1. Function format

• Objective: to turn some opposed codes into functions, provide readability and reusability, and avoid writing the same code repeatedly.
• Function format:
  • Function keyword: function can be omitted without any parameters;
  • The symbol {indicates the beginning of the function body (a separate line after the function name can be used), and the symbol} indicates the end of the function body;
  • Function return value:
    • Explicit return: add return, followed by the value n(0~255);
    • Implicit return: the running result of the last command;

  [function] Function name [(parameter list)] {
      commands;
      [return int;]
  }
  

2. Function call

• Calling function name: all functions must be pre-defined before use, that is, the function is placed at the beginning of the script file, and only the function name can be used when calling the function.

  #! /bin/bash
  
  foo () {
      echo "This is foo"
      return 0
  }
  
  foo  # Call function foo()
  

3. Function parameters in shell

• In shell, when you call a function, you can pass parameters to the function. When taking parameters inside a function, the value of the parameter is obtained in the form of a special variable $n. For example, $1 represents the first parameter, $2 represents the second parameter,..., $100 represents the 100th parameter. When a function is called, the parameters are passed in the form of command line parameters.

  [cdl@h3c/home/cdl/Test]$cat demo.sh
  #!/bin/bash
  
  :<<comment
  
  echo "This is shell How to annotate the whole piece of code in the script,You don't need to use it line by line#Comment“
  
  comment
  
  
  printFunParams() {
      echo "Current file name: $0"
  
      echo "The first parameter in the command line parameters: $1"
      echo "The second parameter in the command line parameters: $2"
      echo "All parameters passed to a script or function: $@"
      echo "All parameters passed to a script or function: $*"
      echo "The total number of all parameters passed to a script or function: $#"
  }
  
  # Calling a function with parameters
  printFunParams A B C D
  [cdl@h3c/home/cdl/Test]$./demo.sh  
  Current file name: ./demo.sh
   The first parameter in the command line parameters: A
   The second parameter in the command line parameters: B
   All parameters passed to a script or function: A B C D
   All parameters passed to a script or function: A B C D
   The total number of all parameters passed to a script or function: 4
  

4.shell programming specification

• The script file name ends with. sh, and try to use UTF-8 file encoding;
• The first line of the script file uses #! / bin/bash without spaces and any options;
• Program header comments: script function, author, modification time, etc;
• The global variable is defined at the beginning of an empty line after the program header comment, in uppercase as much as possible;
• Use ${var} for variable reference, and it is not recommended to use $VaR;
• Single quotation mark (recommended constant), double quotation mark (recommended variable), no quotation mark (recommended number);
• The variables in let and (()) should not be added with $, and the variables in expr should not be added with $;
• Pairs of symbols shall be written at one time to prevent omission, such as: {}, [], "";
• There are spaces at both ends of [] or [[]];
• Command replacement, the ${cmd} format is recommended, and the 'cmd' format is not recommended;
• Script code indentation is not mandatory, or use spaces or TAB keys uniformly;
• Key operations are given an execution result status. If the execution fails, exit n (the execution status result of the previous command is checked with $?);
• Key operations need to be logged to record the success, failure and operation time of the operation;
• You can use & &, |𞓜 instead of simple if then else fi statements;
• If the command is too long, use compliance \ to extend it, except for the command with pipeline;
• Try to use function functions, define different functions as different functions, and call functions directly;
• All file names are named in lowercase. rm -rf is prohibited*
• Separate composite commands (if branch structure and for/while loop), and try to use the following methods:

  if [ condition ]; then
      ......
  fi
  
  while true; do
      ......
  done
  
  for value in {1...10}; do
      ......
  done
  

5. Debugging of shell script

• Debug trace (sh -x script file name)

  • The preferred method for shell script file debugging. After entering the debugging mode, the shell will execute statements in turn to generate debugging information (with a plus sign means that the statement is executed by the shell, and without a plus sign means that the output is generated by the shell)

  [cdl@h3c/home/cdl/Test]$sh -x demo.sh
  + :
  + printFunParams A B C D
  + echo 'Current file name: demo.sh'
  Current file name: demo.sh
  + echo 'The first parameter in the command line parameters: A'
  The first parameter in the command line parameters: A
  + echo 'The second parameter in the command line parameters: B'
  The second parameter in the command line parameters: B
  + echo 'All parameters passed to a script or function: A' B C D
   All parameters passed to a script or function: A B C D
  + echo 'All parameters passed to a script or function: A B C D'
  All parameters passed to a script or function: A B C D
  + echo 'The total number of all parameters passed to a script or function: 4'
  The total number of all parameters passed to a script or function: 4
  

• Check syntax (sh -n script file name)

  • The syntax of the script file is checked, but the script is not executed. If there is a syntax error, the shell will report an error. If there is no error, nothing will be displayed.

  [cdl@h3c/home/cdl/Test]$cat demo.sh
  #!/bin/bash
  
  :<<comment
  
  echo "This is shell How to annotate the whole piece of code in the script,You don't need to use it line by line#Comment“
  
  comment
  
  
  printFunParams() {
      echo "Current file name: $0"
  
      echo "The first parameter in the command line parameters: $1"
      echo "The second parameter in the command line parameters: $2"
      echo "All parameters passed to a script or function: $@"
      echo "All parameters passed to a script or function: $*"
      echo "The total number of all parameters passed to a script or function: $#"
  
  
  # Calling a function with parameters
  printFunParams A B C D
  [cdl@h3c/home/cdl/Test]$sh -n demo.sh
  demo.sh: line 22: syntax error: unexpected end of file
  

• trap syntax (signal capture)

  • The trap command is used to specify the command to be executed after receiving the signal(SIGHUP, SIGINT, SIGQUIT, SIGKILL, SIGTERM, etc.). Common usage:
    • DEBUG signal tracks and analyzes the changes of related variable values;
    • Clean up when the script program is interrupted;
    • Ask the user whether to terminate the script;

• tee syntax (pipe copy tee -a file name)

  • The tee command is used to read the standard input data and output its contents to a file, that is, to the screen and the specified file. For example: ls -l tee file.txt | less