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, StИphane Chazelas.

#!/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").

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 StИphane Chazelas.