bash

$0	My filename
$n	positional parameter n
$n - $m	positional paramters n thru m
${n}	positional parameter n
“${*}”	all positional parameters (single word)
“$@”	all positional parameters (separate)
$#	number of positional parameters
${#{*}}	number of positional parameters
${#@}	number of positional parameters
$?	value returned by last command
$$	PID of current shell
$!	process number of last background cmd
$_	Last argument of previous cmd

-x	print commands as executed
$( )	in-line command substitution (new style)
` `	in-line command substitution (old style)
$((...))	in-line arithmetic evaluation

\h	short hostname
\H	full hostname
\u	username
\w	current working directory
\W	basename of current working directory
\$	# if the effective UID is 0, otherwise $

$name	shell variable name
${name}	delimit shell variable name
${name-word}	variable nameif set, else word
${name=word}	as above but also set name to word
${name?word}	use name if set, otherwise print word and exit
${name+word}	use word if name is set, otherwise use nothing
${namen}	element n in array name
${#name}	length of shell variable name
${#name*}	number of elements in array name
${#name@}	number of elements in array name
${name#pat}	remove shortest leading substring of name that matches pat
${name##pat}	remove longest leading substring of name that matches pat
${name%pat}	remove shortest trailing substring of name that matches pat
${name%%pat}	remove longest trailing substring of name that matches pat
${name:start}	length characters of name starting at start(counting from0); use rest of value if no length. Negative start counts from the end
${name:start:length}
${name/pattern/string}	value of name with first match of pattern replaced with string
${name/pattern}	value of name with first match of pattern deleted
${name/#pattern/string}	value of name with match of pattern replaced with string;match must occur at beginning
${name/%pattern/string}	value of name with match of pattern replaced with string;match occurs at end

Variables