AWK Select Multiple fields

• fileds, $1 $2 ..., a whole line $0
• Printing every line : cat /tmp/a.txt|awk '{ print $0 }'

$ awk -F":" '{ print $1 $3 }' /etc/passwd
stanford7
imcrazy9
test453
user7711
rootadmin4
....etc.

$ awk -F":" '{ print $1 " " $3 }' /etc/passwd

$ awk -F":" '{ print "username: " $1 "ttuid:" $3" }' /etc/passwd
username: stanford          uid:7
username: iamcrazy          uid:9
username: test45            uid:3
username: user77            uid:11
username: rootadmin         uid:4
....etc.