Advanced Bash-Scripting Guide:
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5.1. Quoting Variables

When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents reinterpretation of all special characters within the quoted string -- the variable name [1] -- except $, ` (backquote), and \ (escape). [2] Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see Example 4-1, above).

Use double quotes to prevent word splitting. [3] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators. 
variable1="a variable containing five words"
COMMAND This is $variable1    # Executes COMMAND with 7 arguments:
# "This" "is" "a" "variable" "containing" "five" "words"

COMMAND "This is $variable1"  # Executes COMMAND with 1 argument:
# "This is a variable containing five words"


variable2=""    # Empty.

COMMAND $variable2 $variable2 $variable2
                # Executes COMMAND with no arguments. 
COMMAND "$variable2" "$variable2" "$variable2"
                # Executes COMMAND with 3 empty arguments. 
COMMAND "$variable2 $variable2 $variable2"
                # Executes COMMAND with 1 argument (2 spaces). 

# Thanks, Stéphane Chazelas.

Tip

Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting or preservation of whitespace is an issue.

Example 5-1. Echoing Weird Variables

#!/bin/bash
# weirdvars.sh: Echoing weird variables.

var="'(]\\{}\$\""
echo $var        # '(]\{}$"
echo "$var"      # '(]\{}$"     Doesn't make a difference.

echo

IFS='\'
echo $var        # '(] {}$"     \ converted to space. Why?
echo "$var"      # '(]\{}$"

# Examples above supplied by Stephane Chazelas.

exit 0

Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally. Consider single quotes ("full quoting") to be a stricter method of quoting than double quotes ("partial quoting").

Note

Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result. 
echo "Why can't I write 's between single quotes"

echo

# The roundabout method.
echo 'Why can'\''t I write '"'"'s between single quotes'
#    |-------|  |----------|   |-----------------------|
# Three single-quoted strings, with escaped and quoted single quotes between.

# This example courtesy of Stéphane Chazelas.

Notes

[1]

It also has side-effects on the value of the variable (see below)

[2]

Encapsulating "!" within double quotes gives an error when used from the command line. This is interpreted as a history command. Within a script, though, this problem does not occur, since the Bash history mechanism is disabled then.

Of more concern is the apparently inconsistent behavior of "\" within double quotes.

bash$ echo hello\!
hello!

bash$ echo "hello\!"
hello\!



bash$ echo -e x\ty
xty

bash$ echo -e "x\ty"
x       y

What happens is that double quotes normally escape the "\" escape character, so that it echoes literally. However, the -e option to echo changes that. It causes the "\t" to be interpreted as a tab.

(Thank you, Wayne Pollock, for pointing this out, and Geoff Lee for explaining it.)

[3]

"Word splitting", in this context, means dividing a character string into a number of separate and discrete arguments.

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