Your bash shell understand special characters with special meanings. For example, $var is used to display the variable value. Bash expands variables and wildcards, for example:
echo $PATH echo $PS1 echo /etc/*.conf
However, sometime you do not wish to use variables or wildcards. For example, do not print value of $PATH, but just print $PATH on screen as a word. You can enable or disable the meaning of a special character by enclosing them into a single or double quotes. This is also useful to suppress warnings and error messages while writing the shell scripts.
echo "Path is $PATH"
echo 'I want to print $PATH'
There are three types of quotes
|"||Double Quotes||The double quote ( "quote" ) protects everything enclosed between two double quote marks except $, ', " and \.
Use the double quotes when you want variables and command substitution.
|'||Single quotes||The single quote ( 'quote' ) protects everything enclosed between two single quote marks.
It is used to turn off the special meaning of all characters.
|`||Back quote||Use back quote ( `command-name` ) to execute command and replace a command with its output
within the same command-line. However, $(command-name) is encouraged syntax for substitution as it is recommended
by POSIX standard and it improves script readability.
The backslash ( \ ) alters the special meaning of the ' and " i.e. it will escape or cancel the special meaning of the next character. The following will display filename in double quote:
FILE="/etc/resolv.conf" echo "File is \"$FILE\" "
File is "/etc/resolv.conf"
The following will remove the special meaning of the dollar ( $ ) sign:
FILE="/etc/resolv.conf" echo "File is \$FILE "
File is $FILE