Master Linux User & Permission Management: Commands, Options, and Best Practices

Viewing the Current User

Use whoami to display the logged‑in username, or who am i for the user of the current pseudo‑terminal.

Common who Options

-a

– display all available information -d – show dead processes -m – similar to

am i

-q

– show the number of logged‑in users and their names -u – display login information -r – show run level

Creating a New User

Simple creation:

adduser newname

Difference Between useradd and adduser

useradd

only creates the account; you must set a password afterwards with passwd newname. adduser creates the account, home directory, and prompts for a password, performing a full setup.

adduser Detailed Options

Usage:

adduser [options] login

-b, --base-dir BASE_DIR

– base directory for the new account’s home -c, --comment COMMENT – GECOS field -d, --home-dir HOME_DIR – home directory -D, --defaults – show or change default useradd configuration -e, --expiredate EXPIRE_DATE – account expiration date -f, --inactive INACTIVE – days after password inactivity before lock -g, --gid GROUP – primary group name or ID -G, --groups GROUPS – supplementary groups -h, --help – display help -k, --skel SKEL_DIR – skeleton directory -K, --key KEY=VALUE – override /etc/login.defs defaults -l, --no-log-init – do not add user to recent login databases -m, --create-home – create home directory -M, --no-create-home – do not create home directory -N, --no-user-group – do not create a group with the same name -o, --non-unique – allow duplicate UID -p, --password PASSWORD – encrypted password -r, --system – create a system account -R, --root CHROOT_DIR – chroot directory -s, --shell SHELL – login shell -u, --uid UID – user ID -U, --user-group – create a group with the same name as the user -Z, --selinux-user SEUSER – SELinux user mapping

Deleting a User

Use userdel:

userdel [options] username

-f, --force

– force removal even if the user is logged in -r, --remove – delete the user’s home directory and mail spool -R, --root CHROOT_DIR – operate inside a chroot -Z, --selinux-user – remove SELinux mappings

On systems without userdel, deluser –remove-home newname can be used to also delete the home directory.

Changing a User’s Password

passwd newname

Modifying a User Account

Change primary group: usermod -g siatstudent newname Add the user to multiple groups: usermod -G friends,happy,funny newname Append a group without removing existing ones:

usermod -aG happy newname

Viewing a User’s Groups

groups newname

Group Management

Create a group:

groupadd newname

-f, --force

– succeed if the group already exists -g, --gid GID – specify GID -K, --key KEY=VALUE – override defaults -o, --non-unique – allow duplicate GID -p, --password PASSWORD – set encrypted password -r, --system – create a system group -R, --root CHROOT_DIR – chroot directory

Modify a group name: groupmod -n test2group testgroup Delete a group:

groupdel test2group

File Ownership and Group Inspection

List files with owners and groups: ls -l Show current user’s groups: groups Show a specific user’s groups: groups testnewuser Display all groups:

cat /etc/group

chmod – Changing File Permissions

Basic syntax:

chmod [options]... MODE[,MODE]... FILE...

-c, --changes

– report only when a change is made -f, --silent, --quiet – suppress most error messages -v, --verbose – diagnostic output --no-preserve-root – do not treat '/' specially --preserve-root – fail when operating recursively on '/' --reference=RFILE – use RFILE’s mode as reference -R, --recursive – apply recursively --help – display help --version – display version

Permission symbols: u – user (owner) g – group o – others a – all + – add permission - – remove permission = – set exact permission

Examples:

chmod u+rx file.txt

chmod g+r file.txt

chmod o-r file.txt

chmod g+r o-r file.txt

chmod go-r file.txt

chmod +x file.txt

chmod u=rwx,g=r,o=- file.txt

chmod -R 700 /home/newname

chown – Changing File Owner and Group

Basic usage: chown newname file.txt Change both owner and group: chown newname:friends file.txt Recursive change:

chown -R root /u

-c, --changes

– report only when a change is made -f, --silent, --quiet – suppress most error messages -v, --verbose – diagnostic output --dereference – affect the referent of symbolic links (default) -h, --no-dereference – affect symbolic links themselves --from=CURRENT_OWNER:CURRENT_GROUP – change only if current owner/group match -R, --recursive – operate recursively -H, -L, -P – control traversal of symbolic links when recursive --help – display help --version – display version