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A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ignorespace, lines which begin with a
space character are not saved in the history list. A value of ignoredups causes lines matching the previous history entry to not be saved. A value of
ignoreboth is shorthand for ignorespace and ignoredups. A value of erasedups causes all previous lines matching the current line to be removed from the his-
tory list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL is unset, or does not include a valid value, all lines read
by the shell parser are saved on the history list, subject to the value of HISTIGNORE. The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of HISTCONTROL.