Re: rfc: exposing *at functions to shell

[Eric Blake]

Mike Frysinger <vapier <at> gentoo.org> writes:

> > cd /tmp
> > mkdir -p sub
> > {
> >   ln --at=4 -sf foo bar   # call symlinkat("foo",4,"bar")
> >   readlink --at=4 -m bar  # call areadlinkat(4,"bar")
> > } 4< sub
> > 
> > would output /tmp/sub/foo.
> 
> isnt this possible today under linux by using /proc/self/fd ?  i'm not 
> suggesting this as a clean/portable replacement, just trying to better 
> understand the proposal.

Yes, with sufficient /proc supprt, the above example would be equivalent to:

cd /tmp
mkdir -p sub
{
  ln -sf foo /proc/self/4/bar
  readlink -m /proc/self/4/bar
} 4< sub

Without /proc, it's similar to doing everything via absolute or anchored paths 
(or using shell variables for shorthand), but then you lose time with O(n^2) on 
deep nesting of directories, as well as the robustness that the *at functions 
give in the case that some other process is modifying the same hierarchy that 
you are operating on:

cd /tmp
mkdir -p sub
dir=sub
{
  ln -sf foo "$dir"/bar
  readlink -m "$dir"/bar
} 4< "$dir"

> --at-fd might be a better explicit option without getting too verbose ?

Indeed.

-- 
Eric Blake