print a list of a auxiliary groups assigned to a single user name, using getent(1)

getent_auxgroups.sh

# getent_auxgroups.sh - print auxiliary groups assigned to a single user name
#
# % Environment parameters
# WHOM 
#  - user name to parse for
#  - If unspecified, defaults to $USER
#  - Will be interpreted within a regular expression, in awk(1)
#
# % Depends On
# - getent(1)
# - awk(1)
#
# % See Also
# - pw(8), FreeBSD System Manager's Manual
#
# Public Domain
#
getent group | 
  awk -v UNAME="${WHOM:-${USER}}" \
    'BEGIN { FS=":" } 
     { split(4,subfield,/,/); 
       for (idx in subfield) { 
         if (match($subfield[idx],"^" UNAME "$")) { 
            print $1 
         } 
       } 
     }'