Shell编程笔记

• 赋值语句=前后不能有空格

• 算数表达式使用expr

  // 1+2
  // 注意运算符前后必须要有空格
  expr 1 + 2
  expr 2 - 1
  expr 10 / 5
  expr 20 % 3
  // *是特殊字符，所以要转义
  expr 10 \* 3
  

• 反引号用来执行被反引号括起来的命令

  result=`expr 6 + 3`
  echo $result
  

• 单引号和双引号，与PHP一样，单引号中的变量会被当成字符串，双引号中的变量会被替换成相应的变量

  str=23
  // Str is 23
  echo "Str is $str"
  // Str is $str
  echo 'Str is $str'
  

• 关于exit 1还是exit 0. 每条命令执行完后都有一个退出码(exit status)，Linux中退出码为0表示命令执行成功，不为0表示命令执行失败。使用$?来获取前一条命令的执行状态

  rm unknownfile
  // 1
  echo $?
  ls
  // 0
  echo $?
  

• 读取键盘输入

  echo 'Input your name:'
  read name
  echo "Hi $name"
  

• if语法

  if condition
  then
      command1 if condition is true or if exit status
      of condition is 0 (zero)
      ...
      ...
  fi
  

  或者是

     if condition
     then
                 condition is zero (true - 0)
                 execute all commands up to else statement
  
     else
                 if condition is not true then
                 execute all commands up to fi
     fi
  

  或者是

     if condition
     then
                 condition is zero (true - 0)
                 execute all commands up to elif statement
     elif condition1 
     then
                 condition1 is zero (true - 0)
                 execute all commands up to elif statement  
     elif condition2
     then
                 condition2 is zero (true - 0)
                 execute all commands up to elif statement          
     else
                 None of the above condtion,condtion1,condtion2 are true (i.e. 
                 all of the above nonzero or false)
                 execute all commands up to fi
     fi
  

• test命令

  数字比较

  Mathematical Operator in  Shell Script	Meaning	Normal Arithmetical/ Mathematical Statements	But in Shell
  			For test statement with if command	For [ expr ] statement with if command
  -eq	is equal to	5 == 6	if test 5 -eq 6	if [ 5 -eq 6 ]
  -ne	is not equal to	5 != 6	if test 5 -ne 6	if [ 5 -ne 6 ]
  -lt	is less than	5 < 6	if test 5 -lt 6	if [ 5 -lt 6 ]
  -le	is less than or equal to	5 <= 6	if test 5 -le 6	if [ 5 -le 6 ]
  -gt	is greater than	5 > 6	if test 5 -gt 6	if [ 5 -gt 6 ]
  -ge	is greater than or equal to	5 >= 6	if test 5 -ge 6	if [ 5 -ge 6 ]

  字符串比较

  Operator	Meaning
  string1 = string2	string1 is equal to string2
  string1 != string2	string1 is NOT equal to string2
  string1	string1 is NOT NULL or not defined
  -n string1	string1 is NOT NULL and does exist
  -z string1	string1 is NULL and does exist

  文件或者目录

  Test	Meaning
  -s file	Non empty file
  -f file	Is File exist or normal file and not a directory
  -d dir	Is Directory exist and not a file
  -w file	Is writeable file
  -r file	Is read-only file
  -x file	Is file is executable

• for语法

      for { variable name } in { list }
      do
               execute one for each item in the list until the list is
               not finished (And repeat all statement between do and done)
      done
  

  或

  for ((  i = 0 ;  i <= 5;  i++  ))
  do
    echo "Welcome $i times"
  done
  

• while语法

     while [ condition ]
     do
           command1
           command2
           command3
           ..
           ....
      done
  

  例子

  i=0
  while [ $i -lt 10 ]
  do
      echo $i
      i=`expr $i + 1`
  done
  

• case语法, 注意每个语句结束是两个;;

     case  $variable-name  in
      pattern1)   command
                      ...
                      ..
                      command;;
      pattern2)   command
                      ...
                      ..
                      command;;
      patternN)   command
                      ...
                      ..
                      command;;
      *)             command
                      ...
                      ..
                      command;;
     esac