Applications that fulfilling the purpose of the embedded system
These applications uses library to function
There are high level OS abstractions or the OS components that applications make use of it
Networking: to connect to other devices
File system: for files to live
Low level interface: needed to support function of the device
Hardware: everything sits on

Example: android and raspberry pi

Linux in the Cloud

Fault tolerance: if you have a failure in one place(region), you can move services to another place
Redundancy:
Region: region is collection of AZ
Availability Zone: data centers
Subnet: local network instance for the compute resource in the cloud

How to SSH/Connect to a remote server

Ssh User/Host_Name@RemoteHost_IP (public address)

Port used: 22/TCP

Linux Commands

Directories

https://opensource.com/life/16/10/introduction-linux-filesystems

/etc = stores system configuration
/boot = linux kernel configs and files
/sys = where the data is stored;

sudo file system where kernel gives info about systems

/proc = sudo file system where kernel gives info about processes
/dev = device
/tmp = directory that holds large less important files. Files will be deleted upon booting the system
var/temp = hold more persistence file. Files will be deleted after 30 days.

Basic Commands - 1

Command	What it does
whoami	Return user name
uname -a OR -r	Which Kernel is this
cat /etc/release	Show which distro you are on
pwd	Print working directory
cd ~	Change to home directory
cd /	Changes to root directory of file system
ls	List of files Ls -a // prints hidden files(starts with .)
W > log.txt	Redirect the output into text file
Last >> log.txt	Append output to the same text file
grep -i usb	Prints usb from the file // -i = casing doesn’t matter

Process Commands - 2

Process and PID: In Linux, when an executable stored on disk is called a program, and a program loaded into memory and running is called a process. A process is given a unique number called process ID (PID) that identifies that process to the system, when it is started

w	Anyone logged in
last	Shows last person to login and last time system rebooted
uptime cat /proc/loadavg	Shows howmuch time the server is up What is the current system load
cat /etc/passwd \| wc -l	This file has all the users on a linux
sudo dmesg	sends info about kernel (hardware related info)
htop	Shows running processes on a host Gives processes that using the most CPU Useful for troubleshooting load issues
ps aux ps aux \| grep Gautam \| wc -l ps -eF \| grep Gautam \| wc -l ps -U User_Name \| wc -l	Dump out all of the processes To grab howmany your process is running the Same result as above Howmany process running as given user name
ps aux \| grep process_name	Grab process ID(PID) of given process name
cat /proc/PID/status \| grep threads	Grab howmany threads is the process running
service S_NAME status	Which linux service is running

Log Command - 3

/var/log

Examples:

sudo cat /var/log/httpd/access_log | grep -E “^10.0.1.10”

All general logs saved here

Howmany time has 10.0.1.10 has access the site

last

Shows last person to login and last time system rebooted

Tar Commands - 4

tar cf archive.tar test_file_*	Archive test_file_* into archive.tar
tar tf archive.tar	Look through/List files in the archive.tar
tar rf archive.tar file?	Append/add additional files into archive.tar
tar xf archive.tar file3 tar xf archive.tar // extract everything	Extract specific file from the archive.tar
tar xf archive.tar –wildcard ‘test_file_?’	Extract list of files using globbing(?,*) via wildcard
tar --delete --file=archive.tar file3	Remove file from archive.tar // edge case
tar czf archive.tar.gz file_*	Compress(to reduce file size) file via tag czf // gzip algorithm Why?: if you’re running on low disk space
tar cjf archive.tar.bz2 file_*	Compress(to reduce file size) file via tag cjf // bzip2 algorithm
zip -r archive.zip file_*	Compress(to reduce file size) file via option -r // zip algorithm

Searching data Commands - 5

cat gautam.txt | grep Apple

Prints the total number of word Apple

cat gautam.txt | grep Apple | wc -l

Prints the total number of line that contains Apple word

cat /etc/passwd | grep gautam | cut -d: -f6

Prints the home directory for gautam user

How:

Cut the whole line from : (-d is delimiter where do you want to make this cut)
Takes the field from 6th position

grep -E ^M gautam.txt	Match everything in line that starts with letter M -E = find pattern as an extended regular expression
grep -E “apple?” gautam.txt	Search anyline that ends with apple $ = match end of line
grep -E “apple\|ball” gautam.txt	Search for either result via \|
grep -E “Ap*le” gautam.txt	In line; Match an A followed by zero or more ps followed by le. // doesn’t have to be line start with a, it searches in line and not from the beginning of the line
grep -E “Ap+le” gautam.txt	In line; Match an A followed by one or more ps followed by le. // doesn’t have to be line start with a, it searches in line and not from the beginning of the line
grep -E “Ap?le” gautam.txt	Match an A followed by maybe more ps followed by le.
grep -E “Ap[p-z]le” gautam.txt	Match an A followed by p through z followed by le.

Hardware Command - 6

df -h	How much storage is available Disk free -h(human-readable)
cat /proc/cpuinfo lscpu sysctl -n hw.ncpu OR sysctl -a \| grep machdep.cpu // for mac OS	How many CPUs/cores CPU speed (model name)
sudo dmidecode sudo lshw	Info about motherboard // under memory device you’ll find size = RAM
free -m	Howmuch swap is being used // IMP
sudo lshw	BIOS version Lshw pull infor about montherboard
du -sh DIR_NAME	To check the size of the directory du = disk usage
mount ABSOLUTE_PATH /mnt mount downloads/work /mnt	To mount on disk

Network Commands - 7

Ifconfig	View and change the interface configuration
sudo ip addr show	To determine the IP address Loopback device(internal device used by linux) eth0(actual NIC) return IP address of the host Returns MAC address // link/ether
ping -c1 www.google.com curl -I www.google.com	Connect to google server \| test connectivity To make the connection // make sure the DNS is working correctly
sudo ip route show OR ip r s	Determine the gateway
cat /etc/resolv.conf OR cat /etc/resolv.conf \| grep nameserver	Determine DNS server (Name Server) This is the host that any DNS query going to go to
dig www.nike.com	Translet domain name to IP address with more info (question[give me internet record] section and answer section)
host www.nike.com	Translet domain name to IP address \| DNS lookup using configured DNS Use Host for Lightweight DNS work
dig @1.1.1.1 www.nike.com OR dig www.nike.com 1.1.1.1	If you can’t ping google.com tell some other (external)DNS address to perform the lookup for you AND this also means your own DNS is borken and that is fixed in /etc/resolv.conf
netstat AND ss netstat -tulpn	View listing service and active connections Destination Gateway genmask , flags, etc

Find and determine network configuration

User/Permission Command - 8

cat /etc/passwd	List of users in linux
su - User_Name	Change from one user to another
sudo cat /etc/sudoers	See what permission assigned to all users
sudo visudo	To edit permission file
sudo su -	To go to sudo user from your user
Id gautampambhar OR cat /etc/group OR Groups	What group a user belong to
Getent passwd gautampambhar OR cat /etc/passwd \| grep gautampambhar	What is user’s home directory Return /bin/bash
sudo /etc/shadow	To see the password hash of the users

sudo useradd gautam2 sudo passwd gautam2	To add a new user Asked for the current user password Set a password of a new user
sudo useradd -m gautam3	Ideal way to create a user with having following -m = home directory
sudo groupadd admins sudo usermod -a -G admins gautam3	Add a group Add user to a group -a: add/append -G: group You have to logout and log back in to see yourself into this group or reload profile( . /etc/profile)
sudo passwd -l User_Name	Blocking user account password -l = locking
sudo userdel User_Name	Delete user
sudo chown -R user:group /home/deletedUser	Replace new user to the group with the deleted user’s group

When you create user

Set his home directory // sudo mkhomedir_helper gautam2
Set his password
Specify the login shell // sudo usermod -s /bin/bash gautam2

When you create group

You can set it’s own permission(by vim sudo visudo) without adding this group into root group
Add user to this groups

Applications

Desktop application

Server applications

Server provides client services

Ex: apache, nginx (free open source web server)

Your computer connects to the internet
Apache/another computer having applications connects to internet serve content to your PC
Apache listens on a port ex: 40, 443

Listening for incoming connections

Listen to request through a web browser and serve content
Developers will host theirs files on apache; apache host will host these files and serve them as web content for incoming request

Nginx

Reverse Proxy: proxy request for something else
Load balancing: balance incoming request
Mail proxy
HTTP caching

Programming language, For example, C#, Bash, Python, etc

Chronology of source code to executable file

You write code in C; your source code in C
You then use a compiler to compile that code into binaries
Binary then is executable ex: firefox

Package management tool

How installation packages are tracked and managed

Issue: application needs to install dependencies which also depend on other dependencies

Resolve: package manager like apt-get and yum, which installs the complete dependencies/applications that applications need

How?: It downloads the library and packages from the software repository and installs on the host machine

Ex:

dpkg: debian package

Apt-get: allows automated update and package dependency management to solve problem of dependencies hell for debian linux distro

Yum: allows automated update and package dependency management to solve problem of dependencies hell for redhat linux distro

CLI

How it works

It reads the input from your CLI
Process input by interpreter
Executed via application

$ stands for ready to take user input

CLI Syntax

Command [option] [file]

Ex: ls -l

How it works

Read the input
Divide the input with ignoring comments
Apply quoting
Parse into commands
Apply redirection (by piping)
Wait for exit status
Display output (read output)

Variables

To store value (number, character, string) and reference it later

Use case

a=”hi”
Use variables to store output from certain commands

a=$(ls)

Quoting

Methods of quoting

Escape character: preserves the literal value of the next following character
Single quotes: preserve the literal value of variable

Ex: echo “this is $a” // “this is $a” // It won't print the value of $a

Double quotes: preserves the literal value of most characters contained within quotes, exception includes $, ‘,\

Ex: echo “this is $a” // “this is gautam” // It will print the value of $a
Ex: echo “this is a \”quotes\”” // this is a ”quotes” // preserves “”

Path environment variable

Example for quoting in Bash

This is ‘just’ a “text”

Echo “This is ‘just’ a \”test\”” // escape “” by putting \ in front of “

this is a backslash "\" and this is a single quote'

Echo “this is a backslash \”\\\” and this is a single quote'” // escape “ and \ by putting \ in front of “ and \

3 double quotes “””, and 3 single quotes ‘’’, and three backslashes \\\

Echo “3 double quotes \“\”\”, and 3 single quotes ‘’’, and three backslashes \\\\\\\\\”

Escape “ by putting \ in front of “
Escape \ by putting \\ in front of \

Why \\ instead of \?

Because if you put \ → that will be \\ for each \ → \\ return single \\ → therefor add \\ in front of \

In a nutshell if you want to use \ just the way it is; use \\ for escapting it

This is a newline character \n, it will create a new line

Echo “This is a newline character \\\n, it will create a new line”

Man & Info command for Help

To get more info about command use Man and Info command

Ex:

man ls
/lines // to search a keyword in the man page use /word

Linux directories and file system

We have a hard drive in our computer

When we install linux OS, it will put a file system on a hard drive and place file on that file system

It will put a file system in a specific hierarchy

Linux OS

It’s hardware(a physical component used for computing) and a Linux kernel fitted on hardware

Computing hardware

CPU: processes computer function and performs calculation
RAM: high performance and volatile storage
Secondary storage: SSD/HDD/CVD - persistent storage for data not currently in use
Network interface card: permits connection to a network
Input device: Mouse/Keyboard - send data into the computer via human interaction
Output device: monitor - send info from computer to the user

Drivers:

Drivers needed for application to use hardware

Cron

Is a daemon that runs in the background and it can execute things on an interval as you like it

Internet, Network and Routers

In local network, multiple devices are connected with each other via either wire or wireless to communicate and transfer the data/packets
Switch is carrying data/packets and unable devices to talk with eachother via local network // switches forwards packas on the same network
Switch is then connected to a router
Router(Gateway) is then connected to ISP // router forwards packets between networks

Idea

If you have a server and needs to access to an internet or other networks, it needs to be connected through a router

Command

Ping -c l google.com or dig www.nike.com
Step pings is saying like give as a host name google.com
Next turn that into IP address through DNS query

DNS

Translate domain name to IP address

It’s going to tell your Name Server that(we asked to perform a query) please translate nike.com into IP address

Network configuration

Scenario

If you are on a local network on some device and want to talk to device in the same network, that request doesn’t has to be go through gateway, you can talk directly through the local network (via switch)

User and Groups

3 types of user

Root user
Standard user
System user(service account)

Deployed when applications are installed
Do not have a login shell

Permissions

chown: it lets us change the user & group that owns the file

You can change the file group by using this command and put it onto another group

sudo chwon :Group_Name filename

You can change the file user by using command and assign file to another user

sudo chwon User_Name filename

Change ownership of user and group together

sudo chwon User_Name:Group_Name filename

chmod: let us change the mode read, write & execute across user, groups and everyone

You can change the file mode from read to execute/write

Linux - LFCS

Essential Command

Find/Locate

What: search for files and directories.

How

Find:

Search files and directories
Can search by file name, type, timestamp, and other attribute
Searching in realtime, so it can be slow on machine

find -name “file.txt”	Will search for a file in a current directory
find / -name “file.txt”	Will search for a file in root directory
find /users/hustler -name “file.txt”	Will search for a file in specified directory
find / -iname “file.txt”	To make search case insensitive
find / -type f -name “*.log” type c type d type l type f	Search type of different files C = find input devices D= directories L = Links F = files
find /etc -type f -user “hustler”	Find type by users // which user contains what files in the /etc directory

locate:

Faster search then find
Limited search option; can’t search by attributes or metadata
Relies on a database for the search, which must be refreshed regularly

locate “file.txt”	Will search for a file and will return file with address
locate -i “file.txt”	To make search case insensitive

which/whereis/type

Which

Returns the location of a command based on the PATH settings,

Whereis

Returns the location of the binary, source file(s) and man pages
will return multiole versions of a he it thev exist

Type

Returns information about the command type.
Details are based on how command relates to the shell configuration

which python

which nano

Will return the python location

Will return the nano location

whereis python

whereis python | tr “ ” ‘\n’

Will return the python location with more details

Print all entry in a new line

tr = transmit into new line

type python

Where is python located = same as which

File Systems

Block device: media used to store data(SSD, Hard disk, floppy disk)

File system: method of allowing OS to interact with data on a block device

It sits on top of the block device and where actual computer system and information resides

FS with journaling

What: it helps prevent data loss and file corruption when you lose power.

You have your OS that writes data into your disk
If power went down on your machine, the system would have no idea whether the file was properly written to the drive or what state the data's in.

How

With journaling, when you start to write a file, the operating system will note that the file is being written to disk.
Once the file has been written, the operating system will remove the entry from the journal.
with journaling, when the computer comes back online, Linux would check the journal and resume any partially completed jobs.

Type of file systems

EXT1/2/3: extended file system created for Linux in 1992

Ext4: increased filesytem size, journal checksum

BTRFS: BetterFS or ButterFS created by oracle in 2007

Includes FS expansion and reduction
Replacement of EXT2/3/4

ReiserFS

Founded in 2001
Dev killed his wife and development stalled

ZFS: created by sun microsystem for solaris now owned by oracle

Supports drive pooling, FS snapshots, FS stripping
Each file has a checksum, making it easy to tell if the file has been corrupted

Can be extended dynamically on the fly. However it can’t be reduced dynamically

FS has to be unmounted before you reduce its size

Handles large file well

JFS
SWAP: this is not a file system

Used to format a drive, but not technically a filesystem.
Used for virtual memory (memory swapping) and doesn't have a viewable structure
Temporary place for items in memory to be stored in low RAM situations

FAT/32/exFAT

Manipulate File Content

cat file.txt	Print the file content
cat file.txt \| more/less/sort	Cat with More: advance the output one screen at a time Less: advance the output one screen at a time; scroll up-down one line at a time, can find keyword from the text(by /type keyword), monitor file in a real time(monitor log file at the time of being written) // sudo less +F var/log/syslog
cat file.txt \| sort sort -r file.txt	Sort file content alphabetically // will not modify file content Sort file in reverse oerder
sort file.txt > file.txt	Write output to the file
touch file.txt	Create a new file
nano file.txt	Create and open a new file
diff file1.txt file2.txt diff -c file1.txt file2.txt	View difference between file content C = change A = add D = delete -c = context // will give more output on the file content to review
dif ../shopping ../data/shopping	To perform directory comparison It performed file comparison in one directory to matching files in the second directory and generate report on differences
comm file1.txt file2.txt	Compare two sored files line by line Output will be in 3 colmuln
cmp file1.txt file2.txt	Compares 2 files, byte by byte, returns the position of first difference

Redirection operators

If we want to save the output of a command, or manipulate the output for immediate review, what should we do? Let's look at what redirection offers.

stdin (0) - standard input

stdout (1) - standard output

Stderr (2) - standard error

How data is entered or presented for processing Typically the keyboard or mouse, but could also be a file

The data returned from a command

Error messages that are returned, kept separate from stdout

pipe

Ex: cat file.txt | sort | head -10

Output of the cat will be sent to sort and that will forwarded to head // displays first 10 lines

> (create/overwrite)

Used to write command output to the file

will create a file if it doesn't exist, or will overwrite an existing file:

>> (create/append)

Used to append command output to a file

will create a file if it doesn't exist, or add output to an existing file

< (input)

Ex: less < file.txt

Used to direct the contents of a file to a command

Often used to send data to a script for processing but works with commands too:

Regular Expression - Regex

What: A regular expression, often referred to as regex or regexp, is a specific series of characters which are used to define a search pattern.

Why: A regex is most often used for "find all" or "find and replace" activity. It is also used when you only know part of a search string, or if you are using a wildcard search.

Regex basic

^	The start of a string or line
$	The end of a string or line
.	Wildcard which can match any character, except newline (\n)
\|	Matches a specific character or group of characters on either side (e.g. alb corresponds to a or b)
\	Used to escape a special character
t	The character "t"
az	The string "az"

Regex Examples

grep ‘^The’ file.txt

grep ‘^T[a-z]^e’ file.txt

grep ‘^T[a-z][^e]’ file.txt

grep ‘\<[tT]he\>’ file.txt

Print line that starts with “The”

Print line that starts with “T”, followed by a lower case from a to z, and does not end with “e”

Return every The in the file, doesn’t matter the letter(can be uppercase or lowercase)

grep -E -o "\b[A-Za-z0-9._%+-]+@[A-Za-zo-9.-]+\[A-Za-z]{2,6} \b" file.tx

print out email address in the file

Archive/Backup/Compress/Decompress

sudo tar cvf mybackup.tar DirPath sudo tar cvf mybackup.tar /home/work/toarhive	backup files; FilePath should be full path c=create v=verbose f=file This tar will have same amount of space those files contains.
sudo tar cvfz mybackup.tar.gz DirPath	Compress files z=zip This will be smaller in size
Tar tf archive.tar	Look through/List files in the archive.tar
Tar xvf archive.tar file3 Tar xvf archive.tar // extract everything	Extract specific file from the archive.tar

touch file.txt nano file.txt ls -lhF > file.txt	Create a file. Create and open file Create a file.
cp -v demo(file) folder(folderName) cp demo demo2 cp demo demo2 demo3 folder(folderName) cp -i demo folder(folderName) cp -iv demo folder cp -iv demo ..	Copy the file into the given directory/folder at last. -v stands for verbose, it prints copied file into mentioned folder on the command line Copy the the existing file(demo) and give a new name at last of copied file // rename and create a new file Copy multiple files inside folder name you provide at last If you copy an existing file inside a foldr, -i will ask you to overwrite the file. -i stands for interactive option Use multiple option together (overwrite & verbose) Copy file to previous directory
mv -v play/demo.txt .	Moves the file from play folder into current directory
mv demo NewName.txt	Rename file from demo to NewName.txt
rm -v demo rm -v -r folder rm -i demo	Remove file // remove means it is deleted forever, can’t find in the trash Remove folder Remove with interactive mode

Hard/Soft Links

Task: As an admin, how can you handle the need for a file to be accessible in two places at the same time? Let's look at hard and soft links as an option.

Hard Link

Direct pointer to file
Can only be a file
Shares the same inode as source
Must be on the same filesystem
As long as hard link exists, the data exists

Soft Link (Symbolic Link)

A redirect to file (think shortcut or alias)
Can be a file or directory
Has a unique inode
Can be on a different filesystem or mounted share
If the source file is deleted, the soft link is broken

ln info.txt infohardlink

Create a hard link

This will create a hardlink file(which is copy of the info.txt file)

ls -li = hardlink shares same inode value as source file

So if you update hardlink file, it will automatically update the source file

If you delete source file, you will still has HL file

ln -s info.txt infosoftlink

Create a soft link

File Permissions

Manage access to the root account

Task: The root account has access to manage and modify the entire system. To secure and protect the system, how to we restrict access to root?

Why not root?

The root account has the most access of any account on a Linux system.
Most modern Linux systems do not allow you to log in or change over to root.
root access is not necessary for most daily tasks.

How to avoid root

Always log in as yourself or another non-privileged account!

sudo - superuser do

Grants temporary elevated privileges to run a command

Substitute user
Changes to the root account until logout

How to manage sudo access

Access is granted via entries in the /etc/sudoers file
Access can be managed by account or by group

Taks

Add a new user into /etc/sudoers
Add a user to the existing group in the /etc/sudoers → sudo gpasswd -a username groupname

To open /etc/sudoers file use “sudo visudo”

Delete a user from group in /etc/sudoers → sudo gpasswd -d username groupname

sudo -i	Login as a root user
su UserName	Switch to different user

Tasks

Backup App via archive by Tar

What: Team wants to take a full backup of the custom application in the /opt/myapp/ directory. back will need to be compressed. We need to create a file called myapp.tar.gz in the /home/cloud_user/archive directory.

mkdir archive
cd archive
# Archive the /opt/myapp Directory as myapp.tar.gz
tar -czvf myapp.tar.gz /opt/myapp
# List the Contents of myapp.tar.gz and Redirect It to app.list
tar -tf myapp.tar.gz
# redirect the output to an app.list file:
tar -tf myapp.tar.gz > app.list

Add/Change execute permission to files and view/change attribute

Task: Correct the permissions and attributes of /opt/myapp/start.sh on the system, so the custom application located under /opt/myapp has the permission to execute. In order to prevent problems with this program, /opt/myapp/start.sh, we are also asked to set the attribute of the file, so that the file cannot be altered or removed accidentally.

# View the Current Permissions of /opt/myapp/start.sh
cd /opt/myapp
ls -l start.sh
stat start.sh
# Change Permissions on /opt/myapp/start.sh to Allow Full Privileges for User and Group Only
chmod u=rwx,g+wx,o-r start.sh
chmod 770 start.sh
# Verify That /opt/myapp/start.sh Is Executable
stat start.sh
./start.sh
# Make the /opt/myapp/start.sh Immutable Using a File Attribute
lsattr start.sh
sudo chattr +i start.sh
lsattr start.sh

Finding Files, and Assigning Permissions and Ownership

Task: We need to give any members of the DevOps group write access to /opt/myapp, so that they can work on the application. We also need to give the user cloud_user ownership of the directory. DevOps members and cloud_user need to be able to execute the application, start.sh as well. Finally, we need to make sure that anyone who is not cloud_user or in the DevOps group has absolutely no access to /opt/myapp whatsoever.

# Find Custom Application Files under /opt/myapp and Display a Detailed
find /opt
find /opt/myapp -ls
# Change the /opt/myapp Directory to be Owned by the cloud_user and the Group devop
find /opt/myapp -exec sudo chown cloud_user:devop {} \;
# Set Permissions for /opt/myapp Files
find /opt/myapp -name "d*" -ok chmod 660 {} \;
# Next we'll change permissions on anything that does not start with d (the directory itself and the start.sh script). We're going to also prompt ourselves to confirm each change with the -ok switch:
find /opt/myapp '!' -name "d*" -ok chmod 770 {} \;
# Find a Directory under /home Which Is Not Owned by a User or Group
find /home -nouser -nogroup -ls
# Execute the chown Command with the find Command
find /home -ls
find /home -nouser -a -nogroup -ls
find /home -nouser -nogroup -exec sudo chown cloud_user:cloud_user {} \;

Transfer Files Securely Over the Network

Tasks:

# Push copy of directory from server to server
scp -rp /opt/myapp root@server2:/opt
# Pull copy of directory from server to server
scp -rp root@server:/opt/myapp /opt
# Push copy of directory from server1 to server
rsync -aP /opt/myapi server2:/opt
# Pull copy of directory from server to server1
rsync -aP server2:/opt/myapi /opt

Operation of Running Systems

Boot, Reboot, and Shut Down a System Safely

What: A Linux machine requires maintenance for hardware and OS updates. How should you shut down or reboot the machine?

shutdown: This is the go-to command for shutting down, powering off, or rebooting a machine. It works on system 5 and system-based operating systems

Shutdown -r <TIME>: Reboots the server at the time specified.
shutdown -H <TIME>: Halts all CPU activities at the time specifier
Shutdown: -P <TIME>: Shuts down the OS and sends a command to power off the machine at the time specified

Reboot: It performs the same function as shutdown -r, but can also be used to halt or power off the server when the correct parameters are sent.
Halt: This performs the same function as shutdown -H. And can also be used to reboot or power off the server when the correct parameters are sent
Poweroff: It performs the same function as shutdown -P, and can also be used to reboot or halt the server when the correct parameters are sent

Linux Server Troubleshooting Guide

Methods

USE: Utilization - Saturation - Error
KAVBG: Knowledge - Ask Question - Visualize - Break it to replace it - Google

Commands

Load and CPU Usage

Load: uptime

Find the amount of load on the system

Load average should always be equated to the number of CPU cores. If the ratio of load average to the number of cores > 1, then the system is under load and requires attention.

CPU Usage: top/ps

provides load averages, CPUs, and their percentage utilization breakup in terms of usage with respect to user processes, system processes (executing kernel code), waiting for IO operations to complete, interrupt handling, and CPU time stolen from a virtual machine

Java Thread dumps: jstack

Thread dumps contain a great amount of information about the current state of the application.
These can be extremely useful in ascertaining a range of possible issues such as high CPU consumption, thread deadlocks, application slowdown, etc.

uptime	Know whether our system is overloaded to find the load average values for the last 1, 5, and 15 minutes
cat /proc/cpuinfo \| grep processor \| wc -l	To know the number of cores we can use /proc file system:
top	provides load averages, CPUs, and their percentage utilization
ps	To know the process ID of our Java process will list process IDs of those Java processes for which the tool has access permissions.
jstack	prints the stack traces of all threads that are attached to the JVM. These include application threads, VM internal threads, and optionally native stack frames. Using jstack, it is also possible to force (using –F flag) a thread dump of a hung process or when its output has been redirected such that the dumps are not available through kill –QUIT command

Scenarios

Task: To understand that our application is suffering from high CPU usage

Top: note whether CPU is constantly above 85-90% CPU utilization or not. we need to isolate the root cause(s).
Initiate a few thread dumps: In thread dumps, we are only interested in those threads which are keeping the CPU busy - threads which are in RUNNABLE state.

Once we have isolated those threads in thread dump, we can analyze their stack dumps and focus on those code paths.
how: top or ps

Inspect the threat dump status

we focus only on those whose state shows up as ‘R’
notice threads which consumes high CPU%

To, summarize, we took thread dumps and made a note of RUNNABLE threads. In parallel, we

also took output from ps command. We filtered out a great number of RUNNABLE threads from

thread dump for our analysis because we were able to find those threads which are consuming

maximum CPU.

Kernel Buffer Commands

free -m

Check RAM

top

See running processes

dmesg

dmesg | less

dmesg -T

Check kernel message if it’s printing any logs

Virtual Memory Commands

vmstate

How many resources are in queue, buffer size, swap, free memory and more

PID Status Commands

ps	How many resources are in queue, buffer size, swap, free memory and more