Important files¶

/etc/passwd : all users and accountt data /etc/shadow /etc/group

ID¶

0: Root id

1-99 are traditionally reserved for special system users (sometimes called pseudo-users), such as wheel, daemon, lp, operator, news, mail, etc.

1-200: system processes (Redhat)
201-999: system users that uses system process but don't own file on the system 1000 - : system users

whatis id

id (1)               - print real and effective user and group IDs

id milad

uid=1000(milad) gid=1000(milad) groups=1000(milad),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),114(lpadmin),115(sambashare),119(vboxusers),123(wireshark)

id -u
id -un

1000
milad

id -g
id -gn

1000
milad

id -G
id -Gn

1000 4 24 27 30 46 114 115 119 123
milad adm cdrom sudo dip plugdev lpadmin sambashare vboxusers wireshark

id -g milad

1000

Adding user¶

useradd -mM -c "Milad As" -g [primary group id] --skel SKEL_DIR -s /bin/bash -u UID -e Y-M-D milad

-U create a same group as username and add user to it  
-m create home dir  
-M do not create home directory  
-d home dir  

/etc/skel                Default SKEL_DIR  
/etc/default/useradd     Default settings

usermod -L milad # lock # Adds a ! in shadow file password field
usermod -U milad # unlock

-a, --append                  append the user to the supplemental GROUPS
  -m, --move-home               move contents of the home directory to the

Manage password and login¶

# Change user password

passwd # Current user
passwd username

/usr/sbin/nologin - to prevent login

# have look at:
/etc/shadow
milad:x:17032:0:99999:7:::
user : passHash : lastPassCh : DayToAllowPassCh : DayToRequirePassCh : warnBeforeExpire : activeAfterExpire : expireDate

Manage password policies¶

/etc/login.defs - shadow password suite configuration
ex: how many day to change pass or shoud change pass

man 5 login.defs

chage - change user password expiry information

chage -l milad

Last password change					: Aug 19, 2016
Password expires					: never
Password inactive					: never
Account expires						: never
Minimum number of days between password change		: 0
Maximum number of days between password change		: 99999
Number of days of warning before password expires	: 7

chage -E [`date -d "+10day" +%F` | -1 to remove]

Groups¶

groupadd
groupmod
groupdel
gpasswd

getent group sudo

sudo:x:27:milad

groups milad

milad : milad adm cdrom sudo dip plugdev lpadmin sambashare vboxusers wireshark

cat /etc/group | grep milad | wc -l

10

mkdir test
ls -lhd test

drwxrwxr-x 2 milad milad 4.0K Dec 28 07:15 test

chgrp, chown, newgrp¶

chgrp adm test
ls -lhd test

drwxrwxr-x 2 milad adm 4.0K Dec 28 07:15 test

chown :milad test
ls -lhd test

drwxrwxr-x 2 milad milad 4.0K Dec 28 07:15 test

chgrp adm test
cd test
touch newfile.txt
ls -lhtrFA
newgrp adm
touch newfile2.txt
ls -l

setgid¶

chmod g+s test/
# inharite parent directory group

Lecture notes¶

All lecture notes are available at github in ipynb format: https://github.com/ravexina/linux-notes.

There are html exports of project available at: https://ravexina.github.io/linux-notes.

License¶

Linux Notes by Milad As (Ravexina) is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.