1		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.
