Shell Special variables and TEST flags

Here are some shell special variables:

Special Shell Variables:

Variable	Meaning
$0	Filename of script
$1	Positional parameter #1
$2 - $9	Positional parameters #2 - #9
${10}	Positional parameter #10
$#	Number of positional parameters
"$*"	All the positional parameters (as a single word) *
"$@"	All the positional parameters (as separate strings)
${#*}	Number of positional parameters
${#@}	Number of positional parameters
$?	Return value
$$	Process ID (PID) of script
$-	Flags passed to script (using set)
$_	Last argument of previous command
$!	Process ID (PID) of last job run in background

TEST Operators: Files

Operator	Tests Whether	-----	Operator	Tests Whether
-e	File exists		-s	File is not zero size
-f	File is a regular file
-d	File is a directory		-r	File has read permission
-h	File is a symbolic link		-w	File has write permission
-L	File is a symbolic link		-x	File has execute permission
-b	File is a block device
-c	File is a character device		-g	sgid flag set
-p	File is a pipe		-u	suid flag set
-S	File is a socket		-k	"sticky bit" set
-t	File is associated with a terminal
-N	File modified since it was last read		F1 -nt F2	File F1 is newer than F2 *
-O	You own the file		F1 -ot F2	File F1 is older than F2 *
-G	Group id of file same as yours		F1 -ef F2	Files F1 and F2 are hard links to the same file *
!	NOT (inverts sense of above tests)

* Binary operator (requires two operands).

from: http://www.tldp.org/LDP/abs/html/refcards.html