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
- Operators are
- 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
- 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:
