lk/basics/users.md

title	tags
users	Documentation basics

Basic Information

Let's get some entries with 'getent', e.g. passwd or group.

getent passwd

getent group

Obviously:

getent shadow

will require sudo.

Examples

sudo adduser maestro

add user 'maestro'

This depends upon the settings in the /etc/default/useradd file and /etc/login.defs

sudo useradd -m pinkie

add user 'pinkie' with a home directory

sudo adduser -m -e 2017-04-25 temp

add expiry date to user

userdel maestro

delete maestro

userdel -r maestro

delete maestro and hir homefolder

groups

find which group you are in

id

same

id -Gn maestro

Find which groups maestro is in

deluser --remove-home maestro

delete user maestro

usermod -aG sudo maestro

add user maestro to group sudo

cat /etc/passwd

list users' passwords (and therefore users)

groupadd awesome

create the group 'awesome'

passwords are stored in /etc/shadow.

there are user accounts for processes such as 'bin' and 'nobody' which are locked, so they're unusable.

passwd -l bin

lock the user 'bin'

more /etc/passwd | grep games

we find the name, password and user id of the user 'games'. I.e. the password is 'x', and the user id is '5'. The password is an impossible hash, so no input password could match.

groupdel learners | delete the group 'learners'

gpasswd -d pi games | remove user 'pi' from the group 'games'

id games

find the id number of group 'games' (60)

usermod -aG sudo maestro

add user to group 'maestro'

user info is stored in /etc's passwd, shadow, group and gshadow

Defaults

The default new user profiles are under /etc/skel.

Shells

A list of shells is in /etc/shells.

Only root can run shells not listed in /etc/shells

To change a user's shell:

usermod --shell /bin/bash user1

Alternatively, change the shell in /etc/passwd.

Usermod also lets you change a user's username:

usermod -l henry mark

However, this will not change the home directory.

Lock a user out of an account:

usermod -L henry

More Arguments

-G or -groups adds the user to other groups:

usermod -G sudo henry

-s adds the user to a shell.

-u let's you manually specifiy a UID.

Groups

In /etc/group, a group file may look like this:

sudo:x:27:mike,steve

We can use groupmod, like like usermod, e.g. to change a name:

groupmod -n frontoffice backoffice

Delte a group:

groupdel frontoffice

Logins

See list of logged on users.

w

See last logons:

last

or all logon attempts, including bad attempts:

lastb

List recently accessed files:

last -d

See files opened by steve

lsof -t -u steve

See files opened by anyone but steve

lsof -u ^steve

Fuser can also track people loggingin:

fuser /var/log/syslog

... and fuser can kill everything accessing the home directory:

fuser -km /home

Looking for Dodgy Files

Some files can be executed by people as if they had super user permissions, and that's okay... sometimes.

Let's start with files executable by user:

sudo find / -type f -perm -g=s -ls

And then those executable by the group:

find / -type f -perm -g=s -ls

And finally, worrying files, executable by anyone as if sie were the owner:

find / -xdev -o -nogroup -print

Then have a look at resource usage per user.

#SGID

sudo chmod u+s process.sh

This will modify process.sh to that instead of being simply executable, anyone executing it will have the permissions as if owner while executing it.