Managing Group Accounts (Files and Commands)

/etc/group contains group account information.

Format:
groupname:password: GID:userlist

e.g.:

admin:x:119:diane
ssh:x:109:

/etc/gshadow contains group passwords.

Format:
groupname:password:administrators:members

e.g.:

diane:!::

Commands:

chage – change user password expiry information

gpasswd – change group membership, group passwords

groupadd – create a group

groupdel – delete a group

groupmod – modify a group

grpconv – moves all group password information to /etc/gshadow

grpunconv – creates group from group and gshadow and then removes gshadow

Shared password is not recommended, so do not use group passwords unless necessary.

In adding a user to a group, usermod removes user from any groups not specified, instead use gpasswd. gpasswd adds the new user without removing the new user from any existing group memberships.
syntax:
gpasswd -a user group

Managing User Accounts (Files and Commands)

etc/passwd contains the user account information(for human or non-human users). Non-human users like sshd also owns files and run processes.

Format:
username:password:userid:groupid:GECOS:homedirectory:loginshell

e.g:

diane:x:1000:1000:Diane Palo,,,:/home/diane:/bin/bash
sshd:x:115:65534::/var/run/sshd:/usr/sbin/nologin

All added users will have a userid of greater than or equall to 1000.

To suspend an account, replace the login shell to bin/false or sbin/nologin to prevent the user from logging in to the machine. Setting the login shell to bin/false or sbin/nologin is used to create limited accounts.

/etc/shadow contains user password information.

Format:
username
:password
:days since Jan 1, 1970 that password was last changed
:days before password may be changed
:days after which password must be changed
:days before password is to expire that user is warned
:days after password expires that account is disabled
:days since Jan 1, 1970 that account is disabled
:a reserved field

e.g:

diane:$6$CnGqE0NA$iLrquxZUS6bqnAbffVNtS/BRy2CB/abthemvhyUHm9A5EK8YdZtj0fSAEhUg8B2YConm0hXCuJsaItAr6RU5R1:15165:0:99999:7:::

Password is encypted, a hash of the password.

Add ‘!’ to disable an account temporarily or sudo passwd -l username.
Remove ‘!’ to enable the account or sudo passwd -u username.
To change user password expiry information, use chage username.

Commands:
passwd – change password
syntax:
passwd password

pwconv – moves all user password information from /etc/
passwd to /etc/shadow
syntax:
pwconv

pwunconv – moves all password information from /etc/shadow to /etc/passwd then deletes /etc/shadow
syntax:
pwunconv

useradd – create an account
syntax:
adduser username

userdel – delete an account
syntax:
userdel username

usermod – modify an account
syntax:
usermod username

Other Commands:

finger – display user’s information.
syntax:
finger username

chfn – change user’s information.
syntax:
chfn username

Managing Processes in Linux

When running a computer system, several process are executed. As a System Administrator of your own computer, you have to manage the processes running on your system. Processes have to be monitored so as to check the health and use of system resources. If you are running Linux, I created a summary on creating, monitoring, and killing a process. A process is actually an executable running in the computer’s memory. Each process has an assigned unique number known as Process ID(PID).  PID’s may be up to 32757. PID’s 0 and 1 are reserved for the kernel, the heart of the Operating System and the init, the mother of all processes.

PROCESS MONITORING
ps: displays a a snap shot of the current processes in the working directory.
ps uxa: displays a snapshot of all the current  running processes.

pstree: displays a snapshot of the genealogy of processes.

top: displays all current running processes in real time.

CREATING A PROCESS
executable file: run the executable
e.g.: usr/sbin/httpd – run the httpd process

KILLING A PROCESS
kill -9 PID: terminate the process through a kenrel call
kill -15 PID: terminate a process nicely

FIELD NAMES IN PROCESS MONITORING
USER –  The user who started the process
PID – The process ID
%CPU –  Shows the cputime / realtime percentage.
%MEM – The fraction of RSS divided by the total size of RAM
VSZ – Size of virtual memory used by the process
RSS – Resident set size (Data & Text segments only) in Kb
TTY – The TTY associated with this process
STAT – The current status
TIME – CPU time in MINS:SECS
COMMAND –  The full command line used to start the process

I also included additional commands in controlling a process. to run a process in the background use & command.
e.g.: xclock &
output: [JOB_ID] PID

jobs: Lists all commands stopped, or running in the background
jobs -l: Lists PID
fg JOB_ID: force a suspended or background process to continue running in the foreground
bg JOB_ID: force a suspended process to continue running in the background