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 -- except $, ` (backquote), and \ (escape). [1] 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. [2] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.

    List="one two three"

    for a in $List     # Splits the variable in parts at whitespace.
    do
      echo "$a"
    done
    # one
    # two
    # three

    echo "---"

    for a in "$List"   # Preserves whitespace in a single variable.
    do #     ^     ^
      echo "$a"
    done
    # one two three

A more elaborate example:

    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.

| 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.

    echo

    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.

    echo

    var2="\\\\\""
    echo $var2       #   "
    echo "$var2"     # \\"
    echo
    # But ... var2="\\\\"" is illegal. Why?
    var3='\\\\'
    echo "$var3"     # \\\\
    # Strong quoting works, though.

    exit

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").

| 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] | 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, and especially following an echo -e command.
    bash$ echo hello\!
    hello!
    bash$ echo "hello\!"
    hello\!

    bash$ echo \
    >
    bash$ echo "\"
    >
    bash$ echo \a
    a
    bash$ echo "\a"
    \a

    bash$ echo x\ty
    xty
    bash$ echo "x\ty"
    x\ty

    bash$ echo -e x\ty
    xty
    bash$ echo -e "x\ty"
    x       y
Double quotes following an echo sometimes escape `\`. Moreover, the `-e` option to echo causes the "\t" to be interpreted as a tab. (Thank you, Wayne Pollock, for pointing this out, and Geoff Lee and Daniel Barclay for explaining it.)
[2] | "Word splitting," in this context, means dividing a character string into separate and discrete arguments.

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