Functions

• Functions refer to blocks of code that can be defined and executed within scripts or the command line.

• These functions allow you to modularize your code, making it easier to manage, reuse, and understand.

• Functions can accept parameters and return values, just like in programming languages.

#Syntax:
Function_Name () {
    #Statements
}

#Example:
#A function to print "Hello Linux" on terminal

#!/bin/bash
#Function Defination
print_Hello () {
    echo "Hello Linux"
}    

#Function Call
print_Hello

Function To Add Two Numbers

#!/bin/bash
#Function Defn
addTwoNumbers () {
    echo "Enter the first number:"
    read num1

    echo "Enter the second number:"
    read num2

    sum=$(expr $num1 + $num2)
    echo "The addition of ${num1} + ${num2} is : ${sum}"
} 

#Function Call
addTwoNumbers

#Output:
Enter the first number:
10
Enter the second number:
20
The addition of 10 + 20 is : 30

Case

• In Linux Shell Scripting, "cases" refer to switch/case statements.

• These are control structures that allow you to perform different actions based on the value of a variable or expression.

#Syntax:
case ${variable} in
val1)
    #Statement1
;;
val2)
    #Statement2
;;
*)
    #Default Statement
;;
esac

#Example:
#Write a case which when executes, perform some commands in Linux

#!/bin/bash
echo "1 : File Create"
echo "2 : Display IP Address"
echo "3 : Current Directory"
echo "4 : Make Directory"

echo "Enter the input : "
read input

case ${input} in
1)
    echo "Enter the file name : "
    read fileName
    touch /tmp/${filename}
;;
2)
    ifconfig
;;
3)
    pwd
;;
4)
    echo "Enter the directory name : "
    read dirName
    mkdir /tmp/${dirname}
;;
*)
    echo "I can't do it !!"
;;
esac

#Executing the Script...
1 : File Create
2 : Display IP Address
3 : Current Directory
4 : Make Directory
Enter the input :
3
/home/viraj

This was our last lecture at the Linux Workshop. Pranav Jambare (DevOps @TCS), delivered an exceptional amount of lectures. His guidance led us through intricate Linux landscapes, mastering commands and scripting. His passion and support created a vibrant learning environment, igniting our love for Linux.

A big thank you to Pranav Sir for an amazing workshop.

Loops in Linux

• Loops in shell scripting allow you to execute a series of commands repeatedly, based on a condition or a list of items.

Types of Loops

While Loop

• The while loop repeatedly executes the commands inside the loop as long as the condition remains true.

• If the condition becomes false, the loop stops executing and moves to the next part of the script.

#Syntax:
while <command>;
do
    # Statement execute if command is true
done

#Example: (Create a sample.sh and edit the file with the given script)
#Printing numbers from 1 to 20
#!/bin/bash
A=1
while [ ${A} -le 20 ];
do
    echo "A = ${A}"
    A=`expr ${A} + 1` 
done

#After running script...
A = 1
A = 2
...
A = 19
A = 20

Until Loop

• The until loop is similar to the while loop, but it continues executing the commands inside the loop until the condition becomes false.

• Once the condition is true, the loop stops executing.

#Syntax:
until <command>;
do
    # Statement execute if command is false
done

#Example: (Create a sample.sh and edit the file with the given script)
#Printing numbers from 20 to 1
#!/bin/bash
A=20
while [ ${A} -ge 1 ];
do
    echo "A = ${A}"
    A=`expr ${A} - 1` 
done

#After running script...
A = 20
A = 19
...
A = 2
A = 1

For Loop

• The for loop iterates over each item in the list, and the variable takes the value of each item in each iteration.

• The commands inside the loop are executed for each item in the list.

#Syntax:
for var in word1 word2 .. wordn;
do
    # Statement executed till element end
done

#Example:
#Printing the names of students...
#!/bin/bash
A=1
for studName in Viraj Amogh Suyash;
do
    echo "The name of student ${A} is : ${studName}"
    A=`expr ${A} + 1`
done

#After running the script...
The name of student 1 is : Viraj
The name of student 2 is : Amogh
The name of student 3 is : Suyash

#Example:
#To print alphabets from A to Z
#!/bin/bash
for alpha in {A..Z};
do
    echo "${alpha}"
done

#After running the script...
A
B...
..
Y
Z

#Example:
#To print the names of students from a file
#!/bin/bash
for name in $(cat /home/viraj/namesFile);
do
    echo "The name is : ${name}"
done

Command Line Arguments

• Command Line Arguments are inputs or parameters provided to a command or script when it is executed in the terminal.

• $1 $2 $3 --> The first command-line argument is represented by $1, the second one by $2, and so on.

• $0 --> Represents the name of the script itself.

• $@ --> Passing arguments one-by-one

• $# --> Total number of command line arguments provided

• $* --> Represents all the command-line arguments as a single string

• $$ --> Represents the process ID (PID) of the currently running shell or script

• $? --> Holds the status of the last executed command

• $! --> Hold the process ID (PID) of the last executed command

#Example:
#!/bin/bash
for names in $1 $2;
do
    echo "Your name is : ${names}"
    echo "The name of script is : $0"
    echo "Passing args one-by-one : $@"
    echo "Total number of args provided : $#"
    echo "Args as a single string : $*"
    echo "PID of current script : $$"
    echo "Status of last executed cmd: $?"
    echo "PID of last executed cmd: $!"
done

#After running script...
Your name is : Viraj
The name of script is : sampleFile.sh
Passing args one-by-one : Viraj Amogh
Total number of args provided : 2
Args as a single string : Viraj Amogh
PID of current script : 4920
Status of last executed cmd : 0
PID of last executed cmd : --

Your name is : Amogh
The name of script is : sampleFile.sh
Passing args one-by-one : Viraj Amogh
Total number of args provided : 2
Args as a single string : Viraj Amogh
PID of current script : 4920
Status of last executed cmd : 0
PID of last executed cmd : --

Shell Scripting Question

Q. Create 5 directories and take user input to read name of those directories. Now create two files in each directory and take user input to name those files. Now take user input to add some content in each file.

#Answer:
#!/bin/bash
for dirName in {1..5};
do 
    echo "Enter the name of directory : "
    read dir
    mkdir ${dir}

    for fileName in 1 2;
    do 
        echo "Enter the name of file : "
        read file

        touch /home/viraj/${dir}/${file}

        echo "Enter the content for file : "
        read content >> /home/viraj/${dir}/${file}
    done
done

Filters

• In Linux, filters refer to commands that process input data and produce output data based on specific criteria.

• Used, when we want to see exact data rather than the whole data.

• Some of the common filters will be explained below.

Sort

• This sort filter is used to sort the content of a file based on the specific format given for sorting.

#To sort the content of file in ascending order
#Also display the output on the terminal

#Syntax:
sort <filename>

#Example:
[viraj@localhost ~]$ vim sortTheFile
apple
mango
banana
grapes
jackfruit
pineapple

[viraj@localhost ~]$ sort sortTheFile
apple
banana
grapes
jackfruit
mango
pineapple

Options for the sort filter

#Syntax:

# Used to sort the content of file in decending order
sort -r <filename>

# Used to sort the numbers present in the file in ascending order
sort -n <filename>

# Used to check whether the file content is sorted or not
sort -c <filename>

# Used to sort the content of file & also remove the duplicate content in file
sort -u <filename>

#used to sort the months in order
sort -M <filename>

Tail

• The tail filter is used to display the last few lines of a file or stream of data.

#Syntax:
tail <filename>

#Example:
[viraj@localhost ~]$ tail mySampleFile
This is a sample file
_____________________________________________________________________________

# tail filter with options: 
#To display the last 'n' lines of a file
#Syntax:
tail -n <filename>

#Example:
[viraj@localhost ~]$ vim filterFile
Hello
Welcome
To
My
Blog

[viraj@localhost ~]$ tail -3 filterFile
To
My
Blog
_____________________________________________________________________________

# To merge the content of the files
#Syntax:
tail -q <file1> <file2>

#Example:
#Creating the file1 and file2 and then merging them...
[viraj@localhost ~]$ vim file1
This is my file1 created by me

[viraj@localhost ~]$ vim file2
This is another file2 created by me again!!

[viraj@localhost ~]$ tail -q file1 file2
This is my file1 created by me
This is another file2 created by me again!!
_____________________________________________________________________________

# To get the dynamic (real-time) output on the terminal
#Syntax:
tail -f <filename>

#Example:
# To get the logs output dynamically in myCustom.log file
tail -f myCustom.log

Head

• The head filter is used to display the first few lines of a file or a stream of data.

• It is the counterpart to the tail command, which shows the last few lines of a file.

#To display first 'n' lines of the file
#Syntax:
head -n <filename>

#Example:
#Creating the file...
[viraj@localhost ~]$ vim myHeadFile
Hello 
Again
Nice 
Meeting
You!!

#First 2 lines are printed...
[viraj@localhost ~]$ head -2 myHeadFile
Hello 
Again

#The other head options are very similar to the tail options mentioned above...

WC

• WC is a filter that stands for word count.

#Syntax:
wc <filename>

#Example:
[viraj@localhost ~]$ wc myFile
1 8 37 myFile
____________________________________________________________________________

# wc with options:
#To get the line count in the file
#Syntax:
wc -l <filename>

#Example:
[viraj@localhost ~]$ wc -l myFile
1 myFile
____________________________________________________________________________

# To get the count of characters in the file
#Syntax:
wc -m <filename>

#Example:
[viraj@localhost ~]$ wc -m myFile
37 myFile
____________________________________________________________________________

# To get the count of bytes in the file
#Syntax:
wc -c <filename>

#Example:
[viraj@localhost ~]$ wc -c myFile
37 myFile
____________________________________________________________________________

# To get the count of words in the file
# Syntax:
wc -w <filename>

#Example:
[viraj@localhost ~]$ wc -w myFile
8 myFile

Uniq

• The uniq filter in Linux is used to remove duplicate lines from a sorted file or input stream.

• Its primary purpose is to identify and display unique lines, removing consecutive duplicates and leaving only one occurrence of each unique line.

#To get count of repetition
#Syntax:
uniq -c <filename>

#Example:
# Creating a file...
[viraj@localhost ~]$ vim myUniqFile
viraj
viraj
amogh
suyash
suyash

[viraj@localhost ~]$ uniq -c myUniqFile
2 viraj
1 amogh
2 suyash
____________________________________________________________________________

#To only print the repeated lines
#Syntax:
uniq -d <filename>

#Example:
[viraj@localhost ~]$ uniq -d myUniqFile
viraj
suyash
____________________________________________________________________________

#To print repeated lines with occurence
#Syntax:
uniq -D <filename>

#Example:
[viraj@localhost ~]$ uniq -D myUniqFile
viraj 
viraj
suyash
suyash
____________________________________________________________________________

#To only print unique lines
#Syntax:
uniq -u <filename>

#Example:
[viraj@localhost ~]$ uniq -u myUniqFile
amogh

Comm

#To show the common content between the two files
#Syntax:
comm <file1> <file2>

#Example:
#Creating two files...
[viraj@localhost ~]$ vim commFile1 
ABC
DEF
GHI
JKL

[viraj@localhost ~]$ vim commFile2
ABC
DEF
GHO
JKM

[viraj@localhost ~]$ comm myFile1 myFile2
            ABC
            DEF
GHI                  
      GHO
JKL   
      JKM

Diff

#To show the different content between the two files
#Syntax:
diff <file1> <file2>

#Example:
[viraj@localhost ~]$ diff myFile1 myFile2
3,4c3,4
< GHI
< JKL
------
> GHO
> JKM

GREP

# To search the exact word
#Syntax:
grep -w <filename>

#Example:
[viraj@localhost ~]$ grep computer myFile
____________________________________________________________________________

# To search the word with case insensitivity
#Syntax:
grep -i -w <filename>

#Example:
[viraj@localhost ~]$ grep -i computer myFile
____________________________________________________________________________

# To display only the matched words (whole words)
#Syntax:
grep -o -w <filename>

#Example:
[viraj@localhost ~]$ grep -o computer myFile
____________________________________________________________________________

# To get count of a word in a file
#Syntax:
grep -c -w <filename>

#Example:
[viraj@localhost ~]$ grep -c computer myFile
____________________________________________________________________________

# To search for multiple words
#Syntax:
grep -e -w -e -w <filename>

#Example:
[viraj@localhost ~]$ grep -e computer -e program myFile
____________________________________________________________________________

# To display the matched line and line number
#Syntax:
grep -n -w <filename>

#Example:
[viraj@localhost ~]$ grep -n computer myFile

AWK

# To print the content of the file
#Syntax:
awk '{print}' <fileName>

#Example:
[viraj@localhost ~]$ awk '{print}' myFile
____________________________________________________________________________

# To print the lines that match the given word in the file
#Syntax:
awk '/word/ {print}' <fileName>

#Example:
[viraj@localhost ~]$ awk '/computer/ {print}' myFile
____________________________________________________________________________

# To print specific words of each line in the file
#Syntax:
awk '{print $<lineNumber>, $<lineNumber>}' <fileName>

#Example:
[viraj@localhost ~]$ awk '{print $2, $6}' myFile
____________________________________________________________________________

# To print the last word of each line in the file
#Syntax:
awk '{print $NF}' <fileName>

#Example:
[viraj@localhost ~]$ awk '{print $NF}' myFile
____________________________________________________________________________

# To print specific words of specific lines in the file
#Syntax:
awk 'NR==<line_number> {print $<lineNumber>, $<lineNumber>}' <fileName>

#Example:
[viraj@localhost ~]$ awk 'NR==2, NR==3 {print $2, $4}' myFile

Cut

#Syntax:
cut -d<delimiter> -f<fieldNumber> <fileName>

#Example:
[viraj@localhost ~]$ cut -d"-" -f3 myFile

Sed

#To replace the word by occurrence
#Syntax:
sed 's/<wordNeedToReplace>/<replaceWith>/<occurrenceNum>' <filename>

#Example:
sed 's/ABC/VIRAJ/2' myFile
_____________________________________________________________________________

#To replace the word globally
#Syntax:
sed 's/<wordNeedToReplace>/<replaceWith>/g' <filename>

#Example:
sed 's/ABC/VIRAJ/g' myFile
_____________________________________________________________________________

#To replace the word on the specific line
#Syntax:
sed '<lineNumber> s/<wordNeedToReplace>/<replaceWith>/' <filename>

#Example:
sed '3 s/program/programs/' myFile
_____________________________________________________________________________

#To delete the last line
#Syntax:
sed '$d' <filename>

#Example:
sed '$d' myFile
_____________________________________________________________________________

#To delete particular lines
#Syntax:
sed '<lineNumber>d' <filename>

#Example:
sed '1,3d' myFile
_____________________________________________________________________________

#To delete lines matching a word
#Syntax:
sed '/word/d' <filename>

#Example:
sed '/computer/d' myFile

Shell Script

• A shell script in Linux is a script written in a scripting language that is interpreted by a shell.

• A shell script is a series of commands written in a text file and executed as a single unit.

• In a shell script, you can use the full range of commands and features available in the shell, including loops, conditionals, functions, variables, and input/output redirection.

• Shell scripts allow you to automate tasks, execute multiple commands sequentially, and perform complex operations that would otherwise require manual execution on the command line.

• The file extension commonly used for shell scripts is .sh, but it is not mandatory.

Types of Shell Script

1. Non Interactive :

   • Non-interactive shell scripts are designed to run without any user input during their execution.

   • They execute a predefined set of commands and do not require any interaction with the user while running.

2. Interactive :

   • Interactive shell scripts are designed to interact with the user during their execution.

   • They prompt the user for input and respond accordingly.

3. Command Line :

   • Command-line shell scripts are run from the command line and accept arguments provided by the user when executing the script.

Shebang

• The shebang, also known as a hashbang or pound-bang, is the special character sequence #! at the beginning of a script file in Linux.

• The shebang indicates the interpreter that should be used to execute the script.

#!/pathTo/interpreter

• For example, in a Bash shell script, the shebang would typically be:

#!/bin/bash

Creating and Running a Script in Linux

• Create a file with the extension .sh

#Creating and editing the file in vim editor...
vim Sample.sh
___________________________________________________________________________
#At the beginning of the file use Shebang and start writing the script...
#Syntax:
#!/bin/bash
<setOfCommands>...

#Example:
#!/bin/bash

echo "This is my first script"
echo "Thank you!!"

#Now save the file...

• Running the sample.sh script

#You can give execute permission to user and then run the file
#Then run as shown...
chmod 777 Sample.sh
./Sample.sh

#Else, you can use bash command (if execute permission is missing)
#Then the file name to execute the file
bash Sample.sh

#Output:
This is my first script 
Thank you!!

To declare the variables in Linux (Temporary)

#Syntax:
<variableName>=<value>

#Example:
myName=Viraj

#The above example shown has a temporary existance, i.e
#if bash command is execute the myName variable won't have it's existance and can't be used anymore
#To declare variables permanently use following method...

To declare the variables in Linux (Permanent)

#Open a file called bashrc or bash.bashrc (according to linux distro's) in vim editor
#Now, define the variables at the last
#Save the file
#Now try to use it in any of the commands, it would have its permanent existance
echo "Good Morning, ${myName}"
Good Morning, Viraj

If - Else Statements

1. Simple if

    #Syntax:
    if [ expression ]
    then
        statement
    fi
   

    #Example:
    if [ ${A} -gt ${B} ]
    then
        echo "${A} is greater than ${B}"
    fi
   

2. If - Else

    #Syntax:
    if [ expression ]
    then
        statement
    else
        statement
    fi
   

    #Example:
    if [ ${A} -gt ${B} ]
    then
        echo "${A} is greater than ${B}"
    else
        echo "${B} is greater than ${A}"
    fi
   

3. Else-if Ladder

    #Syntax:
    if [ expression1 ]
    then
        statement1
   
    elif [ expression2 ]
    then
        statement2
   
    else
        statement5
    fi
   

    #Example:
    if [ ${age} -lt 18 ]
    then
        echo "You are not allowed to drive!!"
    elif [ ${age} -ge 18 ]
    then
        echo "You are allowed to drive!!"
    elif [ ${age} -ge 100 ]
    then
        echo "Please don't drive!!"
    else
        echo "Invalid Age!!"
    fi
   

4. Nested If

    #Syntax:
    if [ expression1 ]
    then 
        if [ expression2 ]
        then
            statement1
        else
            statement2
        fi
    else
        statement3
    fi
   

    #Example:
    if [ ${firstName} == Viraj ]
    then
        echo "First name verified!!"
        if [ ${lastName} == Nalbalwar ]
            echo "Verified User!!"
        else
            echo "Last name incorrect!!"
        fi
    else
        echo "First name incorrect!!"
    fi
   

• The booting process refers to the series of steps that take place when a computer starts up and loads the operating system into memory, it involves several stages and those are listed below.

Stages of the Booting Process

1. BIOS (Basic Input Output System)

   • When the computer is powered on, the BIOS is activated.

   • It performs the Power-On Self-Test (POST), detects and initializes essential hardware components, and locates the boot device (usually the hard drive or SSD) that contains the bootloader.

2. MBR (Master Boot Record)

   • When the computer starts up, the BIOS executes the code in the MBR.

   • The structure of the MBR consists of three primary components:

     

     1. Bootloader: The MBR contains a small piece of executable code called the bootloader, which is responsible for locating and loading the main bootable partition.

     2. Partition Table: The MBR also contains a partition table that can hold information about up to four primary partitions on the storage device.

     3. Magic Number (Parity Bits): It acts as a simple form of data validation and if any part of the MBR is corrupted or altered, including the magic number, the BIOS will not consider it a valid bootable device.

3. GRUB (GRand Unified Bootloader)

   • GRUB is a popular bootloader used in the booting process in Linux.

   • It plays a critical role in the boot process by allowing users to choose which operating system to boot when multiple operating systems are installed on the same computer.

   • The different versions of GRUB that have been developed over time:

     • GRUB 1 --> Getting code from MBR

     • GRUB 1.5 --> Allows file system to communicate with hardware

     • GRUB 2.0 --> Main GRUB image, /boot/grub2/grub.conf -->Loading of kernel

4. Kernel

   • It initializes & configures computer memory and hardware.

   • Uncompressed itself & mount it.

   • Loads the required drivers for it.

   • Initialize virtual devices into the system.

   • Create a root device mount into read-only memory.

   • Freeze unused memory.

5. init.d / system.d

   • It is the first process (mother of the process) after the kernel step.

   • Mounting the file system --> /etc/fstab

   • Starting services

   • Device attaching to the system

   • Identify the default target

6. Run Level

#Commands:

#The "who" command is used to display the current run level & system's last reboot time
who -r 
___________________________________________________________________

#The "init" command is used to change the system's run level, which represents a specific operating state
#Syntax:
init <runlevel>
#Example:
init 5 
init 3
___________________________________________________________________

#The default target determines the operating state in which the system will boot by default
systemctl set-default multi-user.target
systemctl set-default graphical.target
___________________________________________________________________

#This command is used to check the default target
systemctl get-default
___________________________________________________________________

#List of Run Levels:
0 - Halt state (poweroff.target)
1 - Single user mode (rescue.target)
2 - Mutli user mode without networking
3 - Mulit user mode with networking
4 - User Definable
5 - GUI (graphical.target)
6 - Reboot

ACL

• ACL stands for Access Control Lists.

• It allows you to give separate permissions to individual users or groups on files and directories in a file system.

• Allowing you to define more fine-grained permissions for specific users or groups.

#To set ACL on a directory:
#Syntax:
setfacl -m u:<user>:<perm> <dirname>

#Example:
[root@localhost tmp]# setfacl -m u:testuser:rwx TestACL/
[root@localhost tmp]# ls -ld TestACL/
drwxrwxr-x+ 2 root root 41 Jul 19 12.00 TestACL/

#To show the list of users and groups with their respective permissions on the specified file or directory:
#Syntax:
getfacl <dirname>
OR
getfacl <filename>

#Example:
[root@localhost tmp]# getfacl TestACL/
#file: TestACL/
#owner: root
#group: root
user::rwx
user:testuser:rwx
user:robot:rwx
group::r-x
mask::rwx
other::r-x

#To remove the ACL (remove a single user at a time):
setfacl -x u:testuser: TestACL/
#Verifying...
getfacl TestACL/
user::rwx
user:robot:rwx
group:r-x
mask::rwx
other::r-x

#To remove the ACL (remove all users at once):
setfacl -b TestACL/
#Verifying...
getfacl TestACL/
user::rwx
group::r-x
other::r-x

Processes

• The Process represents the execution of a specific task or job on the system.

• Every action you perform on a computer involves one or more processes.

Types of Processes

• Parent and Child Processes :

  • When a new process is created, it is referred to as the "child process" and the process that creates it is called the "parent process".

  • Parent processes are processes that create and manage other processes.

  • If the process creates additional processes as part of its execution, those are considered child processes.

• Zombie Processes :

  • The processes that have completed their execution but still have an entry in the process table.

  • Zombie processes do not consume system resources and are usually cleaned up automatically by the system.

• Orphan Processes :

  • Orphan processes are child processes whose parent processes have terminated unexpectedly before collecting their exit status.

Process State

• A process state refers to the current condition or status of a running process.

• The process state indicates what the process is currently doing or waiting for in the system.

Commands Related to Processes in Linux

Top Command

#This command provides real-time dynamic monitoring of system processes, 
#CPU usage, memory utilization, and other system statistics in an interactive shell
[root@localhost ~]# top

To list the processes that have "<word>" in it

#Listing the processes having word "top" in it
#Used to list all running processes and 
# filter the output to display only the lines containing word "top" using grep command.
[root@localhost ~]# ps -ef|grep top

To assign higher or lower priority to a new process

#Syntax:
nice -n <nicenessValue> command

#Example:
[root@localhost ~]# nice -n -5 top

• The "nice" command is used to execute a command to modify a niceness value.

• The nice command is used when starting a new process to set its initial priority.

• The niceness value is an integer that ranges from -20 to +20.

• The lower values indicate higher priority. A process with a higher priority (lower niceness value) will be running more frequently compared to processes with a lower priority.

To assign higher or lower priority to an existing process using its PID

#Syntax:
renice -n <niceness_value> <PID>

#Example:
[root@localhost ~]# renice -5 3030
#Here, 3030 is pid for command top

To terminate, kill, resume or suspend a process

#Syntax:
kill <signal> <PID>

#Signals:
Terminate : -15
Kill : -9
Resume : -18
Suspend : -19

#Example:
kill -9 3030
OR
kill -15 3745

Jobs command

• The command "jobs" is useful when you are running multiple background processes and need to keep track of their status or interact with them.

#Command
[root@localhost ~]# jobs

Alias

• An alias is a user-defined shortcut or nickname for a command or a sequence of commands.

• Aliases allow users to create their shorthand notation for frequently used or complex commands, making it easier and faster to execute them.

#Defining an Alias (Syntax):
alias aliasName="command_or_commands"

#Example:
alias viraj="ls -lrt"

#You can define the alias in the .bashrc or .bash_aliases file
#Defining the alias in the interactive terminal/shell is temporary, to define it permanently write the alias in .bashrc file

Logs

• In Linux, Logs refer to the recorded information about system events, activities, and processes that are stored in log files.

• Logs are essential for developers to troubleshoot issues, monitor system performance, and maintain system security.

Some of the logs in Linux

1. Kernel Log

2. Boot Log

3. CUPS

4. dmesg

5. fail

6. yum

7. cron

8. secure

9. mail

10. wtmp

rsyslog.conf

• The "rsyslog.conf" file is the main configuration file for the rsyslog service in Linux.

• The file is typically located in the "/etc/rsyslog.conf" or "/etc/rsyslog.d/" directory, depending on the Linux distribution.

• The rsyslog.conf file contains directives and rules that how log messages are received, processed, and forwarded by rsyslog.

• It allows administrators to customize the behavior of the logging system according to their specific requirements.

Facilities & Priorities

• Facilities and Priorities are used to categorize and filter log messages.

Facilities:

• Facilities represent different sources that generate log messages.

• Each log message is associated with a facility, which helps identify its source or origin.

• Some common facilities include :

  • auth: Authentication-related messages.

  • cron: Messages from the cron scheduler.

  • authpriv: Authentication-related log messages with enhanced privacy considerations.

  • mail: Mail server-related messages.

  • user: User-level messages.

Priorities:

• Priorities indicate the importance of a log entry.

• They help determine the urgency or criticality of a log message.

• Some common priorities include :

  • debug: Detailed debugging information.

  • info: General information messages.

  • notice: Important but non-error conditions.

  • warning: Indications of potential issues.

  • err: Errors that may impact system functionality.

  • critical: Critical errors requiring immediate attention.

  • alert: Alerts requiring immediate action.

  • emerg: The most severe level, indicating an emergency condition.

To Create Your Custom Log

Creating a custom log file in /var/log/

#Traversing to /var/log/ directory :
[root@localhost ~]# cd /var/log/

#Creating a file myCustom.log :
[root@localhost log]# touch myCustom.log

#Create a facility and priority in rsyslog.conf file :
[root@localhost ~]# vim /etc/rsyslog.conf

#Add your facilities and priorities in the rsyslog.conf file :
#Adding a log selector that matches every facility and priority and give the file name to get the log messages ...
*.*             /var/log/myCustom.log
#Save the file

#Restart the "rsyslog.service: service :
systemctl restart rsyslog.service

#Use tail command to get the dynamic output in the terminal :
#Syntax:
tail -f <pathToFile>
#Example:
tail -f /var/log/myCustom.log

#Try to switch user to another user and check myCustom.log file
[root@localhost ~]# su - vnalbalwar
#You will get the log messages about the another user logged in.

Log Rotation

• Log rotation is the process of managing log files by automatically archiving, compressing, and eventually removing old log files to prevent them from growing too large and consuming excessive disk space.

• We can configure or customize the log rotation according to your specific requirements.

• Just navigate to the file "logrotate.conf" and edit it according to your need.

• Options like rotate files weekly, create new log files after rotating the old ones can be customized.

YUM & RPM

• YUM and RPM are the package installer in Linux.

YUM :

• YUM stands for "Yellowdog Updater, Modified".

• It is a high-level package management utility.

• When you request to install or update a package, Yum checks for and installs any required dependencies as well.

• Yum fetches packages from software repositories.

Yum Commands :

#Syntax:

#Used to install a package from the configured repositories
yum install <packagename>

#The -y option is used to automatically answer "yes" to any prompts during the installation process
yum install <packagename> -y

#Used to remove or uninstall a package from the system
yum remove<packagename>

#The -y option automatically answers "yes" to any prompts during the removal process
yum remove <packagename> -y 

#Lists all installed packages that match the specified <packagename>
yum list <packagename>

#Searches the repositories for packages that match the specified <packagename>
yum search <packagename>

#Displays detailed information about the specified <packagename>
yum info <packagename>

#Updates all installed packages to their latest available versions
yum update -y

RPM :

• RPM stands for Red Hat Package Manager.

• It s a low-level package management system used in RPM-based Linux distributions.

• RPM does not automatically handle package dependencies.

• You must manually install any required dependencies before installing an RPM package.

RPM Commands :

#Syntax:

#Used to install an RPM package, where, 
#'i' option indicates installation, 
#-v enables verbose output during installation, 
#-h displays a hash mark progress indicator during installation
rpm -ivh <packagename>

#Queries an installed RPM package to retrieve a list of files it contains
rpm -qpr <packagename>

#The --nodeps option allows the installation to proceed even if there are unmet dependencies
rpm -ivh --nodeps <packagename>

#Queries the system to check if a package with the specified name is installed
rpm -q <packagename>

#Lists all installed packages sorted by their installation timestamp, with the most recently installed package displayed at the top.
rpm -qa -last

#Lists all installed packages on the system
rpm -qa 

#Used to upgrade an already installed package to a newer version
rpm -uvh <packagename>

#Used to uninstall or remove an installed package
rpm -evv <packagename>

APT Commands :

#Syntax:

#Updates the local package database with the latest information from the repositories
apt-get update -y

#Verifies that the package dependencies are met and the package files are intact
apt-get check <packagename>

#Upgrades the specified packages to their latest available versions
apt-get upgrade <packgame>

#Installs a specific version of a package
apt-get install <pkgname>=<versionno>

#Installs multiple packages in a single command
apt-get install <pkg1> <pkg2>

#Downloads the specified packages without installing them
apt-get install -download-only <pkgname>

Repositories

• A repository is a centralized storage location that contains software packages, metadata, and information about dependencies.

• It serves as a source for obtaining and managing software packages for an operating system.

• The repo's are stored at "/etc/yum.repos.d/redhat.repo", you can navigate there using the cd command and look up the file.

Journalctl

• Journalctl is a command-line utility in Linux that allows you and is responsible to access, view and collect logs from the systemd journal.

• The systemd journal is a centralized logging system that collects and stores log messages generated by various components of the system.

Cron

• Cron is a time-based job scheduling utility, in short, it's a scheduler.

• It allows users to schedule and automate the execution of commands or scripts at specific intervals, such as daily, weekly, monthly, or at specific times of the day.

Cron Commands :

#Syntax:

#Opens the crontab file for editing
#It allows you to add, modify, or remove cron jobs for the current user.
crontab -e

#Lists the current user's crontab entries, displaying the scheduled cron jobs.
crontab -l

#Removes the current user's crontab entries, removing all scheduled cron jobs.
crontab -r

To define cron jobs :

#There are 6 fields in crontab file :
#Syntax:
<minute> <hours> <date> <month> <day> <command>

Crontab Example:

E.g. Write a cron job to print "Hello Linux" in a file every 1 minute forever.

#Create a file sayHelloForever at /tmp/ directory...
[root@localhost tmp]# touch sayHelloForever

#Open the crontab file :
[root@localhost ~]# crontab -e

#Enter the fields in the above manner given...
* * * * * echo "Hello Linux" >> /tmp/sayHelloForever
#Save the file

#To get the dynamic output in terminal use tail command...
[root@localhost ~]# tail -f /tmp/sayHelloForever
Hello Linux
Hello Linux
....

#You can now list the crontab using...
[root@localhost ~]# crontab -l
* * * * * echo "Hello Linux" >> /tmp/sayHelloForever

Pranav Jambare

Find Command:

• The find command is used to search for files and directories based on various criteria such as name, size, type, etc.

• It starts searching from a specified directory provided in the command and traverses the entire directory structure.

• It scans the file system dynamically in real time, examining each file and directory individually.

Thus, it may take longer to complete, especially in large file systems.

#Syntax:
 find <path> <expression>

To find files based on their name in Linux:

#Syntax: (Case-Sensitive Search)
find <path> -name <filename>

#Example:
#To find the file named sampleFile in /tmp directory:
[root@localhost /]# find /tmp -name sampleFile
/tmp/myCollab/sampleFile

#Syntax: (Case-Insensitive Search)
find <path> -iname <filename>

#Example:
#To find the file named sampleFile in /tmp directory:
[root@localhost /]# find /tmp -iname SamPLefile
/tmp/myCollab/sampleFile

To find the files owned by the specific user in Linux:

#Syntax:
find <path> -user <username>

#Example:
# To search for files owned by testuser starting from the root directory:
[root@localhost /]# find / -user testuser

#To search for files owned by vnalbalwar starting from the /tmp directory:
[root@localhost /]# find /tmp -user vnalbalwar

To find files and delete them using the rm command in Linux:

#Syntax:
find <path> -name <filename> -exec rm -rf {} \;

#Example:
# The sampleFile will get removed:
[root@localhost /]# find /tmp -name sampleFile -exec rm -rf {} \;

To find files with specific permission in Linux:

#Syntax:
find <path> -perm <permissionNum> 

#Example:
#To search for files having permission 700 starting from root directory: 
[root@localhost /]# find / -perm 700

To find the directories or files in Linux:

#Syntax: (For searching directories)
find <path> -type d 

#Syntax: (For searching files)
find <path> -type f

#Example:
#Find all the directories starting from root directory :
[root@localhost /]# find / -type d

#Find all the files starting from /tmp directory:
[root@localhost /]# find /tmp -type f

Locate Command:

• The locate command is used to statically and quickly find files and directories by searching in the pre-built database.

• It is faster than the find command but may not provide real-time results as the database needs to be updated periodically.

To update the database :

#Syntax:
updatedb

#Syntax:
locate <keyword>

#Example:
locate sampleFile
/home/vnalbalwar/sampleFile

SSH:

• SSH stands for Secure Shell.

• It provides a secure way to access and manage remote systems.

• It allows you to securely connect to a remote server and execute commands, transfer files, etc.

• When you establish an SSH connection, the data transmitted between the client and the server is encrypted.

• There are two cryptographic keys for SSH :

  • Public Key: The public key is used for encryption and can be freely shared with others. When someone wants to send you encrypted data, they will use your public key to encrypt it, ensuring that only you, with the corresponding private key, can decrypt and access the information.

  • Private Key: The private key is kept secret and should never be shared. It is used to decrypt messages encrypted with your public key and to prove your identity during the SSH authentication process.

Steps to create an SSH connection:

• To setup an SSH connection you need these packages installed on your system: (openssh-server, openssh-client)

#Syntax:
yum install <packagename>

#Installing required packages...
yum install openssh-server
yum install openssh-client

• To check whether port 22 is active/open or not for SSH establishment:

netstat -tunlp|grep 22

• To check the status of the service:

#Syntax:
systemctl status <servicename>

#Checking...
systemctl status sshd

• To start, stop & restart the service:

#Start
systemctl start <servicename>

#Stop
systemctl stop <servicename>

#Restart
systemctl restart <servicename>

• To establish the connection between the client and the remote system:

#Syntax:
ssh <username>@<userIP>

#Example:
ssh vnalbalwar@192.168.0.109

To allow and deny users to connect to the specific server:

#You can navigate to /etc/ssh/sshd_config :
[root@localhost ~]# vim /etc/ssh/sshd_config

#Add these commands there and save the file : (Syntax):
AllowUsers <username>
DenyUsers <username>

#Example
AllowUsers viraj
DenyUsers thisIsMe

#Now restart the service...
[root@localhost ~]# systemctl restart sshd
#And check status...
[root@localhost ~]# systemctl status sshd

• To allow and deny the networks to connect to the specific server:

#You can navigate and edit the file /etc/host.allow or /etc/host.deny
[root@localhost ~]# vim /etc/host.allow

#Add an entry in the hosts.allow file to allow access from the specific IP address:
#Syntax:
<service>: <IP address>
#Example:
sshd: 192.168.0.109
#Restart the ssh service
[root@localhost ~]# systemctl restart sshd
#Same can be done for host.deny file by adding the IP address those which should not connect to the server

Password-Less SSH:

• Password-less SSH is a method of securely accessing remote systems or servers without the need for a password.

• It relies on cryptographic key pairs, consisting of a private key and a public key, to authenticate the connection between a client and a server.

• It can be considered more secure than passwords since it uses public-private key cryptography.

Steps to setup the Password-Less SSH:

• Generating the SSH keys on the local machine:

• The primary purpose of the "ssh-keygen" command is to generate a new SSH key pair.

• It prompts the user for information and creates both a private key and a corresponding public key.

#Command:
ssh-keygen

• The default directories for public and private keys are :

#For public key:
/home/<userdirectory>/.ssh/id_rsa.pub

#For private key:
/home/<userdirectory>/.ssh/id_rsa

Copying the user's public key to the remote server, and enabling the SSH authentication:

#Command:
ssh-copy-id -i id_rsa.pub username@IP

#Example:
ssh-copy-id -i id_rsa.pub root@192.168.0.5

Where,

ssh-copy-id --> It is used to copy a user's public key to a remote server.

-i id_rsa.pub --> Specifies the path to the public key file that will be copied.

root@192.168.0.5 --> Represents the username and IP address of the remote server.

Establishing the connection with the client:

• We can now connect to the remote server using SSH without any password.

#Syntax:
ssh username@IP

#Example:
ssh root@192.168.0.5

Pranav Jambare

Soft Link:

• A soft link is also known as a symbolic link or symlink.

• It is a special type of file that acts as a pointer or as a reference to another file or directory.

• If the original file is deleted, the soft link will be pointing to a file that does not exist anymore and will be considered a corrupted link.

• Creating a Soft Link File :

COPY

#Syntax:
ln -s <mainFile> <SLFile>

#Example:
ln -s myFile mySLFile

• By default, the link count of a file in Linux is always 1.

• Verifying the linking of the soft link file with the original file :

COPY

#Verifying...
ls -l
lrwxrwxrwx 1 root root 8 Jul 14 12.00 mySLFile -> myFile

Hard Link:

• A hard link acts as a copy of the original file.

• It accesses the data available in the original file.

• If the original file is deleted, the hard link to the file will still contain the data of that file.

• After the hard link is created, the link count of both the original file and the hard link file becomes "2".

• Creating a Hard Link File :

COPY

#Syntax:
ln <mainFile> <HLFile>

#Example:
ln myFile myHLFile

• Verifying the Hard Link File :

COPY

#Verifying...
ls -l
#The link count becomes 2 after the hard link file is created...
-rw-rw-r-- 2 root root 27 Jul 14 12:00 myHLFile
-rw-rw-r-- 2 root root 27 Jul 14 12:00 myFile

Permissions:

• Permissions are a set of attributes that determine the access rights and restrictions for files and directories.

• There are three types of permissions: Read, Write, and Execute.

• These permissions are assigned to three different categories of users: the owner of the file, the group associated with the file, and other users who are not the owner or part of the group.

• Read (r): Allows a user to view the contents of a file or list the contents of a directory.

• Write (w): Allows a user to modify or delete a file, or create, delete, or rename files within a directory.

• Execute (x): Allows a user to execute a file as a program or access a directory to traverse its contents.

• Permissions can be displayed using the following command :

COPY

#Syntax:
ls -l <filename>
-rw-rw-r-- 1 myUser myGroup 27 Jul 14 12:00 mainFile

• The permissions are represented as (r, w, x) for each category, placed in the following order: owner, group, and others.

• According to the above statement,

  • (-rw-rw-r--) --> The owner has read and write permissions, the group has read permission, and others have read permission as well.

  • 1 --> Indicates the link count number.

  • myUser --> Owner of the file

  • myGroup --> Group owner of the file

  • 27 --> Size of the file

  • Jul 14 12.00 --> Timestamps

  • mainFile --> Name of the file

Permissions and Corresponding Commands for Files and Directories :

Max Permission for Files and Directories :

For Files :

• Ideally, the max permission for files is "777".

• However, in practice, it is common to assign "666" permissions to files.

• Assigning "666" permissions to files is often sufficient for most regular files, as they do not need to be executed as programs.

• This permission allows the owner, group, and others to read and modify the file contents but does not allow the file to be executed as a program.

For Directories :

• The max permission that can be assigned to directories is "777".

• However, in practice, it is more common to assign directories permission of "755".

Default Permissions of Files and Directories :

For Files as the root user:

• When the root user creates a file, the default permissions are set to "644".

• This means the owner has read and write permissions (rw-).

• The group owner of the file has read permissions (r--).

• Others have read permissions (r--).

For Files as a normal user:

• When a normal user creates a file, the default permissions are set to "664".

• This means the owner has read and write permissions (rw-).

• The group owner of the file has read and write permissions as well (rw-).

• Others have read permissions (r--).

For Directories as the root user:

• When the root user creates a directory, the default permissions are set to "755".

• This means the owner has read, write, and execute permissions (rwx).

• The group owner of the file has read and execute permissions (r-x).

• Others have read and execute permissions (r-x).

For Directories as a normal user:

• When a normal user creates a directory, the default permissions are set to "775".

• The owner has read, write, and execute permissions (rwx).

• The group owner of the file has read, write, and execute permissions as well (rwx).

• Others have read and execute permissions (r-x).

To manually assign permissions to a file in Linux:

COPY

#Syntax:
chmod <permission> <filename>

#Example:
chmod 554 myLinuxFile

#Verify...
ls -l myLinuxFile
-r-xr-xr--

To manually change the user and group of a file in Linux:

COPY

#Syntax:
chown <testuser>:<testgroup> <filename>

#Example:
chown robot:classmate myLinuxFile

#Verifying...
ls -l myLinuxFile
-r-xr-xr-- 1 robot classmate 23 Jul 14 12:00 myLinuxFile

To manually only change the user of a file in Linux:

COPY

#Syntax:
sudo chown <testuser> <filename>

#Example:
sudo chown robot myLinuxFile

#Verifying...
ls -l myLinuxFile
-r-xr-xr-- 1 robot root 23 Jul 14 12:00 myLinuxFile

To manually only change the group of a file in Linux:

COPY

#Syntax:
chgrp <testgroup> <filename>

#Example:
chgrp classmate myLinuxFile

#Verifying...
ls -l myLinuxFile
-r-xr-xr-- 1 robot classmate 23 Jul 14 12:00 myLinuxFile

Umask:

• The umask (user file-creation mode mask) is a value that determines the default permissions assigned to newly created files and directories in Linux.

• The umask value is subtracted from the max permissions to determine the resulting default permissions.

• The umask value is usually set in the shell initialization files such as ~/.bashrc which allows it to be applied automatically when a new shell session is started.

• It can also be modified by using the umask command in the terminal.

COPY

#Syntax:
umask <umaskNumber>

#Example:
umask 055

#Verifying...
umask 
0055

Special Permission:

• Special Permission refers to additional rights assigned to files and directories beyond the read, write, and execute permissions.

• There are three commonly used special permissions: Sticky bit, setuid (SUID), and setgid (SGID).

Sticky Bit

• When the sticky bit is set on a directory, only the owner of a file within that directory, the directory owner, or the superuser can delete or rename the file.

• Other users, even if they have write permissions to the directory, cannot delete or rename files that they do not own.

• Note: The directory should be provided with full permission along with a sticky bit.

# Already created a directory with default permission :
[root@localhost tmp]# ls -ld myDir
drwxr-xr-x. 2 root root 6 Jul 16 12.00 myDir

#Assigning the Sticky Bit (1) and full permission to the directory at once: 
[root@localhost tmp]# chmod 1777 myDir

#Verifying the permission applied...
#The sticky bit is represented by the 't' permission bit.
[root@localhost tmp]# ls -ld myDir
drwxrwxrwt. 2 root root 6 Jul 16 12.00 myDir

• Now create a file "tryToDelete" inside the myDir directory and try to delete it using another user who is not the owner of the file, and not a directory owner too, the operation won't permit.

#Creating a file in myDir
[root@localhost myDir]# touch tryToDelete

#Writing a small message:
[root@localhost myDir]# echo "Hello this is a file created by root user" > tryToDelete

#Switch the user:
[root@localhost myDir]# su - vnalbalwar

#Traversing to the tryToDelete file :
[vnalbalwar@localhost ~]$ cd /tmp/myDir

#Trying to delete the file :
[vnalbalwar@localhost myDir]$ rm -rf tryToDelete
rm: cannot remove 'tryToDelete': Operation not permitted

#Deleting using the root user:
[root@localhost myDir]# rm -rf tryToDelete
[root@localhost myDir]# ls -l
total 0

SUID

• A file with a SUID set always executes as the user who owns the file, regardless of any user executing the command.

• The setuid permission is represented by the 's' permission bit in the user execution field.

• For example, the 'passwd' command needs to update the password file, which requires root privileges.

• By setting the setuid bit on the 'passwd' executable, regular users can execute it and change their passwords.

# Trying to list the files and directories under root with normal user :
[vnalbalwar@localhost ~]$ ls -l /root/
ls: cannot open directory /root: Permission denied

[vnalbalwar@localhost ~]$ ls -l /usr/bin/ls
-rwxr-xr-x. 1 root root 117608 Jul 21 2020 /usr/bin/ls

#Switching to root user and assigning the SUID to /usr/bin/ls :
# SUID = 4
# You can use this command too : chmod u+s <path>
[root@localhost ~]# chmod 4755 /usr/bin/ls
[root@localhost ~]# ls -l /usr/bin/ls
-rwsr-xr-x. 1 root root 117608 Jul 21 2020 /usr/bin/ls

#Switching back to normal user and now trying to list the /root:
[vnalbalwar@localhost ~]$ ls -l /root/
-rw-------. 1 root root 1907 Jul 5 12.00 anaconda-ks.cfg 

#To remove the SUID :
[root@localhost ~]# chmod u-s /usr/bin/ls

#Verifying...
[root@localhost ~]# ls -l /usr/bin/ls
-rwxr-xr-x. 1 root root 117608 Jul 21 2020 /usr/bin/ls

SGID:

• When the SGID permission is set on a directory, any new files or directories created within it inherit the group ownership of that directory instead of the primary group of the user who created them.

• This is useful when multiple users need to collaborate on projects and share files within a common group.

• The SGID permission is represented by the 's' permission bit in the group execution field.

#The directory is myCollab and root is owner and group owner of the file:
[root@localhost tmp]# ls -l
drwxr-xr-x. 2 root root 6 Jul 16 12.00 myCollab

#Creating two files under the myCollab directory:
[root@localhost myCollab]# touch file1 file2
[root@localhost myCollab]# ls -l 
-rw-r--r--. 1 root root 0 Jul 16 12.00 file1
-rw-r--r--. 1 root root 0 Jul 16 12.00 file2

#Changing the group owner to testgroup of the myCollab directory:
[root@localhost tmp]# chown root:testgroup /tmp/myCollab
[root@localhost tmp]# ls -l
drwxr-xr-x. 2 root testgroup 32 Jul 16 12.00 myCollab

#Assigning the SGID, 's' indicates the permission bit in group field
[root@localhost tmp]# chmod g+s myCollab
[root@localhost tmp]# ls -l
drwxr-sr-x. 2 root testgroup 32 Jul 16 12.00 myCollab

 #Create a file called file3 in myCollab and check that the group owner is testgroup now:
[root@localhost myCollab]# touch file3
[root@localhost myCollab]# ls -l file3
-rw-r--r--. 1 root testgroup 0 Jul 16 12.00 file3

#How to remove SGID from directory:
[root@localhost tmp]# chmod g-s myCollab

#Verifying...
[root@localhost tmp]# ls -l
drwxr-xr-x. 2 root testgroup 45 Jul 16 12.00 myCollab

Pranav Jambare

Remote Copying Tools:

1 . SCP :

• SCP stands for secure copy command in Linux.

• It is used to copy the files between the servers securely.

• It allows the secure transfer of files between the local host and the remote host or between two remote hosts.

#Syntax :
scp <filename> <remoteuser>@<remoteuserIP>:<destination>

#Example:
scp myLinuxFile root@192.168.100.7:/root

2 . RSYNC:

• RSYNC stands for remote synchronisation command in Linux.

• It synchronizes files from a source to a destination.

• It is famous for its delta-transfer algorithm, which reduces the amount of data sent over the network by sending only the differences between the source files and the existing files in the destination.

• RSYNC finds files that need to be transferred using a checksum algorithm that looks for files that have changed in size or in the last-modified time.

#Syntax:
rsync -av <filename> <remoteuser>@<remoteuserIP>:<destination>

#Example:
rsync -av myLinuxFile root@192.168.100.7:/root

3 . SFTP:

• SFTP stands for Safe File Transfer Protocol.

• It is a secure variant of FTP.

• It is a part of the SSH Protocol and is designed to securely transfer files between remote systems via a network connection.

#Syntax:
sftp <username>@<userIP>

#Example:
sftp linuxUser@192.168.100.7

Archiving Tools:

• Archiving is the process of combining multiple files into a single package called an archive.

• This archive can then be easily backed up, or simply kept on your machine as a way to organize your file system.

1 . gzip

#Syntax : (Compress)
gzip <filename>

#Example : 
gzip myFile

#Syntax : (Decompress)
gunzip <filename>.gz

#Example : 
gunzip myFile.gz

2 . xz

#Syntax: (Compress)
xz <filename>

#Example:
xz myFile

#Syntax: (Decompress)
unxz <filename>.xz

#Example:
unxz myFile.xz

3 . bzip2

#Syntax: (Compress)
bzip2 <filename>

#Example:
bzip2 myFile

#Syntax: (Decompress)
bzip2 <filename>.bz2

#Example:
bzip2 myFile.bz2

Backup:

Fig. Regular files being added to an archive with tar and then compressed with gzip.

• The tar utility is also able to compress an archive via a compression tool.

• To create an archive file :

#Syntax:
tar -cvzf <destn.backup> <sourcefile>

#Example:
tar -cvzf myLinuxFile.backup file1 file2 file3

• To extract an archive file:

#Syntax:
tar -xvf <destn.backup>

#Example:
tar -xvf myLinuxFile.backup

User Management:

• A user is an entity, that can manipulate files and perform several other operations.

• Each user is assigned an ID that is unique for each user in the operating system.

• The ID 0 is assigned to the root user and the IDs 1 to 999 are assigned to the system users, hence the ids for the local user begin from 1000 onwards.

1 . Adding the User :

• The command "useradd" or "adduser" can be used to create a new user in Linux also, the sudo permissions are required to perform this command.

    #Creating a User:
    root@myLinux:~# useradd testuser
    #OR
    root@myLinux:~# adduser testuser
  

2 . Verifying the User:

root@myLinux:~# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser)

3 . Deleting the User:

root@myLinux:~# userdel testuser

4 . Managing the password policy:

Fields of /etc/password file:

#The sudo permissions are required to perform this command
root@myLinux:~# cat /etc/passwd|tail -1
testuser:x:1001:1001::/home/testuser:/bin/sh

Where,

testuser --> Name of user

x --> Encrpyted password

1001 --> User id

1001 --> Group id

:: --> Nickname (Con-name)

/home/testuser --> Home directory of the user

/bin/bash --> Default shell of user

Setting the password :

#The sudo permissions are required to perform this command
root@myLinux:~# passwd testuser
#Next...
New password: 123 
Retype new password: 123
passwd: password updated successfully

Changing the password policy:

root@myLinux:~# chage -M 2 -m 5000 -W 4 -I 12 -E 2024-12-2

Where,

-M --> maxdays password change date

-m --> mindays password change date

-W --> warning

-I --> Inactivity

-E --> Account Expiry date

5 . Change / Restrict the User:

Give "No Login" to the User:

root@myLinux:~# usermod testuser -s /sbin/nologin

#Verifying...
root@myLinux:~# su - testuser
Cannot change directory to /home/testuser:   
No such file or directory
This account is currently not available.

To revert the changes to get a login:

root@myLinux:~# usermod testuser -s /bin/sh

#Verifying...
root@myLinux:~# su - testuser
Last Login : .....
$whoami
testuser

6 . Lock and Unlock User:

# To Lock the user:
root@myLinux:~# usermod -L testuser

# To Unlock the user:
root@myLinux:~# usermod -U testuser

7 . Adding Group:

#The sudo permissions are required to perform this command
root@myLinux:~# groupadd testgroup

#Verifying the group...
root@myLinux:~# cat /etc/group|tail -1   
testgroup:x:1002:

8 . Adding a user to a group:

root@myLinux:~# usermod -G testgroup testuser 

#Verifying...
root@myLinux:~# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser),1002(testgroup)

9 . Deleting a group:

root@myLinux:~# groupdel testgroup

#Verifying...
root@myLinux:~# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser)

10 . Change the user-id of the user:

root@myLinux:~# usermod -u 1005 testuser

#Verifying...
root@myLinux:~# id testuser
uid=1005(testuser) gid=1001(testuser) groups=1001(testuser)

We started with Linux Introduction:

Operating System

• OS is an interface between computer users and hardware.

• Manages all other application programs on a computer.

• Performs all the basic tasks like file management, memory management, process management, and handling inputs and outputs.

• E.g. Windows, Fedora, Mac, Ubuntu, Redhat, Suse, etc.

Why choose Linux?

• Free and Reliable.

• Open Source and Secure.

• Privacy and Updates.

• Can be customized and has better community support.

Kernel

• The kernel is the heart of any operating system.

• Acts as a middleware (interpreter) between shell and hardware.

• It has all the code required for OS to execute.

• If the kernel crashes, OS crashes too.

Shell

• It is a terminal and allows a user to give commands to the kernel.

• Acts as an interface between a user and the kernel.

Yanking

In the terminal, you can cut, copy and paste a line with the following commands.

E.g.

• yy - Used to copy a line. Move the cursor to the line you want to copy and then press yy.

• p - Paste a copied or cut line after the current line.

• dd (cut) - Deletes current line completely.

Many commands, such as cc, gg, G, :!<command>, and :set nu, can also be executed in the Vi/Vim editor.

Redirection

Imagine you have a command that produces some output on the screen. With redirection, you can choose to send that output or error to a file instead of displaying it on the screen. This way, you can save the output or error for later use or analyze it more conveniently.

Commands :

• Used to search for directories owned by a specific user

find / -user <username>

• With the addition of "2> <errorfile>", any error messages generated by the "find" command during the search process will be redirected to the specific "<errorfile>". This way, you can store the error messages separately while the regular output is displayed on the screen.

find / -user <username> 2> <errorfile>

• With the addition of "> <outputfile>", outputs generated by the "find" command during the search process will be redirected to the specific "<outputfile>". This way, you can store the outputs separately while only the errors will be displayed on the screen.

find / -user <username>> <outputfile>

• "find" will search for directories owned by the specified user, and the output will be redirected to the "<outputfile>". Any errors generated during the search will be redirected to the "<errorfile>".

find / -user <username>> <outputfile> 2> <errorfile>

• Both the regular output and any error messages generated during the search will be removed and not displayed or saved anywhere.

find / -user <username> &> /dev/null

• This command is used to search directories owned by a specific user and save the output to both the terminal and a file.

find / -user <username> | tee <filename>

Modes

• ESC Mode

• Insert Mode

• wq! (Save and Quit)

• v (Visual Mode)

Finally...

I enjoyed the session conducted by Pranav Jambare . Just remember to constantly put in your hard work and believe it will be worth it one day!