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). Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (TODO: see Example 4-1, above).

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.)

Use double quotes to prevent word splitting [1]. 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.

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

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 "$var3"     # \\\\
# Strong quoting works, though.
# ************************************************************ #
# As the first example above shows, nesting quotes is permitted.
echo "$(echo '"')"           # "
#    ^           ^
# At times this comes in useful.
var1="Two bits"
echo "\$var1 = "$var1""      # $var1 = Two bits
#    ^                ^
# Or, as Chris Hiestand points out ...
if [[ "$(du "$My_File1")" -gt "$(du "$My_File2")" ]]
#     ^     ^         ^ ^     ^     ^         ^ ^
then
  ...
fi
# ************************************************************ #

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.