Part 2: System Level Commands and User Management in Linux

System Information Commands

Display system information:
```
 uname
```
Show how long the system has been running:
```
 uptime
```
Display the current date and time:
```
 date
```
Show who is currently logged in:
```
 who
```
Show the current user:
```
 whoami
```

Bash and User ID Commands

Start a new bash session:
```
 bash
```
Locate the bash executable:
```
 which bash
```
Locate the cp command:
```
 which cp
```
Display user ID and group ID:
```
 id
```

Using `sudo`

Show the contents of the /etc/passwd file:
```
cat /etc/passwd
```
Navigate to the root directory (requires root permissions):
```
cd /root
```
Shutdown the system:
```
sudo shutdown
```
Cancel a scheduled shutdown:
```
sudo shutdown -c
```
Reboot the system:
```
sudo reboot
```
Update package lists:
```
sudo apt-get update
```
Upgrade all packages:
```
sudo apt-get upgrade
```

Install Docker:

sudo apt install docker.io
sudo apt-get install docker.io

Remove Docker:
```
sudo apt remove docker.io
```

Package Managers for Different Distributions

Package manager for CentOS:
```
yum
```
Package manager for Fedora:
```
dnf
```
Package manager for Arch Linux:
```
pacman
```
Package manager for Gentoo:
```
portage
```

User and Group Management

Creating and Managing Users

Add a new user named "rider":
```
sudo useradd -m rider
```
Navigate to the home directory and list users:
```
cd ..
ls home/
```
Output:
```
dhruv rider
```
Set a password for the user "rider":
```
sudo passwd rider
```
Switch to the user "rider":
```
su rider
whoami
```
Output:
```
rider
```
Navigate and list directories:
```
ls ..
cd rider
```
Exit the user session and return to the previous user:
```
exit
whoami
```
Output:
```
dhruv
```

Deleting Users

Delete the user "rider":
```
sudo userdel rider
```
Try switching to the deleted user (should fail):
```
su rider
```

Creating and Managing Groups

Add new users "u1", "u2", "u3", and "u4":

sudo useradd u1
sudo useradd u2
sudo useradd u3
sudo useradd u4

Create a new group named "devops":
```
sudo groupadd devops
```
List groups to verify the creation:
```
cat /etc/group
```
Create another group named "tester":
```
sudo groupadd tester
```

Add users "u1" and "u2" to the "devops" group:

sudo gpasswd -a u1 devops
sudo gpasswd -a u2 devops

Add users "u3" and "u4" to the "tester" group:
```
sudo gpasswd -M u3,u4 tester
```
Delete the "tester" group:
```
sudo groupdel tester
```

File Permissions Table

Permission Breakdown

Directory	User (Owner)	Group	Other
d	rwx	rwx	r-x

Permission Values

Value	Permission	User	Group	Other
0	---	-	-	-
1	--x	-	-	x
2	-w-	-	w	-
3	-wx	-	w	x
4	r--	r	-	-
5	r-x	r	-	x
6	rw-	r	w	-
7	rwx	r	w	x

Legend

r: read
w: write
x: execute

In the above tables, the permission breakdown shows the different levels of access for directories, users, groups, and others. The permission values table provides a detailed view of the numerical representation of permissions and what they stand for.

Understanding file permissions:
- Directory: d
- User (Owner): rwx
- Group: rwx
- Other: r-x

Permission values:

r (read): 4
w (write): 2
x (execute): 1

Display file permissions with ls -l:
```
ls -l
```
Change file permissions:
```
chmod 777 one.txt
ls -l
```

Umask Values

Understanding Umask settings:

| Umask | User Access | Group Access | Other | | --- | --- | --- | --- | | 0000 | all | all | all | | 0002 | all | all | read, execute | | 0007 | all | all | none | | 0022 | all | read, execute | read, execute | | 0027 | all | read, execute | none | | 0077 | all | none | none |

Changing Ownership and Group

Create a new file and display its owner:
```
mkdir riderfile.txt
ls -l
```
Change the file owner to "rider":
```
sudo chown rider riderfile.txt
ls -l
```
Change the file group to "devops":
```
sudo chgrp devops riderfile.txt
```

Compression Commands

Compress a directory using zip:

zip -r lfd.zip linuxfordevops/
cp lfd.zip copy/unzipfiles/
ls copy/unzipfiles/
cd copy/unzipfiles/
unzip lfd.zip

Compress a directory using tar:

tar -cvzf linuxfordevops.tar.gz linuxfordevops
ls -l

Extract a tar file:
```
tar -xvzf linuxfordevops.tar.gz
```

`tar` Command Options and Descriptions

Option	Description
-c	Create a new archive
-v	Verbosely list files processed
-f	Specify the filename of the archive
-z	Compress the archive using gzip
-j	Compress the archive using bzip2
-x	Extract files from an archive
-t	List the contents of an archive
-r	Append files to the end of an archive
-u	Only append files newer than copy in archive
-W	Verify the archive after writing it
-k	Do not overwrite existing files
--delete	Delete files from an archive
--exclude	Exclude files matching a pattern
-C	Change to directory
--strip-components	Strip leading components from file names on extraction

Common Examples

Create a compressed archive:

 tar -cvzf archive-name.tar.gz /path/to/directory

Extract a compressed archive:
```
 tar -xvzf archive-name.tar.gz
```
List contents of an archive:
```
 tar -tvf archive-name.tar.gz
```

Append files to an archive:

 tar -rvf archive-name.tar /path/to/file

Delete files from an archive:

 tar --delete -f archive-name.tar /path/to/file

Extract files to a specific directory:

 tar -xvzf archive-name.tar.gz -C /path/to/destination

These options and examples cover a range of typical tar command use cases for creating, managing, and extracting archive files.

File Transfer with SCP

Create a test directory on the local system:
```
cd Documents
mkdir scp_test
```

Transfer a file from a remote server to the local system:

scp -i "/User/dhruv/Downloads/linux-for-devops-key.pem" secret.file.txt ubuntu@ec2-3-15-221-86.us-east-2.compute.amazonaws.com:/home/dhruv

Verify the transferred file:
```
ls
cat secretfile.txt
```

Recursively transfer a directory from a remote server to the local system:

scp -i "/User/dhruv/Downloads/linux-for-devops-key.pem" -r ubuntu@ec2-3-15-221-86.us-east-2.compute.amazonaws.com:/home/dhruv/linuxfordevops .

Synchronization with Rsync

Synchronize files between local and remote systems using rsync:

rsync -avz source_directory/ user@remote_host:/destination_directory/

This guide continues from Part 1, diving deeper into system-level commands, user and group management, file permissions, and essential file transfer techniques in Linux.