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Very useful when you forget a sudo at the beginning of your command, !! acts like an alias of the previous command. Example :

$ mv /very/long/path/for/a/protected/sensible/file/caution.h . (...) Permission denined $ sudo !! sudo mv /very/long/path/for(...) . [sudo] password :
  • How do we call that double !! trick ? Researches over internet are difficult because of that token.
  • How does it work ? I suspect a link with the history command.
  • Where it is defined ? Can I define some other myself ?

EDIT : Some interesting event designators

!!:*

It refers to the arguments of the previous command. Use case :

cat /a/file/to/read/with/long/path nano !!:*

:p

Just print the command without executing it, you have to put it at the end of the event designator.

$ !-5:p sudo rm /etc/fstab -f

More here.

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    Read man history Commented Aug 25, 2016 at 15:41
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    It's a special case of history expansion, in which the shell attempts to expand a word starting with ! to a matching command in the current shell's history list. !! is a special case, equivalent to !-1, where a negative number n following ! refers to the nth previous command. Commented Aug 25, 2016 at 15:48
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    @Costas, more usefully, read LESS='+/^HISTORY EXPANSION' man bash. Commented Aug 26, 2016 at 2:34

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!! is listed in the bash manual under the heading "Event Designators":

 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 blank, newline, carriage return, = or ( (when the extglob shell option is enabled using the shopt builtin). !n Refer to command line n. !-n Refer to the current command minus n. !! Refer to the previous command. This is a synonym for `!-1'. !string Refer to the most recent command preceding the current posi- tion in the history list starting with string. !?string[?] Refer to the most recent command preceding the current posi- tion in the history list containing string. The trailing ? may be omitted if string is followed immediately by a new- line. ^string1^string2^ Quick substitution. Repeat the previous command, replacing string1 with string2. Equivalent to ``!!:s/string1/string2/'' (see Modifiers below). !# The entire command line typed so far.

So !! will be replaced with the previous command.

Note that the shell history will not contain the literal !! but instead the actual command that was executed:

$ ls [some output] $ !! . [same output] $ history 3 645 2016-08-25 17:40:55 ls 646 2016-08-25 17:40:57 ls . 647 2016-08-25 17:41:00 history 3
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