5.2 Creating Users and Groups

5.2 Creating Users and Groups

Once you can identify account types and the /etc databases, the next Essentials skill is creating users and groups safely. You do not need every obscure useradd flag, but you must know the core commands, what a new account needs before login works, and how UID/GID numbering fits the picture.

Creating a user with useradd

The useradd command adds a new entry to the account databases (and, depending on options and distro defaults, may create a home directory).

sudo useradd -m -s /bin/bash maya
sudo passwd maya

Distro defaults differ: some systems create a matching private group (UID=GID naming), others place new users in a shared users group. For the exam, focus on useradd creates the account record, and -m ensures a home directory. Without -m, you may get a passwd entry but no /home/username until you create it yourself.

Worked example: An instructor adds a lab account: sudo useradd -m -s /bin/bash lab22 sudo passwd lab22 Until passwd succeeds, password-based login for lab22 fails even though the account exists in /etc/passwd.

Worked walkthrough: useradd → /etc/passwd fields

Map each flag to the colon-separated passwd line so you can predict the result:

sudo groupadd -g 1600 studio
sudo useradd -m -u 1105 -g studio -G designers,video -c "Maya Lopez" -s /bin/bash maya

Resulting line (field order):

maya:x:1105:1600:Maya Lopez:/home/maya:/bin/bash

Verify with id maya (uid, gid, groups) and ls -ld /home/maya. If -G was omitted, only the primary group appears. If -m was omitted, the home directory may be missing even when field 6 names a path. Run getent passwd maya as another quick confirmation that the account record landed.

Setting passwords with passwd

passwd updates /etc/shadow. Essentials cares about the workflow: create account → set password → user can log in. Password management is separate from merely adding a username.

Creating groups with groupadd

sudo groupadd designers
sudo useradd -m -s /bin/bash sam -G designers

groupadd creates a new GID in /etc/group. Attach users with useradd -G or later membership changes (usermod -aG on many systems). Shared project directories often rely on a common group so members collaborate without sharing one login.

/etc/skel — the new-home template

When useradd -m creates a home, it typically copies files from /etc/skel (for example .bashrc, .profile) into /home/username/.

Worked check: If /etc/skel/.bashrc exists and you run sudo useradd -m demo, expect /home/demo/.bashrc owned by demo. Missing starters → check skel and whether -m was used. A class README placed in skel is copied to every later new home.

UIDs and GIDs: numbers are the real identity

Linux stores ownership as numbers. Recreating username alice with a different UID leaves old files tied to the previous number (or a numeric owner if the name mapping is gone).

Human users are not UID 0 and usually sit above the system range; exact UID_MIN values vary by distro.

Practical creation checklist

1. Human account (home + shell) vs system-style (often no login shell).
2. groupadd first if you need a project GID.
3. useradd -m ... with primary/supplementary groups as required.
4. passwd, then verify with id username.

sudo groupadd projectx
sudo useradd -m -s /bin/bash -G projectx kai
sudo passwd kai
id kai

Common mistakes to avoid on the exam

• Creating a user but forgetting passwd.
• Forgetting -m when a home is expected.
• Confusing /etc/skel (template) with /etc/shadow (hashes).
• Assuming the username string alone owns files forever—UID is what the filesystem stores.

Topic 5.2 creates identities; Topic 5.3 applies them with chmod/chown. A wrong UID at creation becomes a wrong owner later.