即是 1, 2, 3....
while [ "$i" != "100" ]
do
i=$(($i+1)) # 每次 i 都会添加 1
s=$(($s+$i)) # 每次都会加总一次!
done
echo "The result of '1+2+3+...+100' is ==> $s"

for...do...done (固定回圈)

for 这种语法,则是『
已经知道要进行几次回圈』的状态!他的语法是:

for var in con1 con2 con3 ...
do
程序段
done

以上面的例子来说,这个 $var 的变量内容在回圈工作时:

  1. 第一次回圈时, $var 的内容为 con1 ;
  2. 第二次回圈时, $var 的内容为 con2 ;
  3. 第三次回圈时, $var 的内容为 con3 ;
  4. ....

我们可以做个简单的练习。假设我有三种动物,分别是 dog, cat, elephant 三种, 我想每一行都输出这样:『There are dogs...』之类的字样,则可以:

[root@www scripts]# vi sh15.sh
#!/bin/bash
# Program:
# Using for .... loop to print 3 animals
# History:
# 2005/08/29 VBird First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH for animal in dog cat elephant
do
echo "There are ${animal}s.... "
done
[root@www scripts]# vi sh16.sh
#!/bin/bash
# Program
# Use id, finger command to check system account's information.
# History
# 2009/02/18 VBird first release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH
users=$(cut -d ':' -f1 /etc/passwd) # 撷取帐号名称
for username in $users # 开始回圈进行!
do
id $username
finger $username
done
[root@www scripts]# vi sh17.sh
#!/bin/bash
# Program
# Use ping command to check the network's PC state.
# History
# 2009/02/18 VBird first release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH
network="192.168.1" # 先定义一个网域的前面部分!
for sitenu in $(seq 1 100) # seq 为 sequence(连续) 的缩写之意
do
# 底下的程序在取得 ping 的回传值是正确的还是失败的!
ping -c 1 -w 1 ${network}.${sitenu} &> /dev/null && result=0 || result=1
# 开始显示结果是正确的启动 (UP) 还是错误的没有连通 (DOWN)
if [ "$result" == 0 ]; then
echo "Server ${network}.${sitenu} is UP."
else
echo "Server ${network}.${sitenu} is DOWN."
fi
done

上面这一串命令运行之后就可以显示出 192.168.1.1~192.168.1.100 共 100 部主机目前是否能与你的机器连通! 如果你的网域与鸟哥所在的位置不同,则直接修改上头那个 network 的变量内容即可!其实这个范例的重点在 $(seq ..) 那个位置!那个 seq 是连续 (sequence) 的缩写之意!代表后面接的两个数值是一直连续的!

[root@www scripts]# vi sh18.sh
#!/bin/bash
# Program:
# User input dir name, I find the permission of files.
# History:
# 2005/08/29 VBird First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH # 1. 先看看这个目录是否存在啊?
read -p "Please input a directory: " dir
if [ "$dir" == "" -o ! -d "$dir" ]; then
echo "The $dir is NOT exist in your system."
exit 1
fi # 2. 开始测试文件罗~
filelist=$(ls $dir) # 列出所有在该目录下的文件名称
for filename in $filelist
do
perm=""
test -r "$dir/$filename" && perm="$perm readable"
test -w "$dir/$filename" && perm="$perm writable"
test -x "$dir/$filename" && perm="$perm executable"
echo "The file $dir/$filename's permission is $perm "
done

for...do...done 的数值处理
for (( 初始值; 限制值; 运行步阶 ))
do
程序段
done
  • 初始值:某个变量在回圈当中的起始值,直接以类似 i=1 配置好;
  • 限制值:当变量的值在这个限制值的范围内,就继续进行回圈。例如 i<=100;
  • 运行步阶:每作一次回圈时,变量的变化量。例如 i=i+1。

值得注意的是,在『运行步阶』的配置上,如果每次添加 1 ,则可以使用类似『i++』的方式,亦即是 i 每次回圈都会添加一的意思。

[root@www scripts]# vi sh19.sh
#!/bin/bash
# Program:
# Try do calculate 1+2+....+${your_input}
# History:
# 2005/08/29 VBird First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH read -p "Please input a number, I will count for 1+2+...+your_input: " nu s=0
for (( i=1; i<=$nu; i=i+1 ))
do
s=$(($s+$i))
done
echo "The result of '1+2+3+...+$nu' is ==> $s"

 

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