System Administration

This course covers the main or important points from the Udemy Course

File/Text Editor

A text editor is a program that enables you to create and manipulate data in a text file.

There are many standard text editors available by default on most Linux Systems

• vi - Visual Editor
• ed - Standard Line Editor
• ex - Extended Line Editor
• emacs - A Full-screen editor
• pico - Beginner’s editor
• vim - Advanced version of vi

vi vs vim

• Both the editors are the same in terms of functionality, but Vim has more features and an easier learning curve.
• vim is based on vi, when you learn Vim you also learn vi
• vim has a lot of customization and help available online.
• some features that Vim has are,
  1. spell check
  2. completion
  3. comparison
  4. merging
  5. regular expressions
  6. folding
  7. syntax highlighting
  8. vimdiff

vi and vim once learnt are very powerful for editing and programming in the console.

sed

sed (Stream Editor) is a command line utility that can be used to replace a string with a new string in other file operations listed below and much more

• find and delete the line
• remove empty lines
• remove the first or n lines in a file
• to replace tabs with spaces
• show defined lines from a file
• substitute within vi editor

command	description
sed 's/search-word/replace-word/g filename'	replaces all occurences of the search-word with the replace word
sed 's/search-word/d filename	deletes all occurences of the search word
sed -i '/^$/d' filename	removes all the empty lines, -i flag is for inplace edit
sed -i '1,2d' filename	remove lines 1 and 2 from the line
sed -i 's/\t/ /g' filename	replaces tabspace with space
sed -n '12,18p' filename	show lines from 12-18
sed 12,18d filename	show all lines except 12-18

User Account Management

User Account management is an integral part of Linux, here are some commands to manage user and groups

• useradd
• groupadd
• userdel
• groupdel
• usermod

here is an example command that adds the user spiderman to the group spiderman, defines the default shell for the user and also creates a home directory and defines the path for it as well.

useradd -g superheros -s "/bin/bash" -c "user description" -m -d "/home/spiderman spiderman

groupadd and groupdel are the commands to create and delete groups

usermod -aG spiderman superheros` This command adds the user spiderman to superheroes group

there are a few commands that help us with managing users

• who
• last
• w
• finger
• id
• users
• wall
• write

who

this command gives information on who is logged into the system at any given time

last

it gives information about all users from the time of installation, logs all the login, crashes and reboots, and helps with diagnosing when and who was logged in when the event occurred

w

this command is very similar to who gives us more information on idle time, login time and what processes are being run by which user.

id

id command gives information on what the ID of the user is, and the IDs groups the user belongs to.

users

this command gives us the list of all the users logged in

wall

using the wall command we can communicate with all the logged-in users / send a broadcast message

usage - run the command wall

Password Aging

password aging and other parameters related to passwords can be enabled from the config values present in /etc/login.defs file, some parameters are

• PASS_MAX_DAYS
• PASS_MIN_LEN
• PASS_WARN_AGE

chage

the command chage stands for change age, using this we can change the password aging on a per-user basis, the command has a few flags/options that are of significance It has d days since the password was last changed.

• -m the minimum number of days required between password changes --M the maximum number of days that the password is valid.
• -W the number of days before the password is to expire the user needs to be warned about it
• -l gives information about the password age and other information

System Utility Commands

some system utility commands are frequently used are

• date
• uptime
• cal
• bc - (binary calculator)
• hostname
• uname
• which - gives the location of the command
• ps

ps

ps stands for process status and displays all the processes running in the linux system.

usage: ps = shows the processes for the current shell

PID - process ID TTY - terminal type the user is logged into TIME - the amount of CPU in minutes that the process has been running for CMD - the name of the command

command	description
ps -e	shows all the running processes
ps aux	shows all the running processes in BSD format
ps -ef	shows all running processes in full format listing
ps -u username	shows all the proesses running for a user

top

• PID - unique process ID
• USER - username of the owner of the task
• PR - the “PR” field shows the scheduling priority of the process from the perspective of the kernel
• NI - Represents the nice value of a task, lower number = higher priority
• VIRT - total virtual memory used by the task
• RES - memory consumed by the task
• SHR - shows the shared memory used by a task
• S - shows the process state in single-letter form
• %CPU - represents CPU usage
• %MEM - shows the memory usage of the task
• TIME+ - CPU Time, same as TIME but has more granularity to the millisecond

kill

kill command is used to terminate the processes manually, it sends a signal to terminate or kill a process or group of processes.

usage - kill [OPTION] [PID]\

[OPTION] - signal name or signal number / PID

most commonly used signals are

command	description
kill -l	gives the list of signal names or numbers
kill PID	kills the process of the given PID
kill -1	restart
kill -2	interrupt from the keyboard just like CTRL+C
kill -9	forcefully kill the process
kill -15	gracefully kill the process

at

this is a scheduling command just like cronjob, but at command only runs once. when the command is run the console/terminal enters interactive mode and you have to use CTRL+D to exit it.

usage:

• at HH:MM PM = schedule a job
• atq = list all the entries
• atrm = remove at entry
• atd = to manage the at daemon
• at HH:MM PM DDMMYY = schedule a job on a specific date and time.
• at 4PM + 4 days = schedule a job at 4PM 4 days from now
• at now + hours = schedule a job 5 hours from now
• at 8:00 AM Sun = schedule a job on the coming sunday at 8AM

additonal crontabs

there are a few additional ways to run services on a schedule using cron, there are 4 directories for cron - hourly, daily, weekly, and monthly.

all these above files are present in the /etc/cron.____ directory, moving our scripts to these folders will run them without needing to add them to the crontab

Process Management

To send a process to the background which has occupied the current terminal use CTRL+Z to stop the process and keep it in the background and then use jobs to view it in the list of the processes that are running or stopped in the background.

bg, fg, &

to start the stopped process in the background to running use the bg command, and use fg to bring it back to the foreground.

System Monitoring

dmesg

gives information on the system information and warnings, is very important to the system hardware and helps diagnose information.

iostat

gives information on disk usage, use iostat 1 to see the disk information which refreshes every second.

netstat

netstat helps with all the information in the network. use man netstat to learn more about it.

cpu and mem info

the files located at /proc/cpuinfo and /proc/meminfo have all the information related to the CPU and memory.

script

usage script filename This starts recording all the commands typed in the screen to the file, use the exit command to stop the recording process