From Linux Shell Scripting Tutorial - A Beginner's handbook
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Your bash shell understand special characters with special meanings. For example, $var is used to display the variable value. 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 write the scripts.


There are three types of quotes

Quote type Name Meaning Example
" 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.

echo "$SHELL"
echo "Today is $(date)"

' 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.

echo '$SHELL'
echo 'Today is $(date)'

` 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.

echo "Today is `date`"
echo "Today is $(date)"



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:

echo "File is \"$FILE\" "

The following will remove the special meaning of the dollar ( $ ) sign:

echo "File is \$FILE "