What is the zsh builtin `r` command for?

Question

I accidentally typed r into the shell and got what appeared to be a repeat of the last command I ran.

man zshbuiltins, unhelpfully, says this:

r - Same as fc -e -.

The documentation for fc is almost impenetrable but I managed to tease out these quotes:

• fc [ -eename] [ -LI ] [ -mmatch] [old=new... ] [first[last] ]

• the editor program ename is invoked on a file containing these history events....When editing is complete, the edited command is executed.

• If ename is -, no editor is invoked.

This reads to me like the behavior of r is similar to exclamation point !, in that it replays history. Indeed, if I setopt banghist to turn bang back on, things like r man and ! man both seem to replay my last call to man.

What are the similarities and differences between r and !? What's a hypothetical scenario in which I could use r?

Answer 1

You have done a good job quoting the right passages from the man page.

zsh inherited r from ksh, as neither bash or csh know this command. I suppose the implementation is done to be as compatible as possible with these three major shells.

On the other hand the history bang mechanism ! originates from csh which can be disabled with setopt NO_BANG_HIST.

One difference between these two mechanisms which comes to my mind is that r and ! are parsed differently, as ! is a reserved word but r only a (builtin) command. That means you can write e.g.

$ echo my last command was !!

which gets (depending on your settings after pressing SPACE or ENTER) to

$ echo my last command was man zshexpn

which isn't possible with r unless you use command substitution $(r) of course.

So, I thinks it all just boils down to personal preferences or habits (if you used to used ksh or csh)...