An event designator is a reference to a command line entry in the history list. Unless the reference is absolute, events are relative to the current position in the history list.
!
Start a history substitution, except when followed by a space, tab, the end of the line, ‘=’ or ‘(’ (when the extglob shell option is enabled using the shopt builtin).
!n
Refer to command line n.
!-n
Refer to the command n lines back.
!!
Refer to the previous command. This is a synonym for ‘!-1’.
!string
Refer to the most recent command preceding the current position in the history list starting with string.
!?string[?]
Refer to the most recent command preceding the current position in the history list containing string. The trailing ‘?’ may be omitted if the string is followed immediately by a newline.
^string1^string2^
Quick Substitution. Repeat the last command, replacing string1 with string2. Equivalent to !!:s/string1/string2/.
!#
The entire command line typed so far.