mohanpedala/Linux_random.md

## Linux_random.md

      
    Raw
  

              Linux_random.md
            
          
    Getting Started


vi editor cheat sheet
netstat
netcat
View Kernel IP routing table
tcpdump
tcpping
Creating users, group and assigning users to groups
Assign Sudo Permissions
User Accounts
Special Variable Types in advanced bash scripting
Viewing logs with dmesg
Cron job
SCP
PATH
Symlink
Block IP iptables command
CURL
Identify Linux flavour
LSOF
DNS Cahce
MAC DNS Cache Flush
Daily usage
network troubleshooting
Change Source repo list
umask
Disk IO check
Network
Process Priority using Nice
How do I measure separate CPU core usage for a process
YAML-basics

vi editor cheat sheet


Ref

netstat


Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships


You can use 1-65,535 ports on your system in which first 1024 are root privileged.


So Instead of finding the free port, you can get list of used ports using below command
netstat -tunlep | grep LISTEN | awk '{print $4}'


Then you can use any port from 1-65535 except those ports.


Display the kernel routing tables, show numerical addresses instead of trying to determine symbolic host, port or user names.


netstat -rn

Explanation: The -r option specifies that you want the routing table. The -n option is similar to that of the route command.

To we can also view by using

route -n

Explanation: The -n option means that you want numerical IP addresses displayed, instead of the corresponding host names.
Note: You need to be root to execute route.

You can also use the below command to see the ports used

List out all connections - netstat -a

List only TCP or UDP connections - netstat -at (t = tcp) / netstat -au (u = udp)

Disable reverse dns lookup for faster output - netstat -ant

List out only listening connections - netstat -tnl

Get process name/pid - sudo netstat -nlpt

Get process username/uid - sudo netstat -ltpe

Print statistics - netstat -s

Display kernel routing information - netstat -rn

Print network interfaces - netstat -i

Print network interfaces in human friendly format - netstat -ie

Get netstat output continuously - netstat -ct

Display multicast group information - netstat -g

Print active connections - netstat -atnp | grep ESTA

To watch a continous list of active connections - watch -d -n0 "netstat -atnp | grep ESTA"

Check if a service is running - sudo netstat -aple | grep ntp


Show NAT iptable rules:
$ iptables -t nat -L -n -v


Show nat table: netstat-nat command display the natted connections on a Linux iptable firewall:
$ netstat-nat -n


Show source network address translation(SNAT):
$ netstat-nat -S

Fuser:

The fuser command is a very smart unix utility used to find which process is using a file, a directory or a socket. It also gives information about the user owning the process and the type of access. The fuser tool displays the process id(PID) of every process using the specified files or file systems.
# Check Processes Using TCP/UDP Sockets

$ fuser 80/tcp

# Kill Processes that are Using a particular Program

$ fuser -k Port_Number/tcp 

or

$ kill -9 $(fuser 80/tcp 2>/dev/null)

or

$ kill -s TERM <PID>


NetCat (nc):


Test if a port on a remote system is reachable (without telnet)
 Single port:

 nc -zv 127.0.0.1 80
 
 Multiple ports:

 nc -zv 127.0.0.1 22 80 8080
 
 Range of ports:

 nc -zv 127.0.0.1 20-30


Test if a port on a remote system is reachable with bash:

(echo >/dev/tcp/<hostname>/<port>) &>/dev/null && echo "Open <port>" || echo "Close <port>"


Test if a port on a remote system is reachable with telnet:
telnet <hostname> <port>


tcpdump


Capture packets from a particular ethernet interface using tcpdump -i
tcpdump -i eth0
Capture only N number of packets using tcpdump -c
tcpdump -c 2 -i eth0
Display Captured Packets in ASCII using tcpdump -A
tcpdump -A -i eth0
Display Captured Packets in HEX and ASCII using tcpdump -XX
tcpdump -XX -i eth0
Capture the packets and write into a file using tcpdump -w
tcpdump -w 08232010.pcap -i eth0
Reading the packets from a saved file using tcpdump -r
tcpdump -tttt -r data.pcap
Capture packets with IP address using tcpdump -n
tcpdump -n -i eth0
Capture packets with proper readable timestamp using tcpdump -tttt
tcpdump -n -tttt -i eth0
Read packets longer than N bytes
tcpdump -w g_1024.pcap greater 1024
Receive only the packets of a specific protocol type
tcpdump -i eth0 arp
Read packets lesser than N bytes
tcpdump -w l_1024.pcap  less 1024
Capture packets for particular destination IP and Port
tcpdump -w xpackets.pcap -i eth0 dst 10.181.140.216 and port 22
Capture packets for particular destination IP and Port
tcpdump -w xpackets.pcap -i eth0 dst 10.181.140.216 and port 22
Capture TCP communication packets between two hosts
tcpdump -w comm.pcap -i eth0 dst 16.181.170.246 and port 22
tcpdump Filter Packets – Capture all the packets other than arp and rarp
tcpdump -i eth0 not arp and not rarp
tcpdump source to destination packet tracing

tcpdump '(src x.x.x.x and dst x.x.x.x)

Example:

tcpdump '(src 10.0.0.1 and dst 172.32.0.2) or (src 10.0.10.183 and dst 10.0.0.1) or (src 10.0.0.1 and dst 10.0.10.183) or (src 172.32.0.2 and dst 10.0.0.1)'

tcpping


To install tcptraceroute on Debian/Ubuntu:
apt-get install tcptraceroute


To install tcptraceroute on CentOS/REHL, first set up RepoForge on your system, and then:
yum install tcptraceroute


Download tcpping from the web.
cd /usr/bin
wget http://www.vdberg.org/~richard/tcpping
chmod 755 tcpping


To measure network latency by using tcpping, simply run it as follows.
tcpping <fqdn>/<ip> <portnumber>

example:
tcpping example.com 443


Copy file to clip board

cat filename > /dev/clipboard


Paste content from clip board to file
cat /dev/clipboard > file 


List of users

cat /etc/passwd
more /etc/passwd
less /etc/passwd
awk -F':' '{ print $1}' /etc/passwd

Output of a word in a line
awk '/hello/{print $2}' 
Creating users groups and assigning users to groups


Creating user and assigning password

useradd username
passwd


Create a User with Different Home Directory
useradd -d /apps/test test


Change password of a different user (where user is the username for the password you are changing)
passwd <username>


Creating a group

groupadd group_name
groupadd -g 60002 schoolmates


Attaching users to group

gpasswd -a username which_group_to_edit
gpasswd --members username_1,username_2 which_group_to_edit


Add exisisting user to group
sudo usermod -a -G group user_name


Make a group as the primary group
# Check which group is primary
sudo id <username>

# Change group
sudo usermod -g <group> <username>

# verify if the group is primary
sudo id <username>


Assign Sudo Permissions

a. Open visudo
b. Add user under below line
### Allow root to run any commands anywhere
root    ALL=(ALL)       ALL
user	ALL=(ALL)	NOPASSWD: ALL

Output:
[test@instance-1 ~]$ sudo ls
[test@instance-1 ~]$ 

$ visudo

## Allows people in group wheel to run all commands
# %wheel        ALL=(ALL)       ALL
</pre></code>

Enable wheel by uncommenting it
<pre><code>
usermod -aG wheel USERNAME
sudo whoami

Disable tty

a. Open visudo or /etc/sudoers
b. Add the line
Defaults	!requiretty

# Disable "ssh hostname sudo <cmd>", because it will show the password in clear.
#         You have to run "ssh -t hostname sudo <cmd>".
#
#Defaults    requiretty

Defaults    !requiretty

To provide sudo access to groups

1. Add users to /etc/security/access.conf
2. Create a file for group in /etc/sudoers.d/ -- > example_group
3. Add the below content to example_group file
    ```
    # User rules for

    +example_group_users ALL=(ALL)       PASSWD: ALL
    +service_account_group  ALL=(ALL)   NOPASSWD:ALL
    ```

User Accounts

User account lock, unlock, password disable check, check expiry date of account and user shell is set to a valid shell or not


Lock a user to login in to system
sudo passwd -l username

sudo grep nagios /etc/shadow

o/p: nagios:!!:17626:0:99999:7:::


Check if the password is disabled by viewing /etc/shadow file for user entry.
grep ‘username’ /etc/shadow


note: if you are able to see ! in the second field starting that indicates that password is disabled, you have to enable it back by 	using passwd with -u option


Unlock account
passwd -u username

Unlocking password for user temp.

passwd: Success


Delete password
passwd -d username

  Example:

  passwd -d temp

  Removing password for user temp.

  passwd: Success


Once you are done with this change the password field in /etc/shadow field is changed to empty so that without password you can login from any machine to the local machine.


Check if the user expiry date is reached or not by using chage command
chage -l username

Example

chage -l username
Last password change : Jan 05, 2012
Password expires : never
Password inactive : never
Account expires : Jan 01, 2012
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


To set the password to never expire.
chage <username>


Check if the user shell is set to a valid shell or not, if it’s not set it to a valid one.
grep ‘username’ /etc/passwd

Example:

grep ‘surendra’ /etc/passwd

If the user shell in seventh feild is set to /sbin/nologin or /bin/false set it back to /bin/bash or /bin/ksh

usermod -s /bin/bash usrename

usermod -s /bin/bash surendra


Disable automatic password change after x-no.of days
chage -I -1 -m 0 -M 99999 -E -1 <username>


Special Variable Types in advanced bash scripting


Special Parameters
Description


$#
Stores the number of command-line arguments that were passed to the shell program.


$?
Stores the exit value of the last command that was executed.


$0
Stores the first word of the entered command (the name of the shell program).


$*
Stores all the arguments that were entered on the command line ($1 $2 ...).


"$@"
Stores all the arguments that were entered on the command line, individually quoted ("$1" "$2" ...).


Examples

$# = 3
$* = -yes -no /home/username
$@ = array: {"-yes", "-no", "/home/username"}
$0 = ./command, $1 = -yes etc.

Bash scripting operations


Operator
Description


! EXPRESSION
The EXPRESSION is false.


-n STRING
The length of STRING is greater than zero.


-z STRING
The lengh of STRING is zero (ie it is empty).


STRING1 = STRING2
STRING1 is equal to STRING2


STRING1 != STRING2
STRING1 is not equal to STRING2


INTEGER1 -eq INTEGER2
INTEGER1 is numerically equal to INTEGER2


INTEGER1 -gt INTEGER2
INTEGER1 is numerically greater than INTEGER2


INTEGER1 -lt INTEGER2
INTEGER1 is numerically less than INTEGER2


-d
FILE exists and is a directory.


-e
FILE exists.


-r
FILE exists and the read permission is granted.


-s
FILE exists and it's size is greater than zero (ie. it is not empty).


-w
FILE exists and the write permission is granted.


-x
FILE exists and the execute permission is granted.


-f
FILE exists.


Ex:
while IFS='' read -r something_path || [[ -n "$something_path" ]]; do
something_full_name=$(basename ${something_path})

	if [ -f ${DESTINATION_PATH}/${something_full_name} ];
		blah
		blah
	fi

done < "something_list.txt"


IFS takes the list from something_list.txt read the file until the whitespace and store it in something_path and check if the file is empty or not.
If the file has the content it moves to the if condition
if condition checks whether it is a file or not and then excutes the condition

Viewing logs with dmesg

The dmesg command prints the kernel ring buffer. By default, the command will display all messages from the kernel ring buffer.
Cron job


Ref

Scheduling explained

As you can see there are 5 stars. The stars represent different date parts in the following order:

minute (from 0 to 59)
hour (from 0 to 23)
day of month (from 1 to 31)
month (from 1 to 12)
day of week (from 0 to 6) (0=Sunday)
Execute every minute

If you leave the star, or asterisk, it means every. Maybe that's a bit unclear. Let's use the the previous example again:
* * * * * /bin/execute/this/script.sh
Special words


For the first (minute) field, you can also put in a keyword instead of a number:
@reboot     Run once, at startup
@yearly     Run once  a year     "0 0 1 1 *"
@annually   (same as  @yearly)
@monthly    Run once  a month    "0 0 1 * *"
@weekly     Run once  a week     "0 0 * * 0"
@daily      Run once  a day      "0 0 * * *"
@midnight   (same as  @daily)
@hourly     Run once  an hour    "0 * * * *"


Leaving the rest of the fields empty, this would be valid:
@daily /bin/execute/this/script.sh


SCP


Ref
Copy the file "foobar.txt" from a remote host to the local host
$ scp your_username@remotehost.edu:foobar.txt /some/local/directory


Copy the file "foobar.txt" from the local host to a remote host
$ scp foobar.txt your_username@remotehost.edu:/some/remote/directory


Copy the directory "foo" from the local host to a remote host's directory "bar"
$ scp -r foo your_username@remotehost.edu:/some/remote/directory/bar


Copy the file "foobar.txt" from remote host "rh1.edu" to remote host "rh2.edu"
$ scp your_username@rh1.edu:/some/remote/directory/foobar.txt \
your_username@rh2.edu:/some/remote/directory/


Copying the files "foo.txt" and "bar.txt" from the local host to your home directory on the remote host
$ scp foo.txt bar.txt your_username@remotehost.edu:~


Copy the file "foobar.txt" from the local host to a remote host using port 2264
$ scp -P 2264 foobar.txt your_username@remotehost.edu:/some/remote/directory


Copy multiple files from the remote host to your current directory on the local host
$ scp your_username@remotehost.edu:/some/remote/directory/\{a,b,c\} .
$ scp your_username@remotehost.edu:~/\{foo.txt,bar.txt\} .


scp Performance: By default scp uses the Triple-DES cipher to encrypt the data being sent. Using the Blowfish cipher has been shown to increase speed. This can be done by using option -c blowfish in the command line.
$ scp -c blowfish some_file your_username@remotehost.edu:~


It is often suggested that the -C option for compression should also be used to increase speed. The effect of compression, however, will only significantly increase speed if your connection is very slow. Otherwise it may just be adding extra burden to the CPU. An example of using blowfish and compression:
$ scp -c blowfish -C local_file your_username@remotehost.edu:~


PATH


Appending something to PATH, instead of overriding it
PATH="$PATH:<path>"


You may set $PATH permanently in 2 ways.
To set path for particular user : You may need to make the entry in .bash_profile in home directory in the user.

Example in my case I will set java path in tomcat user profile

[tomcat]$ echo "export PATH=$PATH:/path/to/dir" >> /home/tomcat/.bash_profile


To set common path for ALL system users, you may need to set path like this :
$ echo "export PATH=$PATH:/path/to/dir" >> /etc/profile


Symlink


Create symlink
ln -s /path/to/file /destination/


Delete symlink
unlink mySymLink


Block IP iptables command


To block an IP
# iptables -A INPUT -s 65.55.44.100 -j DROP
# service iptables save

-A or --append

-s, --source address

-j, --jump target

-p, --protocol protocol

-D, --delete chain rule-specification

-D, --delete chain rulenum


Block an IP with port number
iptables -A INPUT -s 65.55.44.100 -p tcp --destination-port 443 -j DROP
service iptables save


Unblock an IP
# iptables -D INPUT -s 65.55.44.100 -p tcp --destination-port 443 -j DROP
# service iptables save


CURL


Insecure way to pull
curl -k www.google.com


Secured way with certificate
curl --cacert localmachine.pem:<if_there_is_a_pass> https://www.google.com


Secure way with trace-time
curl -v --trace-time localmachine.pem:<if_there_is_a_pass> https://www.google.com


Secured way with total time take for the request


Create a file curl.txt

    time_namelookup:  %{time_namelookup}\n
       time_connect:  %{time_connect}\n
    time_appconnect:  %{time_appconnect}\n
   time_pretransfer:  %{time_pretransfer}\n
      time_redirect:  %{time_redirect}\n
 time_starttransfer:  %{time_starttransfer}\n
                    ----------\n
         time_total:  %{time_total}\n


Make a request

curl -w "@curl.txt" -o /dev/null -s "http://wordpress.com/"

Or on Windows, it's...

curl -w "@curl.txt" -o NUL -s "http://wordpress.com/"


Secured way with cert

curl -w "@curl.txt" --cacert localmachine.pem https://www.google.com"


To see Access-Control-Allow-Origin header

curl -s -D - -H "Origin: http://example.com" https://api.example.com/my-endpoint -o /dev/null


To test Preflighted requests, add -X option

curl -s -D - -H "Origin: http://example.com" -X OPTIONS https://api.example.com/my-endpoint -o /dev/null


Options:

-s or --silent: Silent or quiet mode. Don't show progress meter or error messages.
-D: --dump-header <filename>
(HTTP  FTP) Write the received protocol headers to the specified file. This option is handy to use when you want to store  the  headers that  an  HTTP site sends to you. Cookies from the headers could then be read in a  second  curl  invocation  by  using  the -b,- -cookie  option! The -c, --cookie-jar option is a better way to store cookies.
-o or --output:
-H or --header: Extra header to include in the request when sending HTTP to a server. You may specify any number of extra  headers.
-X or --request: Specifies a custom request method to use when communicating with the HTTP server. The specified request method will  be used  instead  of  the  method otherwise used (which defaults to GET).


Identify Linux flavour

cat /proc/version


Use lsb_release command To find out Linux distribution name and version - LSB (Linux Standard Base)
lsb_release -a


Once you know that you are running Red Hat for example, you can get to the point with:
cat /etc/redhat-release

Debian:

cat /etc/debian_version

General :

cat /etc/*-release


Also you could use the following command
cat /etc/issue


LSOF


LSOF: List Open Files
# lsof bash: lsof: command not found
# yum install lsof


If you want to find out which process is using particular file you can use below command.
lsof /var/log/messages


Open Files Under Particular Directory – lsof command
# lsof +D /etc


specify particular command/process name to find associated open files using lsof command -c
lsof -c systemd


List All Processes using particular Mount Point
# lsof /home
# lsof +D /home/


See all Opened files and Processes of User
# lsof -u user


Exclude Particular User Opened files and PIDs
lsof -u ^user


Repeat lsof command multiple times
# lsof -u user -c sshd -a -r10


List All IPv4 and IPv6 Network Files
# lsof -i4 -- List All IPv4
# lsof -i6 -- List All IPv6 Network open files


Find which program is using the particular port  number, this option is mostly useful if you would like to find out what are the programs accessing port number
# lsof -i TCP:80 -a -i UDP:80


List Connections to a specific host
# lsof -i@IP-Address


In real time i use below commands to kill and clear disk space. Below commands can save your life when your running out of disk space. Example is “/”  Slash partition is full and i could not able to trace what are the files are using space. Using find command you can find under “/” but those files are deleted but there PID are still in process queue. Without restarting services or server you can simply clear all deleted .TMP files.
# lsof |grep deleted   ##To check deleted Temp Files
# lsof |grep deleted |grep ".TMP" | awk '{ print $2 }' | xargs kill -9 ##Clear Temp Files by killing PID's
# lsof |grep deleted |grep ".TMP" |awk '{print $9}' | xargs rm -f ##To delete Temp Files


lsof port check
lsof -i -P -n | grep LISTEN


lsof command options below
-a AND selections
-b avoid kernel blocks
-c c cmd c ^c /c/[bix]
+c w COMMAND width (9)
+d s dir s files  -d s select by FD set  +D D dir D tree
-i Select IPv[46] files
-K List tasks
-l List UID numbers
-n No host names
-N Select NFS files
-o List file offset
-O No overhead *RISKY*
-P No port names
-R list parent PID
-s List file size
-t Terse listing
-T Disable TCP/TPI info
-U Select Unix socket
-v List version info
-V Verbose search
+|-w Warnings
-X Skip TCP&UDP* files
-Z SELinux Context
+f|-f +filesystem or -file names
-F [f] select fields;
+|-M Port-map registration
-p s Exclude(^)|selected PIDs
-S [t] t second stat timeout (15)
-s p:s exclude(^)|select protocol (p = TCP|UDP) states by name(s).
-u s exclude(^)|select login|UID set s


list open files without lsof with pid
ls -l /proc/<pid>/fd |wc -l


How to get the pid
ps -ef

# or

netstat -tnlp


list open files with total count

lsof -p <pid> | wc -l


DNS Cahce

Distro: Ubuntu


Ubuntu doesn't cache dns records by default so unless you've installed a dns cache there isn't anything to clear. It you want Ubuntu to start caching dns I recommend installing pdnsd together with resolvconf .
Flushing DNS on Ubuntu 18.04 LTS

$ systemd-resolve --statistics
$ systemd-resolve --flush-caches


If the above doesnot work check if systemd has nscd.
locate systemd | grep nscd


If nscd present run the below command.
$ systemctl restart nscd


MAC DNS Cache Flush

$ sudo dscacheutil -flushcache;sudo killall -HUP mDNSResponder;say cache flushed


Daily usage


Start copying file to a different backup folder by appending the file name with bamboo previous release number and the date the file is moved. Remove command will only maintain recent 10 files in the backup folder.
cp -pr nohup.out /backups/nohup-${bamboo.deploy.release.previous}-$(date +%F-%H:%M).out
rm -rf $(ls -1t /backups | tail -n +11)


Displaying current day Files Only
ls -al --time-style=+%D | grep $(date +%D)


If domainname doesnot match between nslookup and domainname command.

Update the sysctl.conf file

vi /etc/sysctl.conf file


Forwarding a particular standard output to a standard input using grep
nginx -V 2>&1 | tr -- - '\n' | grep _module

Output:
http_addition_module 
http_auth_request_module 
http_dav_module 
http_flv_module 
http_gunzip_module 
http_gzip_static_module 
http_mp4_module 
http_random_index_module 
http_realip_module 
http_secure_link_module 
http_slice_module 
http_ssl_module 
http_stub_status_module 
http_sub_module 
http_v2_module 
mail_ssl_module 
stream_realip_module 
stream_ssl_module 
stream_ssl_preread_module 


Change Source repo list

Ubuntu


Navigate to /etc/apt look for /etc/apt/sources.list or incase of docker look for /etc/apt/sources.list.d directory

umask


Octal value
Permission


0
read, write and execute


1
read and write


2
read and execute


3
read only


4
write and execute


5
write only


6
execute only


7
no permissions


if umask is set to 077, the permission can be calculated as follows:
Bit	Targeted at	File permission
0	Owner	read, write and execute
7	Group	No permissions
7	Others	No permissions


TASK: CALCULATING THE FINAL PERMISSION FOR FILES: You can simply subtract the umask from the base permissions to determine the final permission for file as follows:
666 – 022 = 644


File base permissions : 666
umask value : 022
subtract to get permissions of new file (666-022) : 644 (rw-r–r–)


TASK: CALCULATING THE FINAL PERMISSION FOR DIRECTORIES: You can simply subtract the umask from the base permissions to determine the final permission for directory as follows:
777 – 022 = 755


Directory base permissions : 777
umask value : 022
Subtract to get permissions of new directory (777-022) : 755 (rwxr-xr-x)


Limitations of the umask

The umask command can restricts permissions.
The umask command cannot grant extra permissions beyond what is specified by the program that creates the file or directory. If you need to make permission changes to existing file use the chmod command.


umask and level of security: The umask command be used for setting different security levels as follows:


umask
value
Security level	Effective permission (directory)


022
Permissive
755


026
Moderate
751


027
Moderate
750


077
Severe
700


Disk IO check


IO stat for a mount
iostat /dev/mapper/appvg-apps_lv
# output
Linux 2.6.32-754.25.1.el6.x86_64 (VA10PLVPOS322.wellpoint.com)  02/17/2020      _x86_64_        (8 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
        2.18    0.02    0.53    0.09    0.00   97.18

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dm-2              6.26        15.48        35.37   10441066   23850272

Mon Feb 17 11:47:12 EST 2020


dd command taking the usage using a single file (It is used to monitor the writing performance of a disk device on a Linux and Unix-like system) Reference
dd if=/dev/zero of=diskbench bs=1M count=1024 oflag=dsync
# output
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 6.5731 s, 163 MB/s
Explanation


if=/dev/zero (if=/dev/input.file) : The name of the input file you want dd the read from.
of=/tmp/test1.img (of=/path/to/output.file) : The name of the output file you want dd write the input.file to.
bs=1G (bs=block-size) : Set the size of the block you want dd to use. 1 gigabyte was written for the test. Please note that Linux will need 1GB of free space in RAM. - If your test system does not have sufficient RAM available, use a smaller parameter for bs (such as 128MB or 64MB and so on).
count=1 (count=number-of-blocks): The number of blocks you want dd to read.
oflag=dsync (oflag=dsync) : Use synchronized I/O for data. Do not skip this option. This option get rid of caching and gives you good and accurate results
conv=fdatasyn: Again, this tells dd to require a complete “sync” once, right before it exits. This option is equivalent to oflag=dsync.


Network:


Check the network usage based on ethernet interface
cat /proc/net/dev


Process Priority using Nice


Nice command
https://www.tecmint.com/set-linux-process-priority-using-nice-and-renice-commands/

How do I measure separate CPU core usage for a process

ps -p <PID> -L -o pid,tid,psr,pcpu

# PSR is CPU id the thread is currently assigned to.

#Output
   	PID    TID PSR %CPU
 118459 118459   0  0.1
 118459 118465   3  0.0
 118459 118466   0  0.0
 118459 118467   1  0.0
 118459 118556   0  0.0
 118459 118563   0  0.0
 118459 118579   0  0.5
 118459 118606   0  0.5
 118459 118608   2  0.5
 118459 118609   1  0.5
Special Parameters	Description
$#	Stores the number of command-line arguments that were passed to the shell program.
$?	Stores the exit value of the last command that was executed.
$0	Stores the first word of the entered command (the name of the shell program).
$*	Stores all the arguments that were entered on the command line ($1 $2 ...).
"$@"	Stores all the arguments that were entered on the command line, individually quoted ("$1" "$2" ...).
Operator	Description
! EXPRESSION	The EXPRESSION is false.
-n STRING	The length of STRING is greater than zero.
-z STRING	The lengh of STRING is zero (ie it is empty).
STRING1 = STRING2	STRING1 is equal to STRING2
STRING1 != STRING2	STRING1 is not equal to STRING2
INTEGER1 -eq INTEGER2	INTEGER1 is numerically equal to INTEGER2
INTEGER1 -gt INTEGER2	INTEGER1 is numerically greater than INTEGER2
INTEGER1 -lt INTEGER2	INTEGER1 is numerically less than INTEGER2
-d	FILE exists and is a directory.
-e	FILE exists.
-r	FILE exists and the read permission is granted.
-s	FILE exists and it's size is greater than zero (ie. it is not empty).
-w	FILE exists and the write permission is granted.
-x	FILE exists and the execute permission is granted.
-f	FILE exists.
Octal value	Permission
0	read, write and execute
1	read and write
2	read and execute
3	read only
4	write and execute
5	write only
6	execute only
7	no permissions