use the file command to determine a file type:
	$ file $(tty)
	$ file /dev/hda1

[   -c $(tty) ] && echo $(tty) is character special
[   -f $(tty) ] || echo $(tty) will not TEST to be a "regular" file
[ ! -f $(tty) ] && echo $(tty) will not TEST to be a "regular" file


( EXPRESSION )
	EXPRESSION is true

! EXPRESSION
	EXPRESSION is false

EXPRESSION1 -a EXPRESSION2
	both EXPRESSION1 and EXPRESSION2 are true

EXPRESSION1 -o EXPRESSION2
	either EXPRESSION1 or EXPRESSION2 is true

[-n] STRING
	the length of STRING is nonzero

-z STRING
	the length of STRING is zero

STRING1 = STRING2
	the strings are equal

STRING1 != STRING2
	the strings are not equal

INTEGER1 -eq INTEGER2
	INTEGER1 is equal to INTEGER2

INTEGER1 -ge INTEGER2
	INTEGER1 is greater than or equal to INTEGER2

INTEGER1 -gt INTEGER2
	INTEGER1 is greater than INTEGER2

INTEGER1 -le INTEGER2
	INTEGER1 is less than or equal to INTEGER2

INTEGER1 -lt INTEGER2
	INTEGER1 is less than INTEGER2

INTEGER1 -ne INTEGER2
	INTEGER1 is not equal to INTEGER2

FILE1 -ef FILE2
	FILE1 and FILE2 have the same device and inode numbers

FILE1 -nt FILE2
	FILE1 is newer (modification date) than FILE2

FILE1 -ot FILE2
	FILE1 is older than FILE2

-b FILE
	FILE exists and is block special

-c FILE
	FILE exists and is character special

-d FILE
	FILE exists and is a directory

-e FILE
	FILE exists

-f FILE
	FILE exists and is a regular file

-g FILE
	FILE exists and is set-group-ID

-G FILE
	FILE exists and is owned by the effective group ID

-k FILE
	FILE exists and has its sticky bit set

-L FILE
	FILE exists and is a symbolic link

-O FILE
	FILE exists and is owned by the effective user ID

-p FILE
	FILE exists and is a named pipe

-r FILE
	FILE exists and is readable

-s FILE
	FILE exists and has a size greater than zero

-S FILE
	FILE exists and is a socket

-t [FD]
	file descriptor FD (stdout by default) is opened on a terminal

-u FILE
	FILE exists and its set-user-ID bit is set

-w FILE
	FILE exists and is writable

-x FILE
	FILE exists and is executable