Showing posts with label COMMANDS. Show all posts
Showing posts with label COMMANDS. Show all posts

Saturday, May 18, 2024

Using mdadm to Manage RAID and Multipath Storage on Linux: A Practical Guide with Examples

The mdadm command is a powerful tool for managing multiple device sets on Linux systems. It plays a crucial role in creating and maintaining RAID arrays, which provide redundancy and performance benefits, and multipath setups, which ensure data availability in case of hardware failure. Let's delve into how you can use mdadm to harness these powerful storage features, complete with practical examples.

Creating RAID Devices with mdadm

1. Define Your Configuration:

The /etc/mdadm.conf file is where you specify the devices and RAID level for your array.

Example: RAID 1 (Mirroring)

DEVICE /dev/sd[b,c]1  
ARRAY /dev/md0 level=raid1 raid-devices=2 /dev/sdb1 /dev/sdc1

This configuration creates a RAID 1 array (/dev/md0) that mirrors data across two devices (/dev/sdb1 and /dev/sdc1).

Example: RAID 5 (Striping with Parity)

DEVICE /dev/sd[b-d]1
ARRAY /dev/md0 level=raid5 raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1

This configuration creates a RAID 5 array (/dev/md0) that stripes data across three devices with parity information for fault tolerance.

2. Create the RAID Array:

Use mdadm with the -C (create) option and the details from your configuration:

# RAID 1 example sudo mdadm -C /dev/md0 --level=raid1 --raid-devices=2 /dev/sdb1 /dev/sdc1 # RAID 5 example sudo mdadm -C /dev/md0 --level=raid5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1

3. Verify RAID Status:

Check the status of your newly created RAID array:

sudo mdadm --detail /dev/md0

You should see information about the RAID level, state (active, syncing, etc.), device status, and more.

Creating Multipath Devices with mdadm

Multipathing provides an additional layer of reliability by creating multiple paths to access a storage device.

sudo mdadm -C /dev/md1 --level=multipath --raid-devices=2 /dev/mapper/mpatha /dev/mapper/mpathb

This command creates a multipath device (/dev/md1) using two paths (/dev/mapper/mpatha and /dev/mapper/mpathb) that likely correspond to different physical disks.

Key Considerations

  • Choose the Right RAID Level:
    • RAID 0: Best for performance but no redundancy.
    • RAID 1: Offers redundancy with mirroring.
    • RAID 5: Good balance of performance and redundancy.
    • RAID 6: More redundancy than RAID 5 but slightly slower.
    • RAID 10: Combines mirroring and striping for both performance and redundancy.
  • Data Backup: RAID is not a backup solution; always maintain regular backups.
  • Hardware Compatibility: Ensure your hardware (controllers, disks) supports your chosen RAID level.


mdadm empowers you to create robust and fault-tolerant storage solutions on Linux. By mastering its capabilities, you can optimize your server's performance and protect your valuable data.

Let me know if you'd like more in-depth examples or have any specific scenarios you'd like to explore!

Google Apps Wizard cPanel plugin

This tool was developed for web hosting companies, webmasters and website developers that have to setup domains with Google Apps in the everyday work. The purpose of Google Apps Wizard cPanel plugin is to facilitate the steps needed to work with Google Apps services. cPanel end user doesn’t need to have any prior knowledge of DNS records management while working with this tool.


Run the following shell commands as root via SSH:

# wget
# tar -xf gaw-2.0.tar
# cd gaw-2.0
# ./
# cd ..
# rm -Rfv gaw-2.0/ gaw-2.0.tar

Go to WHM >> Main >> Plugins >> Google Apps Wizard and click Update Now.

If you running version 2+ please skip this part.

# wget
# tar -xf gaw-2.0.tar
# cd gaw-2.0
# ./
# cd ..
# rm -Rfv gaw-2.0/ gaw-2.0.tar


# wget
# tar -xf gaw-2.0.tar
# cd gaw-2.0
# ./
# cd ..
# rm -Rfv gaw-2.0/ gaw-2.0.tar

Multi-languages support

The language files folder is located at: /usr/local/cpanel/base/3rdparty/GoogleAppsWizard/lang/
The en.php is the default fallback language file so do not delete it!
To create new language files you must use an existing cPanel prefix.
You can see your available languages by navigating to: Main >> Locales >> View Available Locales.

Templates support

The Google Apps Wizard plugin support third party cPanel templates.
All you need to do is to copy the default template to your third party template folder.
Template path: /usr/local/cpanel/base/frontend/x3/gaw
Example command: # cp /usr/local/cpanel/base/frontend/x3/gaw /usr/local/cpanel/base/frontend/{Your custom template folder name}/

Lynis: Elevate Your Server Security with a Powerful Auditing Tool

In the ever-evolving landscape of cybersecurity, proactive security measures are paramount. One tool that can significantly bolster your server's defenses is Lynis, a comprehensive auditing and hardening tool designed to uncover vulnerabilities and security issues.

What is Lynis?

Lynis is an open-source security auditing tool that meticulously scans your server, assessing its configuration, software components, and potential weaknesses. It provides valuable insights into your system's overall security posture, enabling you to take proactive steps to harden it against potential threats.

Why Choose Lynis?

  • Comprehensive Scanning: Lynis analyzes a wide range of aspects, including operating system settings, network configuration, installed software, user accounts, file permissions, and much more.
  • Customizable Tests: You can tailor Lynis to focus on specific areas of concern, ensuring it aligns with your unique security requirements.
  • Detailed Reports: The tool generates detailed reports highlighting potential vulnerabilities, configuration issues, and recommendations for remediation.
  • Easy to Use: Lynis is designed to be user-friendly, even for those without deep security expertise.

Installing Lynis

  1. Create a Directory: Use the following command to create a directory where you'll store Lynis:

    mkdir /usr/local/lynis
  2. Download Lynis: Navigate to the new directory and download the latest stable version:

    cd /usr/local/lynis
  3. Extract the Files: Unpack the downloaded archive:

    tar -xvf lynis-1.3.0.tar.gz

Running and Using Lynis

  1. Become Root: You'll need root privileges to run Lynis because it accesses system-level information and writes logs.

  2. Run Lynis: Navigate to the Lynis directory and execute the script:

    cd lynis-1.3.0

Lynis will begin its comprehensive scan, analyzing your server's configuration and security settings. The process may take a while, depending on the size and complexity of your system.

Reviewing the Report

Once the scan completes, Lynis will generate a detailed report. Typically, you'll find it in /var/log/lynis.log. This report is a goldmine of information, including:

  • Warnings: Potential vulnerabilities or misconfigurations that need your attention.
  • Suggestions: Recommendations for hardening your system based on Lynis' findings.
  • Details: In-depth explanations of each issue and why it matters.

Take the time to carefully review the report, prioritize the identified issues, and implement the suggested fixes.

Regular Audits

Remember, security is an ongoing process. Schedule regular Lynis scans to keep your server's security posture up-to-date and address any new vulnerabilities that may arise.

Lynis is an indispensable tool in your arsenal for maintaining a secure and resilient server environment. By proactively identifying and addressing vulnerabilities, you'll be well-equipped to protect your data and defend against potential threats.

Thursday, May 9, 2024

Resolving SAR Error: "Cannot open /var/log/sa/sa08"

System Activity Reporter (SAR) is a powerful tool for monitoring system performance, but encountering errors can be frustrating. One common issue users face after installing SAR is the error message "Cannot open /var/log/sa/sa08: No such file or directory" when attempting to run the sar -q command. In this guide, we'll explore why this error occurs and provide step-by-step instructions to resolve it.

Understanding the Error: When executing sar -q, the system is unable to locate the specified SAR data file sa08. This file should be located in the directory /var/log/sa/. The absence of this file indicates that SAR has not been collecting data properly or has encountered an issue during data collection.

Troubleshooting Steps: Follow these steps to troubleshoot and resolve the SAR error:

Check SAR Installation: Ensure that SAR is installed correctly on your system. If not, install it using your package manager.

Verify SAR Data Collection: Confirm whether SAR is actively collecting system activity data. SAR typically collects data at regular intervals and stores it in the /var/log/sa/ directory. Use the command sar -q to check if the data file sa08 exists.

Check Cron Service: SAR relies on the cron service to schedule data collection. Check if the cron service is running by executing

/etc/init.d/crond status
If the service is not running, restart it using

/etc/init.d/crond restart
Restart syslog Service: SAR also depends on the syslog service for logging. Restart the syslog service to ensure proper functioning

/etc/init.d/syslog restart
Verify Data Collection Intervals: SAR collects data at regular intervals defined by cron jobs. Review the cron configuration to ensure that SAR cron jobs are configured correctly and running as expected.

Check File Permissions: Ensure that the /var/log/sa/ directory and SAR data files have appropriate permissions for SAR to read and write data. Correct any permission issues if found.

Conclusion: By following these troubleshooting steps, you can resolve the SAR error "Cannot open /var/log/sa/sa08: No such file or directory" and ensure that SAR functions properly for system performance monitoring. Regular monitoring with SAR is essential for identifying performance bottlenecks and optimizing system resources effectively.

Sunday, May 5, 2024

Resetting the Root Password in Rescue Mode: A Step-by-Step Guide

In critical situations where access to your server's root account is lost or compromised, resetting the root password becomes essential for restoring control over your system. This guide provides a comprehensive walkthrough of resetting the root password in rescue mode, ensuring you regain access to your server without data loss or downtime.

Losing access to the root account on a server can be a daunting scenario, but with the right approach, it's possible to regain control swiftly and securely. In this guide, we'll walk you through the step-by-step process of resetting the root password using rescue mode, a powerful tool that allows you to access and modify your server's filesystem even when it's unable to boot normally.

Step 1: Logging in to Rescue Mode Firstly, access your server through SSH console in rescue mode. This specialized mode provides a safe environment for performing critical operations on your server's filesystem.

Step 2: Identifying the Disk Partition Once logged in, use the fdisk -l command to list all disk partitions and identify the correct one where your root filesystem is located. Typically, this will be indicated by the size of the disk.

Step 3: Mounting the Disk Partition With the correct partition identified, mount it to a temporary directory using the mount /dev/xvda1 /mnt command, replacing /dev/xvda1 with the appropriate partition name.

Step 4: Entering the Mounted Filesystem If the chroot /mnt command doesn't work, try chroot /mnt /bin/bash to enter the mounted filesystem environment, allowing you to execute commands as if you were operating from the root directory.

Step 5: Resetting the Root Password Once inside the mounted filesystem, use the passwd root command to set a new password for the root account. Follow the prompts to enter and confirm the new password securely.

Step 6: Exiting the Chroot Environment After resetting the root password, exit the chroot environment by typing exit in the terminal, returning you to the rescue mode shell.

Step 7: Unmounting the Temporary Partition To finalize the process, unmount the temporary partition with the umount /mnt command, ensuring all changes are properly applied.

Step 8: Rebooting the Server Finally, reboot your server using the reboot command to apply the changes and allow the system to boot normally.

By following this step-by-step guide, you can confidently reset the root password in rescue mode, enabling you to regain control over your server and resume normal operations swiftly and securely. Remember to exercise caution and verify each step carefully to avoid unintended consequences.

Friday, April 26, 2024

Bash Shell Scripting

These tutorials cover various aspects of bash shell scripting, starting with basic script creation and execution, then progressing to more advanced topics such as variables, loops, conditional statements, quoting, arithmetic operations, and file redirections. Each section provides practical examples and explanations to help beginners understand and apply bash scripting concepts effectively.

1. Hello bourne - Bash Shell Scripting
First you need to find out where is your bash interpreter located. Enter the following into your command line:
$ which bash

bash interpreter location:
Open up you favorite text editor and a create file called Insert the following lines to a file:
NOTE:Every bash shell script in this tutorial starts with shebang:"#!" which is not read as a comment. First line is also a place where you put your interpreter which is in this case: /bin/bash.
Here is our first bash shell script example:

# declare STRING variable
STRING="Hello bourne"
#print variable on a screen
echo $STRING

Navigate to a directory where your is located and make the file executable:

$ chmod +x

Make bash shell script executable
Now you are ready to execute your first bash script:


Example of simple bash shell script

2. Simple Backup bash shell script

tar -czf myhome_directory.tar.gz /home/linuxconfig

Simple Backup bash script

3. Variables
In this example we declare simple bash variable and print it on the screen ( stdout ) with echo command.
echo $STRING
Bash string Variables in bash script

Your backup script and variables:
OF=myhome_directory_$(date +%Y%m%d).tar.gz
tar -czf $OF /home/linuxconfig
Bash backup Script with bash Variables

3.1. Global vs. Local variables
#Define bash global variable
#This variable is global and can be used anywhere in this bash script
VAR="global variable"
function bash {
#Define bash local variable
#This variable is local to bash function only
local VAR="local variable"
echo $VAR
echo $VAR
# Note the bash global variable did not change
# "local" is bash reserved word
echo $VAR
Global vs. Local Bash variables in bash script

4. Passing arguments to the bash script
# use predefined variables to access passed arguments
#echo arguments to the shell
echo $1 $2 $3 ' -> echo $1 $2 $3'

# We can also store arguments from bash command line in special array
#echo arguments to the shell
echo ${args[0]} ${args[1]} ${args[2]} ' -> args=("$@"); echo ${args[0]} ${args[1]} ${args[2]}'

#use $@ to print out all arguments at once
echo $@ ' -> echo $@'

# use $# variable to print out
# number of arguments passed to the bash script
echo Number of arguments passed: $# ' -> echo Number of arguments passed: $#'

/ Bash Scripting Tutorial

Passing arguments to the bash script
5. Executing shell commands with bash

# use backticks " ` ` " to execute shell command
echo `uname -o`
# executing bash command without backticks
echo uname -o
Executing shell commands with bash

6. Reading User Input
echo -e "Hi, please type the word: \c "
read word
echo "The word you entered is: $word"
echo -e "Can you please enter two words? "
read word1 word2
echo "Here is your input: \"$word1\" \"$word2\""
echo -e "How do you feel about bash scripting? "
# read command now stores a reply into the default build-in variable $REPLY
echo "You said $REPLY, I'm glad to hear that! "
echo -e "What are your favorite colours ? "
# -a makes read command to read into an array
read -a colours
echo "My favorite colours are also ${colours[0]}, ${colours[1]} and ${colours[2]}:-)"

Reading User Input with bash
7. Bash Trap Command
# bash trap command
trap bashtrap INT
# bash clear screen command
# bash trap function is executed when CTRL-C is pressed:
# bash prints message => Executing bash trap subrutine !
echo "CTRL+C Detected !...executing bash trap !"
# for loop from 1/10 to 10/10
for a in `seq 1 10`; do
echo "$a/10 to Exit."
sleep 1;
echo "Exit Bash Trap Example!!!"

8. Arrays

8.1. Declare simple bash array

#Declare array with 4 elements
ARRAY=( 'Debian Linux' 'Redhat Linux' Ubuntu Linux )
# get number of elements in the array

# echo each element in array
# for loop
for (( i=0;i<$ELEMENTS;i++)); do
echo ${ARRAY[${i}]}
Declare simple bash array

8.2. Read file into bash array

# Declare array
declare -a ARRAY
# Link filedescriptor 10 with stdin
exec 10<&0
# stdin replaced with a file supplied as a first argument
exec < $1
let count=0

while read LINE; do


echo Number of elements: ${#ARRAY[@]}
# echo array's content
echo ${ARRAY[@]}
# restore stdin from filedescriptor 10
# and close filedescriptor 10
exec 0<&10 10<&-

Bash script execution with an output: $ cat bash.txt
Guide $ ./ bash.txt
Number of elements: 4
Bash Scripting Tutorial Guide $

9. Bash if / else / fi statements

9.1. Simple Bash if/else statement

Please note the spacing inside the [ and ] brackets! Without the spaces, it won't work!

# bash check if directory exists
if [ -d $directory ]; then
echo "Directory exists"
echo "Directory does not exists" 
Bash if else fi statement

9.2. Nested if/else

# Declare variable choice and assign value 4
# Print to stdout
echo "1. Bash"
echo "2. Scripting"
echo "3. Tutorial"
echo -n "Please choose a word [1,2 or 3]? "
# Loop while the variable choice is equal 4
# bash while loop
while [ $choice -eq 4 ]; do

# read user input
read choice
# bash nested if/else
if [ $choice -eq 1 ] ; then

echo "You have chosen word: Bash"


if [ $choice -eq 2 ] ; then
echo "You have chosen word: Scripting"

if [ $choice -eq 3 ] ; then
echo "You have chosen word: Tutorial"
echo "Please make a choice between 1-3 !"
echo "1. Bash"
echo "2. Scripting"
echo "3. Tutorial"
echo -n "Please choose a word [1,2 or 3]? "

10. Bash Comparisons

10.1. Arithmetic Comparisons

-lt <
-gt >
-le <=
-ge >=
-eq ==
-ne !=

# declare integers
if [ $NUM1 -eq $NUM2 ]; then
echo "Both Values are equal"
echo "Values are NOT equal"
Bash Arithmetic Comparisons - values are NOT equal

# declare integers
if [ $NUM1 -eq $NUM2 ]; then
echo "Both Values are equal"
elif [ $NUM1 -gt $NUM2 ]; then
echo "NUM1 is greater then NUM2"
echo "NUM2 is greater then NUM1"

Bash Arithmetic Comparisons - greater then

10.2. String Comparisons

= equal
!= not equal
< less then
> greater then
-n s1 string s1 is not empty
-z s1 string s1 is empty

#Declare string S1
#Declare string S2
if [ $S1 = $S2 ]; then
echo "Both Strings are equal"
echo "Strings are NOT equal"
Bash String Comparisons - values are NOT equal

#Declare string S1
#Declare string S2
if [ $S1 = $S2 ]; then
echo "Both Strings are equal"
echo "Strings are NOT equal"
bash interpreter location: /bin/bash

11. Bash File Testing

-b filename Block special file
-c filename Special character file
-d directoryname Check for directory existence
-e filename Check for file existence
-f filename Check for regular file existence not a directory
-G filename Check if file exists and is owned by effective group ID.
-g filename true if file exists and is set-group-id.
-k filename Sticky bit
-L filename Symbolic link
-O filename True if file exists and is owned by the effective user id.
-r filename Check if file is a readable
-S filename Check if file is socket
-s filename Check if file is nonzero size
-u filename Check if file set-ser-id bit is set
-w filename Check if file is writable
-x filename Check if file is executable

if [ -e $file ]; then
echo "File exists"
echo "File does not exists"
Bash File Testing - File does not exist Bash File Testing - File exists

Similarly for example we can use while loop to check if file does not exists. This script will sleep until file does exists. Note bash negator "!" which negates the -e option.

while [ ! -e myfile ]; do
# Sleep until file does exists/is created
sleep 1

12. Loops

12.1. Bash for loop


# bash for loop
for f in $( ls /var/ ); do
echo $f

Running for loop from bash shell command line:
$ for f in $( ls /var/ ); do echo $f; done

Bash for loop

12.2. Bash while loop

# bash while loop
while [ $COUNT -gt 0 ]; do
echo Value of count is: $COUNT

Bash while loop

12.3. Bash until loop

# bash until loop
until [ $COUNT -gt 5 ]; do
echo Value of count is: $COUNT

Bash until loop

12.4. Control bash loop with
Here is a example of while loop controlled by standard input. Until the redirection chain from STDOUT to STDIN to the read command exists the while loop continues.
# This bash script will locate and replace spaces
# in the filenames
# Controlling a loop with bash read command by redirecting STDOUT as
# a STDIN to while loop
# find will not truncate filenames containing spaces
find $DIR -type f | while read file; do
# using POSIX class [:space:] to find space in the filename
if [[ "$file" = *[[:space:]]* ]]; then
# substitute space with "_" character and consequently rename the file
mv "$file" `echo $file | tr ' ' '_'`
# end of while loop

Bash script to replace spaces in the filenames with _

13. Bash Functions

function function_B {
echo Function B.
function function_A {
echo $1
function function_D {
echo Function D.
function function_C {
echo $1
# Pass parameter to function A
function_A "Function A."
# Pass parameter to function C
function_C "Function C."

Bash Functions

14. Bash Select


PS3='Choose one word: '

# bash select
select word in "linux" "bash" "scripting" "tutorial"
echo "The word you have selected is: $word"
# Break, otherwise endless loop

exit 0

Bash Select

15. Case statement conditional

echo "What is your preferred programming / scripting language"
echo "1) bash"
echo "2) perl"
echo "3) phyton"
echo "4) c++"
echo "5) I do not know !"
read case;
#simple case bash structure
# note in this case $case is variable and does not have to
# be named case this is just an example
case $case in
1) echo "You selected bash";;
2) echo "You selected perl";;
3) echo "You selected phyton";;
4) echo "You selected c++";;
5) exit

bash case statement conditiona

16. Bash quotes and quotations
Quotations and quotes are important part of bash and bash scripting. Here are some bash quotes and quotations basics.

16.1. Escaping Meta characters
Before we start with quotes and quotations we should know something about escaping meta characters. Escaping will suppress a special meaning of meta characters and therefore meta characters will be read by bash literally. To do this we need to use backslash "\" character. Example:

#Declare bash string variable
BASH_VAR="Bash Script"

# echo variable BASH_VAR
echo $BASH_VAR

#when meta character such us "$" is escaped with "\" it will be read literally
echo \$BASH_VAR

# backslash has also special meaning and it can be suppressed with yet another "\"
echo "\\"

escaping meta characters in bash

16.2. Single quotes
Single quotes in bash will suppress special meaning of every meta characters. Therefore meta characters will be read literally. It is not possible to use another single quote within two single quotes not even if the single quote is escaped by backslash.

#Declare bash string variable
BASH_VAR="Bash Script"

# echo variable BASH_VAR
echo $BASH_VAR

# meta characters special meaning in bash is suppressed when using single quotes
echo '$BASH_VAR "$BASH_VAR"'

Using single quotes in bash

16.3. Double Quotes
Double quotes in bash will suppress special meaning of every meta characters except "$", "\" and "`". Any other meta characters will be read literally. It is also possible to use single quote within double quotes. If we need to use double quotes within double quotes bash can read them literally when escaping them with "\". Example:

#Declare bash string variable
BASH_VAR="Bash Script"

# echo variable BASH_VAR
echo $BASH_VAR

# meta characters and its special meaning in bash is
# suppressed when using double quotes except "$", "\" and "`"

echo "It's $BASH_VAR and \"$BASH_VAR\" using backticks: `date`"

Using double quotes in bash

16.4. Bash quoting with ANSI-C style
There is also another type of quoting and that is ANSI-C. In this type of quoting characters escaped with "\" will gain special meaning according to the ANSI-C standard.

\a alert (bell) \b backspace
\e an escape character \f form feed
\n newline \r carriage return
\t horizontal tab \v vertical tab
\\ backslash \` single quote
\nnn octal value of characters ( see [ ASCII table] ) \xnn hexadecimal value of characters ( see [ ASCII table] )

The syntax fo ansi-c bash quoting is: $'' . Here is an example:

# as a example we have used \n as a new line, \x40 is hex value for @
# and \56 is octal value for .
echo $'web:\nemail: web\x40linuxconfig\56org'

quoting in bash with ansi-c stype

17. Arithmetic Operations

17.1. Bash Addition Calculator Example


let RESULT1=$1+$2
echo $1+$2=$RESULT1 ' -> # let RESULT1=$1+$2'
declare -i RESULT2
echo $1+$2=$RESULT2 ' -> # declare -i RESULT2; RESULT2=$1+$2'
echo $1+$2=$(($1 + $2)) ' -> # $(($1 + $2))'

Bash Addition Calculator

17.2. Bash Arithmetics


echo '### let ###'
# bash addition
let ADDITION=3+5
echo "3 + 5 =" $ADDITION

# bash subtraction
echo "7 - 8 =" $SUBTRACTION

# bash multiplication
echo "5 * 8 =" $MULTIPLICATION

# bash division
let DIVISION=4/2
echo "4 / 2 =" $DIVISION

# bash modulus
let MODULUS=9%4
echo "9 % 4 =" $MODULUS

# bash power of two
echo "2 ^ 2 =" $POWEROFTWO

echo '### Bash Arithmetic Expansion ###'
# There are two formats for arithmetic expansion: $[ expression ]
# and $(( expression #)) its your choice which you use

echo 4 + 5 = $((4 + 5))
echo 7 - 7 = $[ 7 - 7 ]
echo 4 x 6 = $((3 * 2))
echo 6 / 3 = $((6 / 3))
echo 8 % 7 = $((8 % 7))
echo 2 ^ 8 = $[ 2 ** 8 ]

echo '### Declare ###'

echo -e "Please enter two numbers \c"
# read user input
read num1 num2
declare -i result
echo "Result is:$result "

# bash convert binary number 10001
echo $result

# bash convert octal number 16
echo $result

# bash convert hex number 0xE6A
echo $result

Bash Arithmetic Operations

17.3. Round floating point number

# get floating point number
echo $floating_point_number
# round floating point number with bash
for bash_rounded_number in $(printf %.0f $floating_point_number); do
echo "Rounded number with bash:" $bash_rounded_number

Round floating point number with bash

17.4. Bash floating point calculations

# Simple linux bash calculator
echo "Enter input:"
read userinput
echo "Result with 2 digits after decimal point:"
echo "scale=2; ${userinput}" | bc
echo "Result with 10 digits after decimal point:"
echo "scale=10; ${userinput}" | bc
echo "Result as rounded integer:"
echo $userinput | bc

Bash floating point calculations

18. Redirections

18.1. STDOUT from bash script to STDERR


echo "Redirect this STDOUT to STDERR" 1>&2

To prove that STDOUT is redirected to STDERR we can redirect script's output to file:

STDOUT from bash script to STDERR

18.2. STDERR from bash script to STDOUT


cat $1 2>&1

To prove that STDERR is redirected to STDOUT we can redirect script's output to file:

STDERR from bash script to STDOUT

18.3. stdout to screen
The simple way to redirect a standard output ( stdout ) is to simply use any command, because by default stdout is automatically redirected to screen. First create a file "file1":
$ touch file1
$ ls file1

As you can see from the example above execution of ls command produces STDOUT which by default is redirected to screen.

18.4. stdout to file
The override the default behavior of STDOUT we can use ">" to redirect this output to file:
$ ls file1 > STDOUT
$ cat STDOUT

18.5. stderr to file
By default STDERR is displayed on the screen:
$ ls
file1 STDOUT
$ ls file2
ls: cannot access file2: No such file or directory

In the following example we will redirect the standard error ( stderr ) to a file and stdout to a screen as default. Please note that STDOUT is displayed on the screen, however STDERR is redirected to a file called STDERR:

$ ls
file1 STDOUT
$ ls file1 file2 2> STDERR
$ cat STDERR
ls: cannot access file2: No such file or directory

18.6. stdout to stderr
It is also possible to redirect STDOUT and STDERR to the same file. In the next example we will redirect STDOUT to the same descriptor as STDERR. Both STDOUT and STDERR will be redirected to file "STDERR_STDOUT".

$ ls
$ ls file1 file2 2> STDERR_STDOUT 1>&2
ls: cannot access file2: No such file or directory

File STDERR_STDOUT now contains STDOUT and STDERR.

18.7. stderr to stdout
The above example can be reversed by redirecting STDERR to the same descriptor as SDTOUT:

$ ls
$ ls file1 file2 > STDERR_STDOUT 2>&1
ls: cannot access file2: No such file or directory

18.8. stderr and stdout to file
Previous two examples redirected both STDOUT and STDERR to a file. Another way to achieve the same effect is illustrated below:

$ ls
$ ls file1 file2 &> STDERR_STDOUT
ls: cannot access file2: No such file or directory


$ ls file1 file2 >& STDERR_STDOUT
ls: cannot access file2: No such file or directory

Inode space issue , finding largest inode entry direcotry

To find the directory with the largest inode entry, you can use the following command:
For example:

sudo find / -xdev -printf '%h\n' | sort | uniq -c | sort -nr | head

 Let's break down this command:
  • sudo: Run the command with root privileges to access all directories.
  • find / -xdev -printf '%h\n': Find all files and directories starting from the root directory (/) while excluding other mounted filesystems (-xdev). Print only the directory portion of each file found (%h) followed by a newline (\n).
  • sort: Sort the output alphabetically (directories will be grouped together).
  • uniq -c: Count the occurrences of each unique directory.
  • sort -nr: Sort the counts numerically in reverse order (largest counts first).
  • head: Display the first few lines of output, which will show the directories with the largest number of files.

This command will help you identify the directory with the most files, which could be contributing to the inode space issue. Once you find the problematic directory, you can further investigate and take appropriate actions to manage the inode usage.


Understanding the Boot Process in Redhat Linux & CentOS 5&6

The boot process in Redhat Linux and CentOS involves several stages, each responsible for initializing different components of the system. Here's a breakdown of the boot process:


  1. BIOS (Basic Input/Output System):

    • Performs system integrity checks.
    • Searches, loads, and executes the boot loader program.
    • Detects and loads the MBR (Master Boot Record) boot loader.
  2. MBR (Master Boot Record):

    • Located in the 1st sector of the bootable disk (/dev/hda or /dev/sda).
    • Contains information about the GRUB (Grand Unified Bootloader) boot loader.
  3. GRUB (Grand Unified Bootloader):

    • Displays a splash screen and waits for user input.
    • Loads the default kernel image specified in the grub configuration file (/boot/grub/grub.conf).
    • Has knowledge of the filesystem.
    • Loads and executes the kernel and initrd images.
  4. Init:

    • Reads the /etc/inittab file to determine the default run level.
    • Defines the available run levels (0-6) and their corresponding modes (halt, single user, multiuser, etc.).
    • Sets the default run level based on the configuration.
    • Loads all appropriate programs based on the default run level.
  5. Runlevel Programs:

    • Services and programs started during bootup.
    • Located in directories corresponding to each run level (/etc/rc.d/rc*.d/).
    • Symbolic links are available under /etc/rc*.d/ for convenience.
    • Programs starting with 'S' are executed during startup, while those starting with 'K' are executed during shutdown.
    • The sequence number in the program names determines the order of execution.

    For example:

    • S12syslog starts the syslog daemon with a sequence number of 12.
    • S80sendmail starts the sendmail daemon with a sequence number of 80.
    • Programs with lower sequence numbers are executed before those with higher numbers.


Thursday, December 28, 2023

Mastering Packet Analysis with Tcpdump - Packet Analysing.

Tcpdump is a powerful command-line packet analyzer tool used for network troubleshooting and analysis. It allows the user to intercept and display the packets transmitted or received over a network to which the computer is attached. In this guide, we'll explore how to use tcpdump for various purposes, including capturing packets, filtering traffic, and analyzing packet content.

1. Display Available Interfaces:

To see a list of available network interfaces on your system:

tcpdump -D

2. Capture Packets from a Specific Interface:

To start capturing packets from a specific interface (e.g., venet0):

tcpdump -i venet0

3. Capture Only N Number of Packets:

To limit the capture to a specific number of packets (e.g., 2 packets):

tcpdump -c 2 -i venet0

4. Print Captured Packets in ASCII:

To view the captured packets in ASCII format:

tcpdump -c 2 -A -i venet0

5. Display Captured Packets in HEX and ASCII:

To view the packet's contents in both HEX and ASCII formats:

tcpdump -c 2 -XX -i venet0

Advanced Packet Capturing

6. Capture and Save Packets in a File:

To capture packets and save them to a file for later analysis:

tcpdump -w capture.pcap -i venet0 -c 2

7. Read Captured Packets from a File:

To read packets from a previously saved file:

tcpdump -r capture.pcap

8. Capture Packets from a Specific IP Address:

To capture packets involving a particular IP address:

tcpdump -n -i venet0 -c 2 src

9. Capture Only TCP Packets:

To capture only TCP packets:

tcpdump tcp -n -i venet0 -c 2

10. Capture Packets from a Specific Port:

To capture packets from a particular port (e.g., SSH port 22):

tcpdump -i venet0 -c 2 port 22

Filtering and Analyzing Traffic

11. Capture Packets with a Readable Timestamp:

To capture packets with a more readable timestamp:

tcpdump -i venet0 -c 2 -tttt

12. Read Packets Longer than N Bytes:

To capture and read packets longer than a certain size (e.g., 10 bytes):

tcpdump -i venet0 greater 10 -c 2

13. Filter Packets – Exclude ARP and RARP:

To capture all packets other than ARP and RARP:

tcpdump -i venet0 not arp and not rarp -c 2


Tcpdump is an incredibly versatile tool that can be used for a wide range of network analysis tasks. By understanding how to use its various options and filters, you can diagnose network issues, monitor traffic in real-time, and perform in-depth protocol analysis. Remember, while tcpdump can capture sensitive data, it should be used responsibly and ethically. Happy analyzing!