VAR1="hello world hello world"
# 所谓最长和最短其实就是正则表达式中的贪婪模式和非贪婪模式

# 删除匹配规则为*lo的数据（开头最短删除）
echo ${var1#*lo}
-> world hello world

# 删除匹配规则为*lo的数据（开头最长删除）
echo ${var1##*lo}
-> world

# 删除匹配规则为hel*的数据（尾部最短删除）
echo ${var1%hel*}
->hello world 

# 删除匹配规则为hel*的数据（尾部最长删除）
echo ${var1%%hel*}
->

# 替换第一个旧字符串world为Beijing
echo ${var1/world/Beijing}
->hello Beijing hello world

# 替换第一个旧字符串world为Beijing
echo ${var1/world/Beijing}
->hello Beijing hello Beijing

# x=${y-新值}
echo ${y=$var1}
->hello world hello world

y=
echo ${y-$var1}
->

y=hello Beijing
echo ${y-$var1}
->hello Beijing

# x=${y:-新值}
echo ${y:-$var1}
->hello world hello world

y=
echo ${y:-$var1}
->hello world hello world

y=hello Beijing
echo ${y:-$var1}
->hello Beijing

# x=${y+新值}
echo ${y+$var1}
->

y=
echo ${y+$var1}
->hello world hello world

y=hello Beijing
echo ${y+var1}
->hello world hello world

# x=${y:+新值}
echo ${y:+$var1}
->

y=
echo ${y:+$var1}
->

y=hello Beijing
echo ${y:+var1}
->hello world hello world

# x=${y=新值}
x=${y=$var1}
echo $x
->hello world hello world
echo $y
->hello world hello world

y=
x=${y=$var1}
echo $x
->
echo $y
->

y=hello Beijing
x=${y=$var1}
echo $x
->hello world hello world
echo $y
->hello Beijing

# x=${y:=新值}
x=${y:=$var1}
echo $x
->hello world hello world
echo $y
->hello world hello world

y=
x=${y:=$var1}
echo $x
->hello world hello world
echo $y
->hello world hello world

y=hello Beijing
x=${y:=$var1}
echo $x
->hello Beijing
echo $y
->hello Beijing

# x=${y?新值}
echo ${y?$var1}
->-bash: y: hello world hello world

y=
echo ${y?$var1}
->

y=hello Beijing
echo ${y?$var1}
->hello Beijing

# x=${y:?新值}
echo ${y?$var1}
->-bash: y: hello world hello world

y=
echo ${y:?$var1}
->-bash: y: hello world hello world

y=hello Beijing
echo ${y:?$var1}
->hello Beijing

# 方法一
var1="hello world"
echo ${#var1}
->11

# 方法二
len=`expr length "$var1"`
echo $len
->11

## var1有空格，必须加双引号，否则报错
expr: syntax error

## var2没有空格，可以不需要加双引号
var2=hello
expr length $var2
5

expr index "$var1" llo
->3

expr index "$var1" LOLe
->2

expr match "$var1" hel
->3

expr match "$var1" llo
->0

# 方法一
echo ${var1:2}
->llo world

# 方法二
echo ${var1:2:5}
->llo w

# 方法三
echo ${var1: -2}
->ld

echo ${var1: -2:1}
->l

# 方法四
echo ${var1:(3)}
->lo world

echo ${var1:(3):2}
->lo

echo ${var1:(-2):1}
->l

# 方法五
expr substr "$var1" 2 5
->ello

var2=hello
echo ${var2:2:5}
->llo

expr substr $var2 2 5
->ello

#!/bin/bash

string="Bigdata process framework is Hadoop,Hadoop is an open source project."


function print_tips
{
 echo "=============================="
 echo "1->打印string长度"
 echo "2->删除字符串中所有的Hadoop"
 echo "3->替换第一个Hadoop为Mapreduce"
 echo "4->替换所有Hadoop为Mapreduce"
 echo "q|Q->退出"
 echo "=============================="
}

function len_of_string
{
 echo "${#string}"
}

function del_hadoop
{
 echo "${string//Hadoop/}"
}

function rep_First_hadoop_mapreduce
{
 echo "${string/Hadoop/Mapreduce}"
}

function rep_all_hadoop_mapreduce
{
 echo "${string//Hadoop/Mapreduce}"
}
while true
do
 echo ""
        echo ""
        echo ""
        echo ""
 echo "【string=$string】"
 print_tips
 read -p "Please input your choice (1|2|3|4|q|Q):" choice
 case $choice in
  1)len_of_string;;
  2)del_hadoop;;
  3)rep_first_hadoop_mapreduce;;
  4)rep_all_hadoop_mapreduce;;
  q|Q)exit;;
  *)echo "Error input,only in (1|2|3|4|q|Q)";;
 esac
done

# 获取系统的所有用户并输出
#!/bin/bash

index=1
for user in `cat /etc/passwd | cut -d ":" -f 1`
do
 echo "这是第 $index 个用户，名称为：$user"
 index=$(($index + 1))
        # $(())主要用来进行整数运算，引用变量前面可以加$也可以不加
done

# 根据系统时间计算今年或明年
#!/bin/bash

year=`date +%Y`
echo "今年是$year年，明年是$(($year+1))年"

# 根据系统时间获取今年还剩下多少星期，已经过了多少星期
#!/bin/bash

# 计算现在是今年第几天
#!/bin/bash

days=$(date +%j)
weeks=$((days/7))
remain_weeks=$((52-weeks))
echo "今年还剩$remain_weeks个星期，已经过了$weeks个星期"

# 判定nginx进程是否存在，若不存在则自动拉起进程
#!/bin/bash

nginx_process_num=$(ps -ef | grep nginx | grep -v grep | wc -l)
if [ $nginx_process_num -eq 0 ];then
 echo "nginx未启动，拉起中..."
 Systemctl start nginx
else
 echo "nginx已启动..."
fi

# 设置为只读
var1="hello wordl"
var1="hello shell"
declare -r var1
var1="hello java"
->-bash: var1: readonly variable

# 设置为整形
num1=20
num2=$num1+30
echo $num2
->20+30 #linux默认设置为字符串
declare -i num3
num3=$num1+30
echo $num3
->50

# 定义数组
declare -a array
array=(1 2 3 4)
# 输出全部内容
echo ${array[@]} 
# 输出下标索引为1的内容
echo ${array[1]}
# 数组内元素个数
echo ${#array[@]}
# 指定索引赋值
array[0]=555
# 尾部添加元素
array[4]=666
# 删除元素
# 删除索引为2的元素
unset array[2]
# 清空整个数组
unset array
# 分片访问
# 显示数组下标索引从1开始到3的三个元素（包头不包尾）
${array[@]:1:4}
# 数组遍历
for n in ${array[@]}
do
    echo $n
done

# 显示脚本前定义过的所有函数及内容
# 在前问小试牛刀的print_tips方法后添加declare -f，即可发现只显示了命令前的函数及内容

num1=20
num2=100
expr $num1 \| $num2
expr $num1 \& $num2
expr $num1 \< $num2
expr $num1 \<= $num2
expr $num1 \> $num2
expr $num1 \>= $num2
expr $num1 = $num2
expr $num1 != $num2
expr $num1 + $num2
expr $num1 - $num2
expr $num1 \* $num2
expr $num1 / $num2
expr $num1 % $num2

bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 
23/5
4
scale=2
23/5
4.60

echo "ibase=2;111" |bc
->7

echo "obase=2;192" |bc
->11000000