Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to disable
special treatment for special characters, to prevent reserved
words from being recognized as such, and to prevent parameter
expansion.
Each of the metacharacters listed above under DEFINITIONS has
special meaning to the shell and must be quoted if it is to
represent itself.
When the command history expansion facilities are being used (see
HISTORY EXPANSION below), the history expansion character,
usually !, must be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character, single
quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves
the literal value of the next character that follows, with the
exception of <newline>. If a \<newline> pair appears, and the
backslash is not itself quoted, the \<newline> is treated as a
line continuation (that is, it is removed from the input stream
and effectively ignored).
Enclosing characters in single quotes preserves the literal value
of each character within the quotes. A single quote may not
occur between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value
of all characters within the quotes, with the exception of $, `,
\, and, when history expansion is enabled, !. When the shell is
in posix mode, the ! has no special meaning within double quotes,
even when history expansion is enabled. The characters $ and `
retain their special meaning within double quotes. The backslash
retains its special meaning only when followed by one of the
following characters: $, `, ", \, or <newline>. A double quote
may be quoted within double quotes by preceding it with a
backslash. If enabled, history expansion will be performed
unless an ! appearing in double quotes is escaped using a
backslash. The backslash preceding the ! is not removed.
The special parameters * and @ have special meaning when in
double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The word
expands to string, with backslash-escaped characters replaced as
specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\? question mark
\nnn the eight-bit character whose value is the octal
value nnn (one to three octal digits)
\xHH the eight-bit character whose value is the
hexadecimal value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value
is the hexadecimal value HHHH (one to four hex
digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value
is the hexadecimal value HHHHHHHH (one to eight hex
digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had
not been present.
A double-quoted string preceded by a dollar sign ($"string") will
cause the string to be translated according to the current
locale. The gettext infrastructure performs the message catalog
lookup and translation, using the LC_MESSAGES and TEXTDOMAIN
shell variables. If the current locale is C or POSIX, or if
there are no translations available, the dollar sign is ignored.
If the string is translated and replaced, the replacement is
double-quoted.
