Awk

Pick out the fourth token:

mac=`/sbin/arp -a $ip | awk '{print $4}'`

Separate by = instead of whitespace:

echo "a=b=c" | awk -F= '{print $3}'  # selects "c"

Duplicate entries in the passwd file? Compare these:

awk -F: '{print $1}' /etc/group | sort -u | wc -l
awk -F: '{print $1}' /etc/group | wc -l