Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!mcvax!ukc!warwick!cudcv
From: cudcv@warwick.ac.uk (Rob McMahon)
Newsgroups: comp.unix.questions
Subject: Re: vipw - the program
Message-ID: <63@titania.warwick.ac.uk>
Date: 22 Oct 88 10:55:18 GMT
Article-I.D.: titania.63
References: <218@fantasci.UUCP> <14043@mimsy.UUCP>
Reply-To: cudcv@warwick.ac.uk (Rob McMahon)
Organization: Computing Services, Warwick University, UK
Lines: 24

In article <14043@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>In article <218@fantasci.UUCP> jep@fantasci.UUCP (Joseph E Poplawski) writes:
>>if [ -f "/etc/ptmp" ]; then exit 1; else cat /dev/null >/etc/ptmp ...
>
>This has a window during which it will not notice the lock file.
>You are much better off using links, viz.:
>...
>	if ln $tf $lock 2>/dev/null; then exit 1; else ...

Except that this seems to be silently broken under (at least some versions of)
System V, in which ln(1) deletes the file first, and the command succeeds.
See:

Newsgroups: news.software.b,comp.sources.bugs
Subject: C News Bulletin #8 - fixes for illegal message-IDs, System V ln(1)
Message-ID: <1988Oct18.064611.25833@utstat.uucp>

`mkdir $lock' should still work though ...

Rob
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Rob McMahon, Computing Services, Warwick University, Coventry CV4 7AL, England