#!/bin/bash  
IP_LIST="192.168.18.1 192.168.1.1 192.168.18.2"
for IP in $IP_LIST; do
    NUM=1
    while [ $NUM -le 3 ]; do
        if ping -c 1 $IP > /dev/null; then
            echo "$IP Ping is successful."
            break
        else
            # echo "$IP Ping is failure $NUM"
            FAIL_COUNT[$NUM]=$IP
            let NUM++
        fi
    done
    if [ ${#FAIL_COUNT[*]} -eq 3 ];then
        echo "${FAIL_COUNT[1]} Ping is failure!"
        unset FAIL_COUNT[*]
    fi
done

方法1：
# echo $RANDOM |md5sum |cut -c 1-8
471b94f2
方法2：
# openssl rand -base64 4
vg3BEg==
方法3：
# cat /proc/sys/kernel/random/uuid |cut -c 1-8
ed9e032c

方法1：

# echo $RANDOM |cksum |cut -c 1-8
23648321
方法2：
# openssl rand -base64 4 |cksum |cut -c 1-8
38571131
方法3：
# date +%N |cut -c 1-8
69024815

1）开头加解释器：#!/bin/bash

2）语法缩进，使用四个空格；多加注释说明。

3）命名建议规则：变量名大写、局部变量小写，函数名小写，名字体现出实际作用。

4）默认变量是全局的，在函数中变量local指定为局部变量，避免污染其他作用域。

5）有两个命令能帮助我调试脚本：set -e 遇到执行非0时退出脚本，set-x 打印执行过程。

6）写脚本一定先测试再到生产上。

run command from file in bash command and define after > the filename where the rest of the command is.

常量FUNCNAME，但是有一点区别是，它是一个数组而非字符串，其中数组的第一个元素为当前函数的名称

Two most useful commands in bash

This semicolon character is key for the whole thing to work.

This character should be escaped by a backslash. The complete command would then be:

strings $PWD/bin/myapp | egrep '\.gcda$'

The power of BASH!

可以用来统计代码行

find -print0 经常和 xargs -0 配合使用，处理文件名中换行符这种特殊情况

This is a very good example of why shell scripting does not scale from simple scripts to large projects. This is not the only place where changes in whitespace can lead to scripts that are very difficult to debug. A well-meaning and experienced programmer from another language, but new to bash scripting, might decide to clean up formatting to make it more consistent-- a laudable goal, but one which can lead to unintentional semantic changes to the program.

Flat, short bash scripts are extremely useful tools that I still employ regularly, but once they begin creeping in size and complexity it's time to switch to another language to handle that-- I think that is what (rightly) has driven things likes Python, Puppet, Ansible, Chef, etc.

Despite the syntactic horrors lurking in shell scripts there is still a beautiful simplicity that drives their use which is a testament to the core unix philosophy.