[FW]linux - bash script environment variables

Reference:https://sites.google.com/site/tiger2000/home

$BASH_ENV	absolute path of startup file
$CDPATH	directories searched by cd
$FCEDIT	absolute path of history editor
$HISTCMD	the history number of the current command
$HISFILE	absolute path of history file
$HISTSIZE	number of remembered commands
$HOME	login directory
$IFS	token delimiters
$LINENO	current line number in shell script
$LINES	terminal height
$MAIL	absolute path of mailbox
$MAILCHECK	number of seconds to check mail
$OLDPWD	absolute path of previous directory
$OPTARG	option set by getopt
$OPTIND	option's ordinal position set by getopt
$OSTYPE	the OS on which bash is executing
$PATH	command search path
$PPID	process ID of parent
$PS1	primary prompt
$PS2	secondary prompt
$PWD	absolute path of current directory
$RANDOM	random integer
$REPLY	default variable for read
$SECONDS	number of seconds since shell started
$SHELL	absolute pathname of preferred shell
$TMOUT	seconds to log out after lack of use
$UID	user ID of the current user
$$	process ID of current shell
$?	exit status of most recent statement
$#	參數的數目
$*	代表所有參數
$!	PID of the most recently started backgroup job