A detailed introduction to the basics of Linux scripting

1. Script vim environment

In the script, you usually need to display some script information. This information can be automatically displayed by setting vim
/etc/vimrc This file is the main configuration file of vim. The content of the file is effective globally. ~/.vimrc This file is a sub-file of vim. Editing the configuration file of vim in the user's home directory can also control it, but it is only effective for the current user.
vim ~/.vimrc Edit the configuration file

The meaning of the configuration information:

set nu shows line number
ts=2 means the Tab key is equivalent to 2 spaces
et Convert the Tab key to a space
ai indicates indentation func is the function type mark endfunc is the function end mark call indicates calling append indicates adding statements (0 indicates the first line, and the added content is quoted with "")
autocmd sets the function WESTOS() to automatically call when creating a new file ending with .sh or .script
strftime means automatically identifying the capture time and outputting it in year/month/day format
!/bin/bash magic number; the first command executed when the script is run, usually other specified running environments in the script

  set nu ts=2 et ai
  
  autocmd BufNewFile *.sh,*.script call SHELLTITLE()
   func SHELLTITLE()
    call append(0,"#####################################")
    call append(1,"# Author: lee")
    call append(2,"# Version: 1.0")
    call append(3,"# Create_Time: ".strftime("%Y/%m/%d"))
    call append(4,"# Mail: [email protected]")
    call append(5,"# Info: ")
    call append(6,"#")
    call append(7,"#")
    call append(8,"#####################################")
    call append(9,"")
    call append(10,"#!/bin/bash")
  endfunc

As shown in the figure:

Scripting Exercise:
(1). Create a ip_show.sh script so that the network card IP address can be displayed after the network card is entered, and there should be a blank input prompt

#!/bin/bash
[ -z "$1" ] && {
   echo "Error : Please input interface following script !!"
   exit
   }
ifconfig $1 | awk '/\<inet\>/{print $2}'

The effect is as shown below:

(2). Create host_messages.sh to display the name of the current host and the IP address of the user who logged in to the current host.
hostname:
ipaddress:
username:

#!/bin/bash
  echo "hostname: `hostname`"
  echo "ipaddress: `ifconfig ens3 | awk '/\<inet\>/{print $2}'`"
  echo "username: $USER"

!!!Note the difference between single quotes and back quotes. Single quotes '' are weak quotes and cannot quote \ ` $ !; Back quotes `` are strong quotes and can quote all

(3) Create clear_log.sh to execute the script to clear the log

#!/bin/bash
[ "$USER" != "root" ] && {
  echo "Please run $0 with root !!!"
  exit
  }
[ ! -e "/var/log/messages" ] && {
  echo "No logfile here!!!"
  exit
}
> /var/log/messages && {
  echo logfile has been cleared !!!
  exit
}

2. How to define the environment in shell scripts

Environmental Level
export a=1
The variable disappears after the environment is closed.
User Level
vim ~/.bash_profile User environment variable configuration file
export a=1
Invalid after switching users
System Level
vim /etc/profile main configuration file
export a=2
vim /etc/profile.d/westos.sh sub-configuration file
export b=3
After this variable is set, all users in the system can use

When the variable value specified by the shell command export a=1 is used, it cannot be recognized in the script because the two are not opened in the same shell.
Therefore, you can edit the required variables in the variable configuration file source ~/.bash_profile to take effect the current changes
vim ~/.bash_profile User environment variable configuration file User environment variable configuration file is only effective for the user to whom it is set. The variable becomes invalid after switching users, because after switching users, when you open the shell, you read the .bash_profile in your home directory.
vim /etc/profile System-level environment variable configuration file (generally no longer edit information in this configuration file)
vim /etc/profile.d/westos.sh System-level environment variable custom sub-configuration file (custom name, does not exist originally) source
The /etc/profile.d/westos.sh file takes effect. After setting the variables in the sub-profile, all users in the system can use it.

3. Translated characters in shell scripts

Translate a single character: \
Batch translation characters: '' ""
'' Single quotes are weak quotes and cannot be quoted \ ` $ !
`` Backsingle quotes are strong quotes that can quote all

4. Arrays of variables in scripts

a=(1 2 3 4 5)
echo ${a[0]} displays the first element
echo ${a[-1]} displays the last element
echo ${a[@]:0:3} starts from the first element and displays a total of 3 elements
echo ${a[@]:2:3} starts from the second element and displays 3 elements
echo ${#a[@]} displays all elements
echo ${#a[*]} displays all elements
unset a deletes the array
unset a[0] deletes the specified element

5. Alias ​​settings for system commands

alias xie='vim' # Temporary setting to switch off shell failure

vim ~/.bashrc #User-level configuration file
alias xie='vim'
source ~/.bashrc #The file is only effective for the current user and becomes invalid after switching users

vim /etc/bashrc #system level
alias xie='vim'
source ~/.bashrc #The file takes effect for all users of the system

To remove an alias:
After deleting the system configuration file contents
unalias xie #Delete the alias in the current environment

6. Passing parameters in scripts

Non-interactive mode:
sh /mnt/test.sh westos linux redhat
$0 is /mnt/test.sh #the script itself
$1 is westos #The first string of characters entered after the script
$2 is linux #The second string of characters
$3 is redhat #The third string of characters
$* is westos linux redhat #All characters entered after the script "westos linux redhat"
$@ is westos linux redhat #All characters entered after the script "westos" "linux" "redhat"
$# is 3 #The number of strings entered

Interactive Mode:
read -p enters interactive mode; WORD represents a variable

#!/bin/bash
read -p "Pleas input word:" WORD
echo $WORD

The effect is as shown below:

-s means hiding the entered WORD

#!/bin/bash
read -p "Pleas input word:" -s WORD
echo $WORD

There will be hidden effects when entering.

As shown in the figure:

Scripting Exercise:
Write a script that can create a new user and remind you when the user already exists, and ask you to enter the new user's password if it does not exist.

  #!/bin/bash
  [ -z $1 ] && {
    echo "Please input username: !!" 
    exit
    }
    id $1 &> /dev/null && {
    echo "$1 is exit !!"
  }||{
    useradd $1 &> /dev/null
    read -p "Please input user's password: " -s PASS
    echo " "
   echo $PASS |passwd --stdin $1 &> /dev/null && {
    echo "user is created success !!"
      }
  }

The effect is as shown below:

7. Loop function in script

#!/bin/bash
ECHO()
{
 [ "$WORD" = "exit" -o "$WORD" = "EXIT" ] && {
    echo bye 
    exit
  }
    read -p "Please input word:" WORD
    echo $WORD
    ECHO
}
ECHO

Functions can make script actions execute in a loop.

The effect is as shown below:

Scripting Exercise:
Write a loop script that can detect file types. When the file does not exist, it will display that the file does not exist. If it exists, it will output the file type. When exit is entered, bye is output to exit the script

#!/bin/bash
FILE()
{
  [ $1 "$FILENAME" ] && {
  echo $FILENAME is "$2"
  echo ""
  check
  }
}
check()
{

  read -p "Please input filename :" FILENAME
  [ "$FILENAME" = "exit" ] && {
  echo bye
  exit
  }
  FILE "! -e" "not found"
  FILE -L "link file"
  FILE -f "common file"
  FILE -d "directory"
  FILE -S "socket"
  FILE -b "block file"
  FILE -c "cahr file"
  check
  }
check

Script Exercise 2:
Write a script that creates users in a loop. If the user exists, the output information is that the user already exists. If it does not exist, a new user is created and a password is entered. The password is hidden during input, and it is displayed that the user has been created. And keep prompting to enter the username to create the next user until you exit actively

#!/bin/bash
Create_user()
{

  read -p "Please input username : " USER
  echo ""
  [ "$USER" = "exit" ] && {
  echo bye
  exit
  }
  
  id $USER &> /dev/null && {
  echo "$USER is exit !!"
  Create_user
}||{
  useradd $USER  
  read -p "Please input password :" -s PASS
  echo ""
  echo $PASS |passwd --stdin $USER &> /dev/null && {
  echo ""
  echo $USER is created !!
  }
  
  }
  Create_user
}
create_user

The effect is as shown below:

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