    Common shell variable names and usage.
    
    BASH_VERSION	Version information for this Bash.
    CDPATH	A colon-separated list of directories to search
    		for directories given as arguments to `cd'.
    GLOBIGNORE	A colon-separated list of patterns describing filenames to
    		be ignored by pathname expansion.
    HISTFILE	The name of the file where your command history is stored.
    HISTFILESIZE	The maximum number of lines this file can contain.
    HISTSIZE	The maximum number of history lines that a running
    		shell can access.
    HOME	The complete pathname to your login directory.
    HOSTNAME	The name of the current host.
    HOSTTYPE	The type of CPU this version of Bash is running under.
    IGNOREEOF	Controls the action of the shell on receipt of an EOF
    		character as the sole input.  If set, then the value
    		of it is the number of EOF characters that can be seen
    		in a row on an empty line before the shell will exit
    		(default 10).  When unset, EOF signifies the end of input.
    MACHTYPE	A string describing the current system Bash is running on.
    MAILCHECK	How often, in seconds, Bash checks for new mail.
    MAILPATH	A colon-separated list of filenames which Bash checks
    		for new mail.
    OSTYPE	The version of Unix this version of Bash is running on.
    PATH	A colon-separated list of directories to search when
    		looking for commands.
    PROMPT_COMMAND	A command to be executed before the printing of each
    		primary prompt.
    PS1		The primary prompt string.
    PS2		The secondary prompt string.
    PWD		The full pathname of the current directory.
    SHELLOPTS	A colon-separated list of enabled shell options.
    TERM	The name of the current terminal type.
    TIMEFORMAT	The output format for timing statistics displayed by the
    		`time' reserved word.
    auto_resume	Non-null means a command word appearing on a line by
    		itself is first looked for in the list of currently
    		stopped jobs.  If found there, that job is foregrounded.
    		A value of `exact' means that the command word must
    		exactly match a command in the list of stopped jobs.  A
    		value of `substring' means that the command word must
    		match a substring of the job.  Any other value means that
    		the command must be a prefix of a stopped job.
    histchars	Characters controlling history expansion and quick
    		substitution.  The first character is the history
    		substitution character, usually `!'.  The second is
    		the `quick substitution' character, usually `^'.  The
    		third is the `history comment' character, usually `#'.
    HISTIGNORE	A colon-separated list of patterns used to decide which
    		commands should be saved on the history list.
