Getting Group Names for List of Users

Question

I have a text files with a list of users formatted like:

name1   
name2   
name3   
...

Calling the command groups gives the primary group name for that user. ex:

bash$ groups name1
name1: group_Name

I want to get the groups of all the users of the textfile and put this list of groups into a new text file.

Does anyone know a command for this? Maybe a script the iterates the first file with the names and calls groups on each name then stores the output in a new file. I just need a little help with the specifics.

Answer 1

Read the file line by line, call groups for each item:

while read name ; do groups "$name" ; done < list.txt > with_groups.txt

Loops can often be replaced by xargs:

xargs groups < list.txt > with_groups.txt

Answer 2

How about something like this:

#!/bin/bash
while read ID
do 
    groups $ID
done < input.file |
awk '{print $3}' | sort | uniq > list_of_groups

That gets you a lexically-sorted unique list of group names.