What does !#:3 mean in a shell command

Question

The install guide for ack suggests installing the ack script using this command:

curl http://beyondgrep.com/ack-2.14-single-file > ~/bin/ack && chmod 0755 !#:3 

I assume that the !#:3 at the end is some kind of back-reference, but what does it mean? Is there an equivalent in zsh? Google has not been helpful.

Answer 1

This is a special syntax, expanded by bash. It also works for zsh.

According to the bash man page (section HISTORY EXPANSION), the pattern expands as following:

• The event designator !# refers to the entire command line typed so far which is curl http://beyondgrep.com/ack-2.14-single-file > ~/bin/ack && chmod 0755
• : splits between the event designator (this case the entire line) and the word designator (selects a sub-part)
• the word designator 3 which selects the third word/argument (counting of words starts at zero), in this case ~/bin/ack.

The final command line (usually displayed before executed) is: curl http://beyondgrep.com/ack-2.14-single-file > ~/bin/ack && chmod 0755 ~/bin/ack.

For details, see the bash manual or very similar the zsh manual

Answer 2

In bash, it is history substitution (all history substitutions start with !). Specifically, !# means everything on the command line so far, and the addition ':3' means the third word (starting count at 0).

So, the above command translates !#:3 into ~/bin/ack.