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Quoting

Quoting removes the special meaning of certain characters or words to the shell, such as operators, whitespace, or keywords.

Backslash

  • Preserves the literal meaning of the next character, except for newline
  • A Backslash preceding a newline is treated as line continuation
  • Backslash is the "strongest" quote

Single Quotes ' '

  • Preserves the literal meaning of each character within the quotes. The single quote acts as a "toggle" switch. Reading left to right, each ' toggles quoting on or off
  • A literal single quote may not occur in single quoted text, even when preceded by a backslash
  • Backslashes within single quotes are literal

ANSI C Quotes `$'..'

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e \E an escape character (not ANSI C)
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
' single quote
" double quote
? question mark
\nnn the eight-bit character whose value is the octal value nnn (one to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)
\cx a control-x character

Double Quotes " "

  • Preserves the literal value of all characters within the quotes, except: $ \ ` (dollar, backslash, backquote)
  • Backslash can escape the following characters: $ \ " ` or newline.
  • A double quote may be quoted within double quotes like so: \"
  • history expansion (if enabled) will be performed when ! is encountered inside double quotes. This can be bypassed with \! But the backslash preceding the ! is NOT removed. This only applies to the interactive shell, not scripts.
  • The parameters $* and $@ have special meaning when in double quotes.

Quoting Examples

greeting=hello

echo $greeting          # hello
echo \$greeting         # $greeting
echo '$greeting'        # $greeting
echo "$greeting"        # hello
echo "'$greeting'"      # 'hello'
echo ""$greeting""      # hello
echo "\"$greeting\""    # "hello"
echo ~fred              # /home/fred
echo "~fred"            # ~fred
echo '~fred'            # ~fred

To insert a single quote in single quoted text,

$ # Escape literal ' with backslash
$ echo 'He doesn'\''t have $10'
He doesn't have $10

$ # Place literal ' within double quotes
$ echo 'He doesn'"'"'t have $10'
He doesn't have $10

$ # Another way is to use double quotes and escape the $.
$ echo "He doesn't have \$10"

The $* and $@ variables

These access all the positional parameters

  • passed into the script from the command line
  • passed into a funtion
  • put into the environment with the "set" command

These behave differently than regular variables.
Unquoted, $* and $@ behave the same when expanded.

$ set -- fred barney "Mr. Slate"
$ for i in $*; do echo $i; done
fred
barney
Mr.
Slate

The shell word-splits Mr. Slate, even though:

  • it was quoted originally
  • is stored as a single "word" by the set command.

With quoted "$*", the shell will make the entire set one word, giving

fred barney Mr. Slate

With quoted "$@", the shell will expand the set without word-splitting, giving

fred
barney
Mr. Slate

You usually want to use "$@"

In the example above:

for i in "$@";

can be replaced by simply;

for i;

The same rules regarding * and @ apply to Bash arrays.

${my_array[*]}      all elements, with word splitting.
${my_array[@]}      all elements, with word splitting.

"${my_array[*]}"    all elements, as one word.
"${my_array@*]}"    all elements, no word splitting, 
                    preserves whitespace in elements.