Will there be always a pair of same UID and GID?

Question

I have to change a file/directory's ownership using chmod. I know only the owner's username but want to change the default group name simultaneously. At this stage the group name is not decided so I am putting same as the username:

chown -R username:username path_to_dir

Command may run for various systems/users. So my concern is, will Linux always have a pair username(user):username(group) OR at some point this default group_name may give error.

Although I have started to find GID from the user belongs

id username | tr '(=)' ':' | awk -F: '{print $3}

But for knowledge I want to know. Till now analysis of /etc/passwd /etc/groups except few users like "shutdown,halt,sync,operator" all users have listed in groups as well.

EDIT 1: id -gn username is also good option

Answer 1

There is no guarantee that the groupname = username will exist.

The most common scenario is that system administrators use on Linux is creating a new user locally on the system is without an explicit specification for the group, which means that the group will be created by default same as the user name and assign the user to have the default GID to be of the newly created group.

This doesn't mean that another system administrator will follow this procedure and if you throw other naming services into the mix like NIS or LDAP the group assigned to be default will likely not be same as the new GID with groupname same as username.

So you shouldn't rely on the fact that username=groupname

Answer 2

Although the user private group idiom is often used in modern Linux systems, *nix does not actually require a unique primary group for each user, and it's quite possible for several user accounts to share a common primary group such as users or staff.

Even in cases where the UPG idiom is enforced, there is no particular reason that the numeric GID of a particular UPG should be equal to the owner's UID, although most account-creation tools will try to assign UIDs and GIDs in order, if they are available. The IDs can easily get out of step if the users and/or groups need to be made consistent across multiple systems (for example to accommodate pre-existing NFS shares).

Answer 3

As Ramesh has indicated by default the UID and the GID are equal for ordinary users. However this is not always the case so you must not make such an assumption as this may wreak havoc to your system. Assigning file/folder permissions to a wrong group is somewhat messy, isn't it?

So, the short answer: no, UID is not always equal to GID. Still, /etc/passwd contains both the UID and GID of the default group on the same line so it's rather easy to extract them.

Answer 4

From this link, I see the below information.

A user ID (UID) is a unique positive integer assigned by a Unix-like operating system to each user. Each user is identified to the system by its UID, and user names are generally used only as an interface for humans.

UIDs are stored, along with their corresponding user names and other user-specific information, in the /etc/passwd file, which can be read with the cat command as follows:

cat /etc/passwd

The third field contains the UID, and the fourth field contains the group ID (GID), which by default is equal to the UID for all ordinary users.

EDIT

However, as @Karlson points out it is not true that the UID will be equal to GID by default.

The statement is true only if no ordinary groups are added separately and no groups are explicitly specified on any user creation. By default gid for the next added user = max(gid) + 1. There is no correlation between UID and GID required by any *NIX system they just happen to coincide in some cases.

More references

http://www.thegeekstuff.com/2009/06/useradd-adduser-newuser-how-to-create-linux-users