All About Linux

Linux folder usage

File Permissions

Umask

The umask (short for "user file creation mode mask") is a Linux/Unix system setting that determines the default permissions for newly created files and directories. It effectively "masks out" certain permission bits, preventing them from being set when new files or directories are created.

Understanding File and Directory Permissions
In Linux, file and directory permissions are represented by three sets of bits, each corresponding to the user (owner), group, and others. These are usually represented as three digits in octal (base-8), where each digit can be a combination of:

4: Read (r)
2: Write (w)
1: Execute (x)

For example:

7 (4 + 2 + 1) means read, write, and execute permissions.
6 (4 + 2) means read and write permissions.
5 (4 + 1) means read and execute permissions.

666 defined as read and write for everyone.

Default Permissions Before umask
Files: The default permissions for a newly created file are usually 666 (read and write for everyone). Files do not have execute permissions by default.
Directories: The default permissions for a newly created directory are 777 (read, write, and execute for everyone).
How umask Works
The umask subtracts (or masks out) permissions from these defaults. The umask value is subtracted from the default permissions to determine the actual permissions of a new file or directory.

For example:

A umask of 022 masks out the write permission for the group and others:
Files: 666 (default) - 022 = 644 (read and write for the owner, read-only for group and others).
Directories: 777 (default) - 022 = 755 (read, write, and execute for the owner, read and execute for group and others).

Common umask Values 022: Common default value, resulting in 755 for directories and 644 for files.
002: Allows group members to have write permissions, resulting in 775 for directories and 664 for files.
077: Highly restrictive, giving full access only to the owner, resulting in 700 for directories and 600 for files.

chmod

Chmod is use for Modify permission of file/directory

#drwxrwxrwx > rwx abbreviates as Read, Write , Delete/Execute

chmod 777 "file-name/folder-name"

Chown is use for providing permissions or changing the ownership to other User

#change Owner/User
chown "user-name" "file/folder-name"

#change Owner and Group
chown "user-name":"group-name" "file/folder-name"

Chmod calculator

Steps to Add New User

# To add user in linux
sudo adduser <user-name>  #it will ask for password enter new password

# After adding the user, you can switch to the new user's shell environment using the following command:
su - <user-name>

# after login with new user, try to execute
apt-get update #you may be get error as incident will be reported.

# update the permissions for new user, intially logout from the new user
exit

# from the root user, execute the following command
sudo visudo

# update members of the admin group

%<username> ALL=(ALL) NOPASSWD:ALL     #add this line

Change host name

sudo hostnamectl set-hostname <enter-hostname>
/bin/bash

Linux commands

Mostly Used Commands

Description	Command
Help for commands	man <command>
Change directory	cd <directory>
List directory contents	ls
Print working directory	pwd
Create a new directory	mkdir <directory>
Remove an empty directory	rmdir <directory>
Delete directory (recursively)	rm -r <directory>
Remove files	rm <file>
Concatenate and display file content	cat <file>
Create an empty file	touch <file>
Edit/create file with content	vim <file>
Vim commands	Insert: i, Save and exit: ESC :wq, Exit without saving: ESC :q!
Move or rename files and directories	mv <source> <destination>
Copy file	cp <source-file> <destination-file>
Copy file to a directory	cp <file> <directory>
Modify file/directory permissions	chmod <permissions> <file>
Change file/directory ownership	chown <user> <file>
Change ownership and group of file	chown <user>:<group> <file>

File Compression and Archiving

Description	Command
Create a tar archive	tar cf <archive.tar> <file/dir>
Extract a tar archive	tar -xvf <archive.tar>
Compress files using gzip	gzip <file>
Decompress files using gunzip	gunzip <file>
Create a compressed tar file	tar -czvf <archive.tar.gz> <file/dir>
Extract a compressed tar file	tar -xzvf <archive.tar.gz>
Compress files using bzip2	bzip2 <file>
Extract a ZIP file	unzip <file.zip>

Networking Commands

Description	Command
Test network connectivity	ping <host>
Display network configuration	ifconfig
Display network connections	netstat
Display system routes	route
DNS lookup	nslookup <domain>
Detailed DNS lookup	dig <domain>
Trace network path	traceroute <host>
Secure copy files	scp <source> <user>@<host>:<destination>
Synchronize files/directories remotely	rsync <source> <destination>
SSH into a remote machine	ssh <user>@<host>

System Monitoring and Management

Description	Command
Display disk usage	df -h
Show directory/file disk usage	du -sh
Display CPU usage	top
Display CPU, RAM usage	htop
Display RAM memory usage	free -h
Display uptime and load averages	uptime
Display running processes	ps aux
Display real-time system processes	top
Terminate a process	kill <PID>
Force terminate a process	kill -9 <PID>
Display kernel messages	dmesg
View system journal logs	journalctl

User and Group Management

Description	Command
Add a new user	useradd <username>
Change user password	passwd <username>
Switch to another user	su <username>
Display current user	whoami
Add a new group	groupadd <group>
Delete a group	groupdel <group>
Modify a user	usermod
Modify a group	groupmod

Utilities

Description	Command
Search for files	find <path> -name <pattern>
Search for text in files	grep <pattern> <file>
Count lines, words, characters in file	wc <file>
Display first N lines of a file	head <file>
Display last N lines of a file	tail <file>
Sort lines in a file	sort <file>
Remove duplicate lines in a file	uniq <file>
Redirect output to multiple files	tee <file>
Monitor command output	watch <command>
Display a calendar	cal

Miscellaneous

Description	Command
Display system date and time	date
Display hardware information	lshw
Display PCI devices	lspci
Display USB devices	lsusb
Locate files	locate <file>
Find command binary path	which <command>
Display environment variables	env
Print a message to terminal	echo <message>
Display command history	history
Schedule recurring tasks	crontab -e
Schedule one-time tasks	at <time>
Mount a filesystem	mount <device> <directory>
Unmount a filesystem	umount <device>
Create symbolic link	ln -s <target> <link>
Change root directory	chroot <directory>
Generate checksum	sha256sum <file>

Other commands

• Clean up the packages:

  rm -rf /var/lib/apt/lists/* - This command used to clean up the package list cache after installing packages, especially in Docker, to reduce image size.

  Advantages: Reduces image size and keeps environments clean.

  Disadvantages: Deletes package metadata, requiring you to run apt-get update again before installing or upgrading packages. Not suitable for long-running environments where package management might be needed later.

• dev/null

  In Linux, /dev/null is a special device file known as the "null device" or "null file." It discards anything written to it and immediately returns an end-of-file (EOF) to any process that reads from it. You can think of /dev/null as a "black hole" for data—it simply deletes anything sent to it. This can be useful in several scenarios, especially when you want to suppress output/error or ignore specific data streams.

  Common Uses of /dev/null:

  1. Suppress Command Output.
     You may want to run a command without displaying any output. By redirecting the output to /dev/null, you effectively ignore it.

     Example: Suppresses standard output

     ls > /dev/null
     

     In this example, ls would normally list directory contents, but by redirecting > it to /dev/null, the output is discarded and not shown in the terminal.

  2. Suppress Error Messages.

     Sometimes, you only want to discard error messages, which are typically directed to the standard error (stderr) stream (file descriptor 2).

     Example: Suppresses only the error output

     ls nonexistentfile 2> /dev/null
     

     This command tries to list a file that doesn’t exist. Normally, it would produce an error message, but by redirecting 2> to /dev/null, you discard only the error output and keep the standard output unaffected.

  3. Suppress All Output (Standard and Error)

     In some cases, you may want to suppress both standard output and error messages from a command.

     Example: Suppresses both standard output and error output

     ls nonexistentfile > /dev/null 2>&1
     

     Here, > redirects standard output, and 2>&1 redirects standard error to the same place as standard output, which is now /dev/null. This effectively hides all output from ls, including any error messages.

  4. Run Command in Background Without Output

     When running a command in the background, the output can clutter the terminal. Sending output to /dev/null avoids this.

     Example: Runs ping in the background without showing output

     ping -c 4 google.com > /dev/null 2>&1 &
     

     This runs ping in the background for 4 packets, discarding all output, so you won’t see any results or messages in the terminal.

  5. Check if a File or Directory Exists Without Output

     Sometimes, you only need to check if a file or directory exists without producing any output. /dev/null helps here by discarding the output of the command.

     Example:

     # Checks if a file exists without output
     if ls /path/to/file > /dev/null 2>&1; then
         echo "File exists."
     else
         echo "File does not exist."
     fi
     

     In this example, ls checks if the file exists. If it does, the script echoes "File exists." If not, it says "File does not exist." The command's output is sent to /dev/null, so you don’t see anything unless there is a specific message in the if or else statement.

     Summary
     Suppress command output: command > /dev/null
     Suppress error messages: command 2> /dev/null
     Suppress both output and error: command > /dev/null 2>&1
     Run in background without output: command > /dev/null 2>&1 &