[OT] Does "~" always point to $HOME?

Matthew Seaman m.seaman at infracaninophile.co.uk
Sat Jan 27 10:53:00 UTC 2007

Bill Campbell wrote:
> On Fri, Jan 26, 2007, James Long wrote:
>>> Message: 24
>>> Date: Fri, 26 Jan 2007 09:22:44 -0800
>>> From: Bill Campbell <freebsd at celestial.com>
>>> Subject: Re: [OT] Does "~" always point to $HOME?
>>> To: freebsd-questions at freebsd.org, Bill Campbell
>>> 	<freebsd at celestial.com>
>>> Message-ID: <20070126172244.GB6575 at ayn.mi.celestial.com>
>>> Content-Type: text/plain; charset=us-ascii
>>> My point isn't whether the FreeBSD /bin/sh expands it, but that not all
>>> systems are FreeBSD, and that one can have problems on other *NIX systems.
>>> Knowing where there may be differences, and avoiding the assumptions that a
>>> program behaves the same on all systems, can help writing code that's
>>> portable without surprises.
>> This begs the rookie question:
>> What is the portable way to determine an aribtrary user's home directory
>> then, if ~username is not portable across shells?
>> Does one just have to grep and awk /etc/passwd?  Is the format of
>> /etc/passwd portable, such that one standard grep/awk sequence will 
>> portably return the home directory for user "username"?
> Probably the most portable way to do this would be to use awk.  A
> simple script, homedir, might look like this:
> #!/bin/sh
> # getting the backwhacks correct is sometimes ``interesting''
> homedir=`awk -F: "/^$1:/{print \\$6}" /etc/passwd`
> [ -z "$homedir" ] && {
> 	echo 'empty home for ' $1 2>&1
> 	exit 1
> }
> echo $homedir
> exit 0

That does assume that all the user information is stored within the
local /etc/passwd -- if you're using NIS or LDAP or anything
like that, then you need a method that calls getpwnam(3) for you. 


pw user show -n $username | cut -d: -f 9   (But pw(8) is FreeBSD specific)

perl -le "print +(getpwnam("$username"))[7];"

su  $username -c 'echo $HOME'  (But only if the script is running with
                                root privileges)

Of course, none of these methods are guaranteed to work in all
circumstances.  In which case, you might as well choose to program
in a language or for an interpreter that is readily available on the
systems you are writing the code for and that provides the functionality
you need.  On FreeBSD that probably comes down to using ~username with
/bin/sh (with liberal comments warning of uportable assumptions, of
course) -- which should work unmodified with any of the *BSDs or MacOS X.
If you program the rest of the script carefully it should also work with
bash under Linux (and others) or even (I think) ksh on Solaris.

Portability is hard.



Dr Matthew J Seaman MA, D.Phil.                       7 Priory Courtyard
                                                      Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey         Ramsgate
                                                      Kent, CT11 9PW

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