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Re: invoke tilde expansion on quoted string

From: Greg Wooledge
Subject: Re: invoke tilde expansion on quoted string
Date: Tue, 9 Apr 2013 08:46:52 -0400
User-agent: Mutt/

On Mon, Apr 08, 2013 at 05:13:46PM -0700, Linda Walsh wrote:
> Greg Wooledge wrote:
> > getent(1) is fine where it's available, but it's not a standard tool,
> > so you can only use it on systems that have it.

> Have you encountered it on other linux systems?

It is present on Debian 3.1 (the oldest Debian system to which I have
access), OpenBSD 5.2, and Red Hat Enterprise Linux 5.7.  It would require
some digging to find out exactly when it first appeared on each of those
operating systems.

For comparison, it is NOT present on Red Hat 5.2 ("Apollo").  The one
from the 1990s, not RHEL 5.2.  I utterly despise the fact that it takes
me three sentences to explain a version number.

> Certainly, I'd think that parsing /etc/passwd would be more portable as
> it could likely be adapted to the other methods given programs that
> are similar to getent that provide database entries (passwd  lines)
> like with yp/nis.

That's just asking for trouble.  I don't know every arcane Unix
authentication system out there, but I'd imagine at least one of them
can't be used that way.  Plus, there's the issue of determining which
authentication system you're actually supposed to pull the account
information from.

imadev:~$ grep '^wooledg' /etc/passwd

Oh, look.  I'm not in /etc/passwd.  No, wait, maybe I am:

imadev:~$ grep '+\{0,1\}wooledg' /etc/passwd

Right, the plus sign means I'm really in NIS:

imadev:~$ ypmatch wooledg passwd.byname | awk -F: '{$2="xxxx"; OFS=":"; print}'
wooledg:xxxx:563:22:Greg Wooledge:/net/home/wooledg:/bin/ksh

So to get my home directory by mimicking the operating system's account
lookup procedures, I have to make at least 3 separate checks, and I may
have to merge data from 2 of those.

P.S. /net/home/wooledg is another virtualization.  At the shell level
it can be used as if it were a literal directory.  If you wanted the
physical location you'd either have to cd there and then ask where
you really are, or do yet another NIS lookup and parsing.

I don't know how NIS+ or LDAP work, but I know they're even more
complicated than NIS.

Now you can see why I decided that   eval "home=~$sanitized_username"
was the better choice.

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