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Re: simple prob?

From: Greg Wooledge
Subject: Re: simple prob?
Date: Tue, 29 Jun 2021 17:02:16 -0400

On Tue, Jun 29, 2021 at 09:47:30PM +0100, Kerin Millar wrote:
> On Tue, 29 Jun 2021 16:35:28 -0400
> Greg Wooledge <greg@wooledge.org> wrote:
> > unicorn:~$ njobs() { local _n=$(jobs | wc -l); eval "$1=\$_n"; }
> > unicorn:~$ njobs walsh
> > unicorn:~$ echo "$walsh"
> > 3
> > 
> > Now you just need to add sanity-checking on the argument of njobs, to
> > avoid whatever code injection the malicious caller wants to perform.
> I can't fathom the switch to eval there. Why not printf -v "$1" %s "$_n", for 
> example? It even rejects invalid identifiers.

declare, printf -v, local -n, eval -- they're mostly equivalent.  Some
of them may prevent *some* possible code injections, but none of them
prevent *all* possible code injections.

unicorn:~$ njobs2() { printf -v "$1" %s 42; }
unicorn:~$ njobs2 'x[0$(date >&2)]'
Tue Jun 29 17:00:29 EDT 2021

No matter which one of these you choose, you still have to sanity-check
the input.  Or else declare that you do not care if the user shoots their
own foot off (which is a valid stance as long as your code is never used
in a context where the user can elevate their privileges/capabilites).

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