Quotes

Simple quotes ('...') or double quotes ("...") have specific meanings.

Double quotes gather a set of words (separated by white spaces) in a whole, but the shell keeps interpreting special characters ($, *, etc.).

Simple quotes gather a set of words with shell interpretation.

Example:

#!/bin/bash

i=toto
echo  $i         j
echo "$i         j"
echo '$i         j'

"Inverse" quotes (`...`) executes a command in a child process. They are the equivalent of $(...).

Exemple:

i=toto.com
j=`basename $i .com`
k=$(basename $i .com)
echo with suffix: $i
echo without suffix: $j
echo without suffix: $k