1. 重定向输入和输出

• 输出重定向

bash shell用大于号(>)来完成这项功能：command > outputfile

如果不想覆盖文件原有内容，而是想要将命令的输出追加到已有文件中，比如你正 在创建一个记录系统上某个操作的日志文件。在这种情况下，可以用双大于号(>>)来追加数据。

• 输入重定向

输入重定向符号是小于号(<): command < inputfile

(vent) szbddeMacBook-Pro-3:shell $ wc < test6
       3      16      99

wc命令可以对对数据中的文本进行计数。默认情况下，它会输出3个值:

 文本的行数
 文本的词数 

 文本的字节数

       2.退出脚本

查看Linux退出状态码：echo $?

 3.数值比较

4.字符串比较

 5.文件比较

6.双括号命令符号

双圆括号：可以是任意的数学赋值或比较表达式

双方括号：针对字符串的高级特性

 7.case命令

#!/bin/bash

case $USER in
v_suxue | barbara)
  echo "Welcome, $USER"
  echo "Please enjoy your visit";;
testing)
  echo "Special testing account";;
jessica)
  echo "Do not forget to log off when you're done";;
*)
  echo "Sorry, you are not allowed here";;
esac



# 打印
Welcome, v_suxue
Please enjoy your visit

8.更改字段分隔符

• 空格
• 制表符
• 换行符

 9.break命令

• 跳出单个循环

#!/bin/bash

for var1 in 1 2 3 4 5 6 7 8 9 10
do
  if [ $var1 -eq 5 ]
  then
    break
  fi
  echo "Iteration number: $var1"
done
echo "The for loop is completed"

Iteration number: 1
Iteration number: 2
Iteration number: 3
Iteration number: 4
The for loop is completed

• 跳出内部循环

#!/bin/bash

for (( a = 1; a < 4; a++))
do
  echo "Outer loop: $a"
  for (( b = 1; b < 100; b++ ))
  do
    if [ $b -eq 5 ]
    then
      break
    fi
    echo "    Inner loop: $b"
  done
done

Outer loop: 1
    Inner loop: 1
    Inner loop: 2
    Inner loop: 3
    Inner loop: 4
Outer loop: 2
    Inner loop: 1
    Inner loop: 2
    Inner loop: 3
    Inner loop: 4
Outer loop: 3
    Inner loop: 1
    Inner loop: 2
    Inner loop: 3
    Inner loop: 4

• 跳出外部循环

停止外部循环，break命令接受单个命令行参数值：break n

默认情况下，n为1，表明跳出的是当前的循环。如果你将n设为2，break命令就会停止下一级的外部循环。

#!/bin/bash

for (( a = 1; a < 4; a++))
do
  echo "Outer loop: $a"
  for (( b = 1; b < 100; b++ ))
  do
    if [ $b -gt 4 ]
    then
      break 2
    fi
    echo "    Inner loop: $b"
  done
done

Outer loop: 1
    Inner loop: 1
    Inner loop: 2
    Inner loop: 3
    Inner loop: 4

10.continue命令

#!/bin/bash

for(( var1 = 1; var1 < 15; var1++ ))
do
  if [ $var1 -gt 5 ] && [ $var1 -lt 10 ]
  then
    continue
  fi
  echo "Iteration number: $var1"
done

Iteration number: 1
Iteration number: 2
Iteration number: 3
Iteration number: 4
Iteration number: 5
Iteration number: 10
Iteration number: 11
Iteration number: 12
Iteration number: 13

#!/bin/bash

for(( a = 1; a <= 5; a++ ))
do
  echo "Iteration $a:"
  for (( b = 1; b < 3; b++ ))
  do
    if [ $a -gt 2 ] && [ $a -lt 4 ]
    then
      continue 2
    fi
    var3=$[ $a * $b ]
    echo "    The result of $a * $b is $var3"
  done
done

Iteration 1:
    The result of 1 * 1 is 1
    The result of 1 * 2 is 2
Iteration 2:
    The result of 2 * 1 is 2
    The result of 2 * 2 is 4
Iteration 3:
Iteration 4:
    The result of 4 * 1 is 4
    The result of 4 * 2 is 8
Iteration 5:
    The result of 5 * 1 is 5
    The result of 5 * 2 is 10