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/passwd | sort -u | wc -l > temp1
awk -F: '{print $1}' /etc/passwd | sort    | wc -l > temp2
diff temp1 temp2
rm temp1 temp2