A quick guide on how to copy group member ship in linux to another user. Which can be useful when setting up new users on a linux machine to make sure that users. It is also a way to find out what groups a user is a member of.
Part 1 - Get a list of groups
To get a list of groups as user is a member of can be done by reading the /etc/group file and doing a little bit of processing. This can be done using the following command.
grep -E "(:|,)<username>(:,|$)" /etc/group|cut -f1 -d:
The complex expression above is built to match specific username in each line of the file. Of which there is 3 different cases ":<username>" ",<username>," ",<username>". So we search for the specific username beginning with a ":" or a "," and also ending in a "," or a "$" which is a newline. Then it cuts on the first field using ":" as a separator.
This will output a list of group that "<username>" is a member of
Part 2 - Add another user to the same list of groups
Since we know that addgroup can be used to add a user to a group by doing "addgroup <username> <groupname>" then we can expand the above to add a username to each group in the list like this
for i in `grep -E "(:|,)<username>(:,|$)" /etc/group|cut -f1 -d:` ; do
addgroup <newuser> $i
This will then make sure that the user <newuser> is a member of all the same groups that <username> is.
You should take care using the above as you may give somebody access well beyond what you thought you might have by giving them access to additional groups.