3.4. Quoting

Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in the string from reinterpretation or expansion by the shell or shell script. (A character is "special" if it has an interpretation other than its literal meaning, such as the wild card character, *.)

When referencing a variable, it is generally advisable in enclose it in double quotes (" "). This preserves all special characters within the variable name, except $, ', and \. This allows referencing it, that is, replacing the variable with its value (see Example 3-5, above). Enclosing the arguments to an echo statement in double quotes is usually a good practice (and sometimes required, see Section 3.28).

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").

Escaping is a method of quoting single characters. The escape (\) preceding a character will either toggle on or turn off a special meaning for that character, depending on context.

\n

means newline

\r

means return

\t

means tab

\v

means vertical tab

\b

means backspace

\a

means "alert" (beep or flash)

\0xx

translates to the octal ASCII equivalent of 0xx

# Use the -e option with 'echo' to print these.
echo -e "\v\v\v\v"   # Prints 4 vertical tabs.
echo -e "\042"   # Prints " (quote, ASCII character 42).

\"

gives the quote its literal meaning

echo "Hello"                  # Hello
echo "\"Hello\", he said."    # "Hello", he said.

\$

gives the dollar sign its literal meaning (variable name following \$ will not be referenced)

echo "\$variable01"  # results in $variable01

\\

gives the backslash its literal meaning

echo "\\"  # results in \

The escape also provides a means of writing a multi-line command. Normally, each separate line constitutes a different command, but an escape at the end of a line escapes the newline character, and the command sequence continues onto the next line.

(cd /source/directory && tar cf - . ) | \
(cd /dest/directory && tar xvfp -)
# Repeating Alan Cox's directory tree copy command,
# but split into two lines for increased legibility.