special characters

#	Comments(a quoted or an escaped # in an echo statement does not begin a comment. Likewise, a # appears in certain parameter-substitution constructs and in numerical constant expressions(BASE#NUMBER).)
;	Command separator [semicolon]
;;	Terminator in a case option [double semicolon]
;;&, ;&	Terminators in a case option (version 4+ of Bash)
.	"dot" command [period]. Equivalent to source.
.	"dot", as a component of a filename.
.	"dot" character match. As part of a regular expression, a "dot" matches a single character.
"	partial quoting(soft quoting)
'	full quoting(hard quoting)
,	comma operator. Link together a series of arithmetic operations. All are evaluated, but only the last one is returned. The comma operator can also concatenate strings.
,, ,	Lowercase conversion in parameter substitution(added in version 4 of Bash).
\	escape.
/	Filename path separator
`	command substitution. The `command` construct makes available the output of command for assignment to a variable.
:	null command. It may be considered a synonym for the shell builtin true. The ":" command is itself a Bash builtin, and its exit status is true(0). The ":" also serves as a field separator.
!	reverse (or negate) the sense of a test or exit status. From the command line, the ! invokes the Bash history mechanism.
*	wild card. The character serves as a "wild card" for filename expansion in globbing. The * also represents any number(or zero) characters in a regular expression.
*	arithmetic operator.
**	A double asterisk can represent the exponentiation operator or extended file-match globbing.
?	test operator.
?	wild card. The ? character serves as a single-character "wild card" for filename expansion in globbing, as well as representing one character in an extended regular expression.
$	Variable substitution(contents of a variable).
$	end-of-line. In a regular expression, a "$" addresses the end of a line of text.
${}	Parameter substitution
$*, $@	positional parameters.
$?	exit status variable.
$$	process ID variable.
()	command group. A listing of commands within parentheses starts a subshell.
()	array initialization.
{xxx,yyy,zzz,...}	Brace expansion. No spaces allowed within the braces unless the spaces are quoted or escaped.
{a..z}	Extended Brace expansion.
{}	Block of code. Unlike a command group within (parentheses), as above, a code block enclosed by {braces} will not normally launch a subshell.
{}	placeholder for text. Used after xargs -i(replace strings option).
{}\;	pathname. Mostly used in find constructs. The ";" ends the -exec option of a find command sequence. It needs to be escaped to protect it from interpretation by the shell.
[]	test. Note that [ is part of the shell builtin test(and a synonym for it), not a link to the external command /usr/bin/test.
[[]]	test. More flexible than the single-bracket [] test, this is a shell keyword.
[]	array element.
[]	range of characters. As part of a regular expression.
$[...]	integer expansion. Note that this usage is deprecated, and has been replaced by the ((...)) construct.
(())	integer expansion.
> &> >& >> < <>	redirection.
<<	redirection used in a here document.
<<<	redirection used in a here string.
<, >	ASCII comparison.
\<, \>	word boundary in a regular expression.
|	pipe.
>|	force redirection
||	OR logical operator.
&	Run job in background. Within a script, commands and even loops may run in the background.
&&	AND logical operator.
-	option, prefix.
--	The double-dash -- prefixes long (verbatim) options to commands.
-	redirection from/to stdin or stdout.
-	previous working directory. A cd - command changes to the previous working directory.
-	Minus.
=	Equals. Assignment operator. In a different context, the "=" is a string comparison operator.
+	Plus. In a different context, the + is a Regular Expression operator.
+	Option.
%	modulo. In a different context, the % is a pattern matching operator.
~	home directory.
~+	current working directory.
~-	previous working directory.
=~	regular expression match.
^	beginning-of-line. In a regular expression, a "^" addresses the beginning of a line of text.
^, ^^	Uppercase conversion in parameter substitution (added in version 4 of Bash).
whitespace	In variable assignment context, whitespace is not permitted. To preserve whitespace within a string or in a variable, use quoting.