SEEING THE WORLD AS SHELL SEES

• Lets look at some of the magic that occurs in the command line when we press ENTER key.

Expansion

• Each time we type a command & press Enter key, bash performs several substititutions upon text before command is executed. The process that makes it happen is called expansion
• To demonstrate this lets try

echo Welcome to Linux by Khaja

• Note: IMages with terminal in black (git bash) is connected to ubuntu 18 and Images with terminal in blue (powershell) is connected to centos 8.1
• Now lets execute the following
• What happened? * is a wild card character, so your bash will try to find/match any characters in the files present in current working directory (cwd).
• So initially when we executed there were no files in cwd, so it printed *. Then we created two files and execute echo * again, now * will match any file name in cwd so it printed the file names
• Pathname Expansion: The mechanism by which wild cards work is called as pathname expansion
  • Lets experiment with the following

  create two directories Videos and Documents in the cwd
  

  
  • Lets use pathname expansions

  echo D*
  echo *s
  echo *t
  

  
• Tilde Expansion: Tilde ~ expans into the name of home directory of the current user
• Arithmetic Expansion: Shell allows you to expand arithmetics and the syntax would be $((expression))
  • Operators are
    • + : Addition
    • – : Subtraction
    • * : Multiplication
    • / : Division
    • % : Modulo (remainder)
    • **: Exponentation
• Brace Expansion: Brace expansion can create multipe text strings
  • Using brace expresiion lets try to create folder for every month from year 2017 to 2020 in the format of yyyy-mm
• Parameter Expansion:
• Command Substitution: this allows us to use output of a command as expansion
• Quotes
• Supress expansion: