Path: utzoo!attcan!uunet!husc6!purdue!haven!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.unix.questions
Subject: Re: vipw - the program
Message-ID: <14043@mimsy.UUCP>
Date: 18 Oct 88 12:34:45 GMT
References: <218@fantasci.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 55

In article <218@fantasci.UUCP> jep@fantasci.UUCP (Joseph E Poplawski) writes:
>if [ "n$EDITOR" = "n" ]
>then
>	EDITOR=/usr/bin/vi
>fi

Handy sh programming tip number 3,141: use

	${EDITOR-/usr/bin/vi}

But here is the real reason for the followup:

>if [ -f "/etc/ptmp" ]
>then
>	echo ""
>	echo "/etc/passwd being used, please try again in a few minutes..."
>	echo ""
>	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.:

	# The following is so that we do not remove a ptmp lock file
	# that we did not create, yet we never ignore signals.
	# At worst we will leave a dummy file in /etc.
	lock=/etc/ptmp
	tf=$lock$$
	rm -f $tf
	>$tf
	trap "if [ -f $tf ]; then rm -f $tf $lock; fi; exit 1" 1 2 3 15
	if ln $tf $lock 2>/dev/null; then
		echo '
	/etc/passwd busy, please try again
	' 1>&2
		exit 1
	else
		# cp /etc/passwd /etc/passwd.old # depending on paranoia level
		(trap 1 2 3 15; ${EDITOR-/usr/ucb/vi} /etc/passwd)
		rm -f $tf $lock
	fi

Of course, this script should not be used on systems that already
have a `vipw' program, since it may (and probably does) use some
other form of locking.

(From C code one can use open(O_CREAT|O_EXCL) to atomically create
a file and fail if it already exists, but the shells do not provide
this facility.  Another approach is to write a small tool that does
this.  But unless you have reliable signals you will still have
windows of vulnerability.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris