Comments

A single line comment is written using a #.

#! /bin/bash
# This is a single line comment
echo "Hello world" # This is also a comment

Variables

<aside> 📌 The name of a variable can contain only letters (a to z or A to Z), numbers (0 to 9) or the underscore character (_). Also, the variable name cannot start with a number.

</aside>

System Variables

• Created and maintained by the Linux/Unix OS.

• Conventionally these are written using uppercase.

• Examples of system variables -

  #! /bin/bash
  echo "Location of bash : $BASH"
  echo "Bash Version  : $BASH_VERSION"
  echo "Path to the home directory (/home/user) : $HOME"
  echo "Present working directory : $PWD"
  

  $ ./system-variables.sh 
  Location of bash (/bin/bash) : /bin/bash
  Bash Version  : 4.4.20(1)-release
  Path to the home directory (/home/user) : /home/shathin
  Present working directory : /mnt/g/Dev/Linux-Subsystem/shell-scripting
  

User Variables

• Created by the user.

• Conventionally these are writting using lowercase.

• Defining and echoing a variable

  #! /bin/bash
  my_var=10 # don't put a whitespace on either side of equal symbol (=)
  echo The value is $my_var 
  

  $ ./user-defined-variable.sh 
  The value is 10
  

Deleting a variable

The unset command can be used to delete a variable.

Example:

#! /bin/bash
my_var=10
echo "Value: $my_var"

unset my_var
echo "Value: $my_var"

$ ./delete-variables.sh 
Value: 10
Value:

Arrays

• Arrays are defined as follows: array_var=(val1 val2 ... valn).
• An element of the array can be accessed via its index i.e, $array_var[index]