shell script question

[soongwoo]

I'm a new to use shell script. (learning shell script from www.tldp.org guides)

The question is from /etc/init.d/bootmisc.sh file:
1) : > /var/run/utmp
if grep -q ^utmp: /etc/group
then
2) : exit 0

Could anyone explain what it means? What does ":" mean?
Furthermore, the first one, ": > /var/run/utmp", looks ...

Thanks in advance
soongwoo