Bulk Assign Multiple Users to Multiple Groups

Question

I have 2 text files, users.txt with a list of 1000 users and groups.txt with a list of 50 groups. I want to run a command that adds 30 users to each group (ex: users 1-30 to group 1, users 31-60 to group 2, etc.). What would be the most practical way of doing this?

This is what I have so far:

for i in `cat users.txt` ; do useradd $i; echo Pass$i | passwd $i -- stdin; done
for i in `cat groups.txt` ; do groupadd $i; done

Answer 1

Assuming your files have one user and one group per line, here's a pseudo-oneliner:

sed = groups.txt | while true ; do 
  read line_no
  read group_name
  [ -z $line_no ] && break
  (( from = (line_no - 1) * 30 + 1 ))
  (( to = line_no * 30 ))
  sed -n "${from},${to} p" users.txt | xargs -r -n 1 usermod -a -G $group_name 
done

This code calls sed(1) to print line number (starting with 1) and the line itself (i.e. the group name) for each line in groups.txt, then pipes that output into an endless loop, which does the following:

• Reads line number from the piped input into $line_no variable
• Reads group name from the piped input into $group_name variable
• If $line_no is empty, assumes we've reached an end of groups.txt and break-exits the loop
• Calculates starting and ending line numbers for users.txt using $line_no as index
• Calls sed(1) to print all lines from users.txt between those lines
• That list of usernames is piped into xargs(1), which runs usermod(8) for each single username, appending it to a $group_name from above. '-r' switch tells xargs(1) to not run usermod(8) if the username/stdin is empty.

edit: replaced semicolons with line breaks for legibility