Display all line numbers for user names starting with "admin" from passwd file

Question

How do I find all the occurance of a particular text in a file with a shell script. (Not using grep command)

Answer 1

awk '/^admin/ {print NR;}' /etc/passwd

Answer 2

A POSIX shell-only solution could be

n=0
while n=$((n+1)); IFS=':' read -r u _
do
  case "$u" in
    admin*) echo $n;;
  esac
done < /etc/passwd