shell 实用tips， sed，grep正则表达式

最近用shell做个小工具，发现很多东西在使用的时候，反而没有那么清楚的印象。总结一下最近的小技巧，希望拯救下自己就贫瘠的记忆

1. shell [[]]判断后，多条语句 &&
2. shell 清空文件：
    way 1:  echo "" >$file 不好，会多一行空白行
    way 2:   > $file    好，解决了way1的问题

3. read file by line:
      while read line;do
      done< $file
    
4. diference between unset and "" ：

5. grep 多个pattern：   grep -E 'PATTERN1.*PATTERN2' FILE

6. 取出第二个字段： "a b"    
   s="a b"
   echo ${s##* }
   
7. grep的参数里引用其他变量中，但是里面有dollar符号的：
            '^'"$user"'\s+\$'"$hashNum"`
8.  $() and `` 斜单引号
    $（）POSIX command substitution syntax ，you can
    https://www.quora.com/Bash-shell-What-does-the-sign-mean-when-one-puts-date-in-the-dollar-sign
    But I recommend using $() all the time because you can nest other subcommands inside it. So, you can do things like $(command1 -params $(command2 -params)) or go completely haywire with more nestings.
    For example: ret=$(ls /root)

如果希望取得命令的返回结果，必须用$()的形式, 单反冒号是不行的
    
9. file replace one line which contrains some substring: sed -i -e "s/^${user}.*/${replaced}"  $file
      
10. shell boolean: var=true, if [ $var = true ]

11. get last line of file: tail -1 $file
12.  ”sed“ match space or tab:
         sed -i "/^DenyUsers[[:space:]].*$user.*/ s/$user/meas/g" /etc/ssh/sshd_config
         sed -i "/^DenyUsers\s.*$user.*/ s/$user/something/g" /etc/ssh/sshd_config
   
13. sed, add one file to another file after matched lines，非常实用:
       sed -i  '/\/bin\/ksh/r'" $file" $script  note: add content from $file to $script after line "/bin/ksh"

14. shell 逻辑运算，条件判断：http://www.tldp.org/LDP/abs/html/comparison-ops.html
15.  shell, difference between "print" "echo":

 

未完待续