Internal Variables for BASH

http://tldp.org/LDP/abs/html/internalvariables.html

• $BASH
• path of bash binary
• $BASH_ENV
• 
  
• $BASH_SUBSHELL
• $BASHPID
• $BASH_VERSINFO[n]
• $BASH_VERSION
• $CDPATH
• $DIRSTACK
• $EDITOR
• $EUID
• $FUNCNAME
• $GLOBIGNORE
• $GROUPS
• $HOME
• $HOSTNAME
• $HOSTTYPE
• $IFS
• $IGNOREEOF
• $LC_COLLATE
• $LC_CTYPE
• $LINENO
• $MACHTYPE
• $OLDPWD
• $OSTYPE
• $PATH
• $PIPESTATUS
• $PPID
• $PROMPT_COMMAND
• $PS1
• $PS2
• $PS3
• $PS4
• $PWD
• $REPLY
• $SECONDS
• $SHELLOPTS
• $SHLVL
• $TMOUT
• $UID
• $0, $1, $2, etc.
• $#
• $*
• $@
• $-
• $!
• ${COMMAND1} &
• $_
• $?
• $$

Another must-read for bash scripting is:
http://tldp.org/LDP/abs/html/parameter-substitution.html

In general, all chapters in the ABS book are recommended:
http://tldp.org/LDP/abs/html/index.html