Linux Classroom Series – 08/Jul/2020


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


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

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