The first knowledge of shell "operator and judgment"

Little White's way to the shell

System version Red Hat Enterprise Linux Server Release 7.0

[Basic articles] Understanding the shell and basic operators 1. What is a shell script:

The shell is a program written in C that is a bridge for users to use Linux. The shell is both a command language and a programming language.
A Shell is an application that provides an interface through which users access the service of the operating system kernel.

2. View the existing interpreter of this machine:
Cat /etc/shells

        3. Modify the user's shell interpreter:
  Useradd -s /bin/ksh zhangsan [Specify the login shell when creating user Zhang]
    Usermod -s /bin/ksh zhangsan [Modify the login shell of the existing user Zhang San]

        4. View history commands:
  History
    Vim /etc/profile [Enter configuration file]
    HISTSIZE=1000 [Can modify the default history number]
    History -c [clear history command]

           5. Application of historical commands:
    ! [Serial number] [Command to execute the serial number again]
    ! Cat [Re-execute the command that started with cat recently]

             6. Redirect standard input / output / error output
             > Redirect normal output from command execution to file
             >>: only collect the correct output of the previous command
         2>: only collect the error output of the previous command
         &>: Correct and error output of previous commands
         >&2: Turn the correct output into the wrong output

       Operator:

                     1.expr [Note: There are spaces between numbers]

      [root@server0 ~]# expr 10 % 3 [To find the remainder of 10 and 3]
      1

           2.$[]
     [root@server0 ~]# echo $[5*3] [See 5 times 3]
     15

              [root@server0 ~]# x=5 [Assign x and y respectively and find the product]
      [root@server0 ~]# y=10
      [root@server0 ~]# echo $[x*y]
      50

             3.let [Do not display results]

         [root@server0 ~]# x=6
         [root@server0 ~]# y=6
         [root@server0 ~]# let i=x+y
         [root@server0 ~]# echo $i
         12

               Shorthand operation:
           [root@server0 ~]# x=5
           [root@server0 ~]# let x++ [add 1 to x value]
           [root@server0 ~]# echo $x
           6

            [root@server0 ~]# x=5
            [root@server0 ~]# let x+=5 [add 5 for x value]
            [root@server0 ~]# echo $x
            10
                Let x*=5 [for x value morning multiply 5]
                Let x/=5 [divide the value of x by 5]
                Let x%=5 [for x value]

                4.bc [Calculator]
           5+5
           10

                                5. Take the small tree point after the x bit
            Scale=x
                                    Take two decimal places
             [root@server0 ~]# echo "scale=2;2/10" | bc
            .20

               6. Compare: test or [parameter = parameter] [must have spaces in parentheses]

                             [root@server0 ~]# [ a = a ]
             [root@server0 ~]# echo $? [Judge whether to execute]
             0

                 7. Judging null -z
              [root@server0 ~]# [ -z $p ]
              [root@server0 ~]# echo $?
              0
              [root@server0 ~]# p=5
              [root@server0 ~]# [ -z $p ]
              [root@server0 ~]# echo $?
              1

                      1. One line to play multiple commands
           a && b Execute b only when a is successful
           a || b Execute b only if a fails
           a ; b execute a, execute b

       Example:
            Create user login for more than two email alerts
            #!/bin/bash
            Num=`who | wc -l`
            [ $num -gt 2 ] && mail -s error root < /root/mail

           Edit the message content vim /root/mail

                       2. Determine the status of the file/directory
          -e determines whether the object exists, and the existence is true
          -d determines whether the object is a directory, and the existence is true
          -f determines whether the object is a file, and the existence is true

          -r read
          -w write
         -o execute this

                  Note: root is true for all objects except "script"

          [Moring Morning Light]#chmod -x who.sh
          [Moring Morning Light]#ll who.sh
          -rw-r--r--. 1 root root 81 April 24 11:52 who.sh
          [Moring Morning Light]#[ -o who.sh ]
          [Moring Morning Light]#echo $?
          1

           [Judgement] && command Execute command
           [Judgement] || Command Execute command without it

               !  Negate

            Single branch if can only judge the pair
            Double branch if to judge right and wrong
            Multi-branch if multiple judgment 

First knowledge of the shell operator and judgment