Re: Command substitution (backtick) and tab completion

[Eric Blake]

On 01/05/2011 09:57 AM, Greg Wooledge wrote:
> On Wed, Jan 05, 2011 at 08:21:18AM -0800, chengiz wrote:
>> So if I get this right, the only time this is a problem is when the
>> command substitution runs more than once.
> 
> I'd actually characterize it differently: it's unsafe to run arbitrary
> commands during tab completion, because bash doesn't know what those
> commands might do.
> 
>> When does this happen? Not
>> in my "ls `pwd`/<tab>" example where the command runs once and
>> replaces itself with its output. Does it only run more than once when
>> the ticks are not complete?
> 
> You might realize you made a mistake, hit Ctrl-U, and start over.  But
> the backticked command has already been executed.
> 
> You might hit ESC # to comment out the command line because you suddenly
> realize that you need to do something else first.  Then you come back to
> it (ESC k k ...), remove the # sign, finish typing the command, and run
> it.  But the backticked command was already executed much earlier than
> you might have wished (two commands ago).

Here's another argument why I feel that completion should NEVER rewrite
your command line (not even to replace `pwd` with its value):

I have been known to do things like this for testing parallel
development trees:

cd ~/dir<TAB>
run tests via a single command
HOME=/path/to/alternate/
<up><up><enter> (to rerun cd)
<up><up><enter> (to rerun tests, in a different directory)

As long as tab completion on the cd command preserved the ~, then this
scenario works.  But as soon as completion "helpfully" rewrites ~ into
/path/to/home, it breaks my work flow.  Okay, so that work flow of
abusing the definition of $HOME to swap between parallel work trees
might not be the most common, but I hope it points out to you why
completion should not be in the business of rewriting users commands,
but only appending completions.  And it can be applied to any other
situation (substitute ~/$HOME with your choice of shell variable - if
completion ever rewrites a command line with the expansion of $var
rather than keeping literal $var in place, then you cannot alter $var in
between repetitions of a command - even if completion had to temporarily
expand $var in order to form better context about what I was completing
after the point that $var appeared in my command line).

-- 
Eric Blake   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

signature.asc
Description: OpenPGP digital signature