Linux allows multiple users to access and use the system at the same time, and each user can have their own settings and configurations.

Multi-tasking: 

Linux supports multi-tasking, meaning that multiple programs can run simultaneously without interfering with each other.

Command-line interface: 

Linux has a robust file permission system that allows users to control access to files and directories. This can be used to protect sensitive information and ensure that only authorized users can access it.

Customizability: 

Linux is highly customizable, allowing users to tailor the system to their specific needs. This includes the ability to modify the source code and create a custom distribution. 

Stability and reliability: 

Linux is known for its stability and reliability, with many users reporting that their systems run for months or even years without needing to be rebooted.

Large software repository: 

Linux has a wide range of software that is available through repositories, which makes it easy to find and install new software.

Networking support: 

There are certain exceptions in a Linux file system:

• Directories—Files that are lists of other fles.
  
• Special file—The mechanism used for inout and output. /dev are special fles.
  
• Links—A system to make fle or directory visible in multiple parts of the systems.
  
• Sockets—A special fle type, similar to TCP/IP sockets providing inter-process networking.
  
• Pipes—More or less like sockets; they form a way for process to communicate with each other with out using network socket.

 Subdirectories of the root directory:

In Linux, the file system is organized in a hierarchical structure, with the root directory (/) being the top-level directory. All other files and directories are contained within the root directory.

Where '[OPTIONS]' are optional flags that modify the behavior of the 'ls' command and '[DIRECTORY]' is the name of the directory to be listed (the current directory by default).

Here are some common options for "ls":

• -a: List all files, including hidden files and directories (those starting with a dot .).
• -l: Display the contents in a long format, showing detailed information about each file or directory.
• -h: Display file sizes in human-readable format (e.g. KB, MB, GB, etc.).
• -r: Reverse the order of the listing.
• -t: Sort the contents by modification time (newest first).

Example usage:

Inside a folder you can list all the files that the folder contains using the ls command:

If you add a folder name or path, it will print that folder's contents:

This command lists the contents of the directory /bin/ in a long format with human-readable file sizes.

"ls" accepts a lot of options. One of my favorite options combinations is -al. Try it:

compared to the plain ls, this returns much more information. 

You have, from left to right:

• the file permissions (and if your system supports ACLs, you get an ACL flag as well)
• the number of links to that file
• the owner of the file
• the group of the file
• the file size in bytes
• the file modified datetime
• the file name

This set of data is generated by the l option. The “a” option instead also shows the hidden files. 

cd

The cd (change directory) command is used to change the current working directory in Linux. The basic syntax for cd is as follows:

Where [DIRECTORY] is the name of the directory to change to.

• ".": The current directory.
• "..": The parent directory of the current directory.
• "~": The home directory of the current user.

Once you have a folder, you can move into it using the cd command. “cd” means to change the directory. You invoke it by specifying a folder to move into. You can specify a folder name or an entire path.

Now you are in the "newdir" folder.

You can use the ".." special path to indicate the parent folder:

The # character indicates the start of the comment, which lasts for the entire line after it's found.

You can use it to form a path:

There is another special path indicator which is “.”, and indicates the current folder. You can also use absolute paths, which start from the root folder “/”:

This command changes the current working.

cp

mv

The mv (move) command is used to move or rename files and directories in Linux. The basic syntax for mv is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the mv command, SOURCE is the file or directory to be moved, and DESTINATION is the location to move the SOURCE to or the new name for the SOURCE if it is being renamed.

Here are some standard options for mv:

• -v: Verbose, display progress, and other information during the move operation.
• -i: Interactive, prompt for confirmation before overwriting an existing file.
• -f: Force move, overwrite existing files without confirmation.

Example usage:

Once you have a file, you can move it around using the "mv" command. You specify the file's current path and its new path:

This is how you rename files and folders. If you have two directories, you want to move, use the following command:

If the last parameter is a folder, the file located at the first parameter path is going to be moved into that folder. In this case, you can specify a list of files and they will all be moved to the folder path identified by the last parameter:

rm

The rm (remove) command is used to delete files or directories in Linux. The basic syntax for rm is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the rm command and [FILE] is the name of the file or directory to be deleted.

Here are some common options for rm:

• -f: Force deletion, ignoring any prompts or errors.
• -r or -R: Recursively delete a directory and its contents.
• -v: Verbose, display progress and other information during deletion.
• --interactive: Prompt for confirmation before deleting each file.

It is important to use "rm" with caution, as once a file or directory is deleted, it cannot be recovered without a backup or data recovery tool.

Example usage:

Once you have a file, you can remove it using the "rm" command:

You can also remove a directory using the "rm" command:

mkdir

The mkdir (make directory) command is used to create new directories in Linux. The basic syntax for mkdir is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the mkdir command and DIRECTORY is the name of the directory to be created.

Here are some common options for mkdir:

• -p: Create intermediate directories as necessary.
• -v: Verbose, and display progress and other information during the directory creation.

Example usage:

You create folders using the "mkdir" command:

You can create multiple folders with one command:

You can also create multiple nested folders by adding the-poption:

This command creates a directory structure a/b/c with intermediate directories "a" and "b" if they do not already exist.

Options in UNIX commands commonly take this form. You add them right after the command name, and they change how the command behaves. You can often combine multiple options, too.

You can find which options a command supports by typingman <command-name>. Try now withman mkdir for example (press theqkey to esc the man page). Man pages are the amazing built-in help for UNIX.

rmdir

The rmdir (remove directory) command is used to remove empty directories in Linux. The basic syntax for rmdir is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the rmdir command and DIRECTORY is the name of the directory to be removed.

Here are some common options for rmdir:

• -v: Verbose, display progress, and other information during the directory removal.

Example usage:

Just as you can create a folder usingmkdir, you can delete a folder usingrmdir:

You can also delete multiple folders at once:

Note: The folder you delete must be empty

To delete folders with files in them, we'll use the more generic "rm" command which deletes files and folders, using the "-rf" options:

Be careful as this command does not ask for confirmation and it will immediately remove anything
you ask it to remove. 

There is nobinwhen removing files from the command line, and recovering lost files can be hard.

Note: Directories that contain files cannot be removed with "rmdir". To remove a non-empty directory, use the "rm" command with the "-r" option to remove the directory and its contents recursively.

touch

The touch command is used to create new empty files or update the modification timestamps of existing files in Linux. The basic syntax for touch is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the touch command and FILE is the name of the file to be created or updated.

Here are some common options for touch:

• -a: Only update the access time of the file.
• -m: Only update the modification time of the file.
• -c: Do not create a file if it does not exist (no-create).

Example usage:

You can create an empty file using the touch command:

This command creates a new empty file named "newfile.txt" if it does not already exist, or updates its modification timestamp if it exists.

This command updates the access timestamp of the file "file.txt" without modifying its modification timestamp.

chmod

The chmod (change mode) command is used to change the permissions of files and directories in Linux. The basic syntax for chmod is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the chmod command, MODE is the new permission set for the file or directory, and FILE is the name of the file or directory whose permissions will be changed.

The permissions on a file or directory in Linux can be represented as a three-digit octal number, where each digit represents the permissions for the owner, group, and others, respectively. Each digit is a combination of read (r), write (w), and execute (x) permissions. 

For example, rwx means to read, write, and execute permissions, while r-x means to read and execute but not write permission.

The weird strings you see on each file line, likedrwxr-xr-x, define the permissions of the file or folder. 

Let's dissect it. 

The first letter indicates the type of file:

• "-" means it's a normal file
• "d" means it's a directory
• "l" means it's a link

Then you have 3 sets of values:

• The first set represents the permissions of the owner of the file
• The second set represents the permissions of the members of the group the file is associated to
• The third set represents the permissions of everyone else

Those sets are composed of 3 values. rwx means that a specific persona has read, write, and execution access.

Anything that is removed is swapped with a "-", which lets you form various combinations of values and relative permissions: rw-, r--, r-x, and so on.

You can change the permissions given to a file using the chmod command. chmod can be used in 2 ways. The first uses symbolic arguments, and the second is using numeric arguments. Let's start with symbols first, which is more intuitive. 

You type chmod followed by a space and a letter:

• "a" stands for all
• "u" stands for user
• "g" stands for the group
• "o" stands for others

Then you type either '+' or '-' to add permission or to remove it. Then you enter one or more permissions symbols ( r, w, x ). All are followed by the file or folder name. 

Example Usage:

We have a file named "newfile.txt" which has read and write permission for users and groups and only readable permission for others.

To add executable permission for everyone use the following command:

To add read and write to everyone use the following command:

To remove read, write, and Executable permissions from other permissions use the following command:

You can apply the same permissions to multiple permissions by adding multiple letters before the+/-:

In case you are editing a folder, you can apply the permissions to every file contained in that folder using the -r (recursive) flag.

Numeric arguments are faster but I find them hard to remember when you are not using them day to day. You use a digit that represents the permissions of the persona. This number value can be a maximum of 7, and it's calculated in this way:

• 1 if has execution permission
• 2 if has to write permission
• 4 if has read permission

This gives us 4 combinations:

• 0 no permissions
• 1 can execute
• 2 can write
• 3 can write, execute
• 4 can read
• 5 can read, execute
• 6 can read, write
• 7 can read, write and execute

We use them in pairs of 3, to set the permissions of all the 3 groups altogether:

This command adds execute permission for all users for the file "file.sh".

chown

The chown (change owner) command is used to change the owner and/or group of files and directories in Linux. The basic syntax for chown is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the chown command, OWNER is the username or numeric user ID of the new owner of the file or directory, GROUP is the name or numeric group ID of the new group for the file or directory, and FILE is the name of the file or directory whose owner and/or group will be changed.

Here are some standard options for chown:

• -R: Recursively change the owner and/or group of files and directories in a directory and its subdirectories.

Example Usage: 

If you have a file that's owned by root, you can't write to it as another user:

You can use chown to transfer the ownership to you:

Files/directories don't just have an owner, they also have a group. Through this command, you can change that simultaneously while you change the owner:

This command changes both the owner and group of the file "testfile.txt" to user and group respectively.

It's rather common to have the need to change the ownership of a directory, and recursively all the files contained, plus all the subdirectories and the files contained in them, too. 

You can do so using the -R flag:

You can also just change the group of a file using the chgrp command:

ln

The ln (link) command is used to create hard links or symbolic links in Linux. The basic syntax for creating a hard link is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the ln command, TARGET is the name of the original file or directory, and NEW_LINK is the name of the new hard link to be created.

Here are some standard options for ln:

• -s: Create a symbolic link instead of a hard link.

The ln command is part of the Linux file system commands.

It's used to create links. 

What is a link? It's like a pointer to another file. A file that points to another file. You might be familiar with Windows shortcuts.

We have 2 types of links: 

1. Hard links
2. Soft links

1. Hard links

Hard links are rarely used. They have a few limitations: you can't link to directories, and you can't
link to external filesystems (disks).

A hard link is created using the following command:

Now any time you edit any of those files, the content will be updated for both. 

If you delete the original file, the link will still contain the original file content, as that's not removed until there is one hard link pointing to it.

2. Soft links

Soft links are different. They are more powerful as you can link to other filesystems and to directories, but when the original is removed, the link will be broken. You create soft links using the-soption of "ln" :

Example Usage:

Say you have a file called file.txt. You can create a soft link to it using the following command:

In this case, you can see there's a special “l” flag when you list the file using ls -al, and the file name has a @ at the end, and it's colored differently if you have colors enabled:

Now if you delete the original file, the links will be broken, and the shell will tell you "No such file or
directory" if you try to access it:

find

The "find" command is used to search for files and directories in a specified location. The basic syntax for the "find" command is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the find command, [LOCATION] is the starting directory for the search, and [EXPRESSION] is a set of conditions that specify the files and directories to be found.

Here are some standard options for "find":

• -name: Search for files and directories with a specific name.
• -type: Search for files and directories of a specific type (e.g. f for files, d for directories).
• -mtime: Search for files and directories modified within a certain number of days ago.
• -print: Display the names of the found files and directories.

Example usage:

Find all the files under the current tree that have the " .js" extension and print the relative path of each file matching:

It's important to use quotes around special characters like '*' to avoid the shell interpreting them.

Find directories under the current tree matching the name "src":

Use-type fto search only files, or -type lto only search symbolic links. 

-nameis case sensitive. use-inameto perform a case-insensitive search.

You can search under multiple root trees:

Find directories under the current tree matching the name "node_modules" or 'public':

You can also exclude a path, using -not -path:

You can search files that have more than 100 characters (bytes) in them:

Search files bigger than 100KB but smaller than 1MB:

Search files edited more than 3 days ago:

Search files edited in the last 24 hours:

You can delete all the files matching a search by adding the-deleteoption. This deletes all the files
edited in the last 24 hours:

You can execute a command on each result of the search. In this example, we runcatto print the file
content:

notice the terminating " \; ".{}is filled with the file name at execution time.

locate

The 'locate' command is used to find files and directories quickly on a Linux system. It works by searching an index of filenames, which is generated by the updatedb command, rather than by scanning the file system itself. The basic syntax for the locate command is as follows:

Where [OPTIONS] are optional flags that modify the behavior of the locate command, and [PATTERN] is a string that specifies the name of the files and directories to be found.

Here are some standard options for "locate":

• -i: Perform a case-insensitive search.
• -c: Display the number of matching results, but not the results themselves.

Example usage:

If you want to search any file use the following command:

To finds all files and directories named file.txt or FILE.TXT on the system, ignoring the case:

To display, the number of files and directories named file.txt on the system, but not the names themselves.

which

The "which" command is used to determine the location of an executable file in the system's PATH. The basic syntax for the which command is as follows:

Where [COMMAND] is the name of the executable file to be found.

Example usage:

You can do so using 'which'. The command will return the path to the command specified:

whichwill only work for executables stored on disk, not aliases or built-in shell functions.

whereis

whereis command is used to find the location of the source/binary file of a command and manuals sections for a specified file in a Linux system. If we compare the "whereis" command with the find command they will appear similar to each other as both can be used for the same purposes but the "whereis" command produces the result more accurately by consuming less time comparatively.

Where [OPTIONS] are optional flags that modify the behavior of the locate command.

Here are some standard options for whereis:

• -b : This option is used when we only want to search for binaries.
• -m : This option is used when we only want to search for manual sections. 
• -B: This option is used to change or otherwise limit the places where whereis searches for binaries.
• -M: This option is used to change or otherwise limit the places where whereis searches for manual sections. 
• -S: This option is used to change or otherwise limit the places where whereis searches for sources.

Example Usage:

 To find the location of the source/binary file of a command uses the following command:

To locate the binary of a Linux command let’s say gunzip:

Each of these commands has various options and flags that can be used to customize their behavior, so it is important to consult the manual pages for these commands for more information.

Example: 

You can compress a file using the gzip compression protocol namedLZ77using the gzipcommand. Here's the simplest usage:

This will create a compressed file "newfile.txt.gz" and delete the original file.

To prevent this, you can use the-coption and use output redirection to write the output to the "newfile.gz" file:

The-coption specifies that output will go to the standard output stream, leaving the original file intact, or you can use the -koption:

There are various levels of compression. The more the compression, the longer it will take to compress
(and decompress). Levels range from 1 (fastest, worst compression) to 9 (slowest, better compression), and the default is 6.

You can choose a specific level with the -<NUMBER> option:

• -c: create an archive
• -v: verbose output
• -f: file, used to specify the name of the archive file

To extract the contents of a tar archive:

tar -xvf archive.tar

• -x: extract files
• -v: verbose output
• -f: file, used to specify the name of the archive file

To extract them to a specific directory, use:

You can also just list the files contained in an archive:

'tar' is often used to create acompressed archive, gzipping the archive. This is done using the -zoption:

This is just like creating a tar archive and then running gzipon it. 

To unarchive a gzipped archive, you can usegunzip, orgzip -d, and then unarchive it, buttar -xfwill recognize it's a gzipped archive and do it for you:

bzip2

'bzip2' is a lossless compression tool in Linux used to compress and decompress files. The compression method used by bzip2 is known as the Burrows-Wheeler algorithm and it provides better compression ratios than gzip. To use bzip2, one can run the following command in the terminal:

Syntax: 

Options: 

This will compress the file "file.txt" and create "file.bz2".

This will decompress the file "file.txt.bz2" and create "file.txt".

zip

The zip command is used to compress and archive files and directories in a zip archive. It works by compressing each file individually, then combining all the compressed files into a single archive file. The syntax for using the zip command is:

Where:

• options are various optional switches that can modify the behavior of the zip command.
• archive.zip is the name of the archive file that will be created.
• file1 [file2 ...] are the names of the files and/or directories to be included in the archive.

• include all files and subdirectories within a specified directory.
• -9 (or --best): use the highest level of compression.
• -u (or --update): update an existing archive file, adding or replacing files as necessary.

For example:

To extract files from a zip archive, use the unzip command.

unzip

• -l: List the contents of an archive without actually extracting the files.
• -o: Overwrite files without asking for confirmation.
• -d: Specify the destination directory for the extracted files.
• -p: Extract files and preserve the original file permissions.
• -q: Quiet mode, do not display any messages or error messages.

If the [file(s) ...] argument is not specified, "unzip" will extract all the files in the archive. If an [destination] argument is not specified, "unzip" will extract the files to the current working directory.

Some common text-processing commands in Linux include:

cat

cut

'cut' is a text processing command in Linux that is used to extract specific columns or fields from input data (i.e., from a file or from the standard input). It splits the input into lines and then cuts out specified fields from each line.

Here are some key features of the cut command:

Options:

• -b, --bytes: selects the specified byte positions
• -c, --characters: selects the specified character positions
• -d, --delimiter=DELIM: specifies the delimiter that separates fields in the input data
• -f, --fields=LIST: specifies the fields to be extracted (a comma-separated list of field numbers)

Usage:

cut -f1,3 -d: /etc/passwd: Extracts the first and third fields (separated by a colon) from the /etc/passwd file.

grep

sort is a Linux command used to sort the contents of a file or input stream in ascending or descending order. The sorted output can be redirected to a file or displayed on the terminal. By default, sort sorts the input lines in lexicographic order. Options can be provided to sort the input based on different criteria such as numeric value, reverse order, field-based sorting, etc.

Syntax: 

uniq

Syntax: 

awk

tr

'tr' is a Linux command used for text translation and manipulation. It translates or deletes specified characters in the input text and writes the result to standard output.

'tr' can be used to perform operations such as character substitution, deletion, and compression. For example, it can be used to convert lowercase characters to uppercase, remove duplicates, or squeeze repeated characters.

Syntax: 

tac

'tac' is a Linux command used to concatenate and print files in reverse order. It reads the input files in reverse order, line by line, and writes the result to the standard output.

The tac command is useful for viewing the last lines of a file, for example, the latest entries in a log file.

Syntax: 

Run the following command:

head

'head' is a Linux command that prints the first n lines (10 by default) of a file to the standard output. It can also be used to view the first part of a large file to see what it contains before reading the whole file.

Syntax: 

Options:

• -n: Specify the number of lines to display.
• -c: Specify the number of bytes to display.
• -q: Quiet, never print headers giving file names.

Example: 

To print the first 5 lines of the file 'dogs' use the following command:

tail

•  -f, --follow: Output appended data as the file grows 
• -n, --lines=[+|-]NUM: Number of lines to output. A '+' symbol shows lines starting from NUM 
• -q, --quiet, --silent: Don't print headers when multiple files are outputting 
• -v, --verbose: Always print headers when multiple files are outputting

VI is a screen-oriented text editor originally designed for UNIX and UNIX-based operating systems. VI is the most popular and classic text editor in the Linux family. Below, are some reasons which make it a widely used editor?

• It is available in almost all flavors of Linux Distributions,
• It works the same across different platforms and Distributions,
• It is user-friendly.

Hence, millions of Linux users love it and use it for their editing needs.

In recent times there be an advanced version of the VI editor available, and the most popular one is Vim which is VI Improved. Some of the other ones are Elvis, Nvi, Nano, and Vile. It is wise to learn VI because it is feature-rich and offers endless possibilities to edit a file.

To work on the VI editor, you need to understand its operation modes. They can be divided into two main parts.

• Command mode: In commands mode, VI only understands commands, you can only move the cursor and cut, copy, and paste the text. In the command, you can modify and change the file. Remember that Commands are always case Sensitive. Insert mode is always used to insert text into the file.
• Insert Mode: To switch to the Insert mode from the command mode by pressing 'i' on the keyboard. Once you are in Insert mode, any key would be taken as input for the file on which you are currently working.

To return to the command mode and save the changes you have made you need to press the Esc key.

VI-Editor Usage

1. To launch the VI Text-Editor, Open the Terminal and type vi <file name>.

It will open a new interface:

As you see VI editor opens in command mode. 

Always remember that if you specify an existing file, then the editor would open it for you to edit, else, you can create a new file.

2. Press “I” to enter the insert mode.

Here I type Hello World.

3. Press ‘ESC’ to Enter Command mode. Type :wq to save and Quit.

To check the content of the file we use cat the file name here the file name is first_file.

VI editing Commands

There are several commands available in the VI text editor.

Note: The "command mode" to execute these commands. VI editor is case-sensitive so make sure you type the commands in the right letter case.

Here are all the commands used in the VI Text editor.

Keystrokes	Action
I	Insert at cursor (goes into insert mode)
A	Write after the cursor (goes into insert mode)
A	Write at the end of the line (goes into insert mode)
ESC	Terminate insert mode
U	Undo last change
U	Undo all changes to the entire line
O	Open a new line (goes into insert mode)
dd 3dd	Delete line Delete 3 lines.
D	Delete the contents of the line after the cursor
C	Delete the contents of a line after the cursor and insert new text. Press the ESC key to end insertion.
dw 4dw	Delete word Delete 4 words
cw	Change word
x	Delete the character at the cursor
r	Replace character
R	Overwrite characters from cursor onward
s	Substitute one character under the cursor and continue to insert
S	Substitute the entire line and begin to insert at the beginning of the line
~	Change case of individual character

Make sure, you have pressed the right command otherwise you will end up making undesirable changes to the file. You can also enter the insert mode by pressing a, A, or o, as required.

Moving within a file

You need to be in command mode to move within a file. The default keys for navigation are mentioned below else; you can also use the arrow keys on the keyboard.

Keystroke	Use
k	Move cursor up
j	Move cursor down
h	Move cursor left
l	Move cursor right

Saving and Closing the file

You should be in the command mode to exit the editor and save changes to the file.

Keystroke	Use
Shift+zz	Save the file and quit
:w	Save the file but keep it open
:q	Quit without saving
:wq	Save the file and quit

In a short time, I hope, I have completely described the whole thing, if there are any doubts then write them down in my comment session.

Man Pages

The man command is used to display the manual pages for a Linux command. It stands for manual.

Syntax: 

• Section 1: User commands
• Section 2: System calls
• Section 3: Library functions
• Section 4: Devices and special files
• Section 5: File formats and conventions
• Section 6: Games
• Section 7: Miscellaneous
• Section 8: System administration commands and daemons

You can also use the option -s followed by a section number to display manual pages from a specific section.

For example, man ls displays the manual page for the ls command and man -s 2 fork displays the manual page for the fork system call.

This is a man (from the manual) page. Man pages are an essential tool to learn, as a developer. They contain so much information that sometimes it's almost too much.

Command Line Help

The "help" command is a shell built-in command in most Unix-like operating systems. It displays information about built-in shell commands and functions. The information provided by "help" varies by shell and system, but typically includes a brief description of the command and its options. 

The "help" command can be useful for quickly looking up information on shell commands without having to switch to a manual page or online documentation. For example, in a Bash shell, typing "help echo" will provide information on the "echo" command.

Which

The "which" command in Linux is used to find the location of a command in the system PATH. It displays the full path of the executable file associated with the given command. It is commonly used to determine the location of a binary file to ensure it is not a malicious file.

Usage:

The "which" command takes one argument: the name of the command for which the location is being searched.

Example:

In the above example, the "which" command was used to find the location of the ls command. The output indicates that the ls command is located at /bin/ls.

Whereis

"whereis" is a Linux command that can be used to locate the source, binary, and manual page files for a specified command. It is used to search the standard Linux directories for specific executables, libraries, and manual pages, and returns the path to these files.

Syntax:

Options: 

• -b, --binary: Search only for executable files 
• -m, --man: Search only for manual pages 
• -s, --source: Search only for source files 
• -B path, --block-size=path: Use a blocksize of path 
• -M path, --manpath=path: Specify man search path 
• -f, --fill-holes: Fill in any holes in the manual section with system manuals 
• -u, --update: Update the database used by whereis

Examples:

These commands are used to search for files or directories based on name, content, or other attributes. They provide quick and efficient ways to search and locate information within a Linux system.

Example usage:

Print "Hello World!"

Print "Hello World!" along with the shell:

Enable interpolation of a backslash:

Example usage:

Callingunamewithout any options will return the Operating System codename:

To display all information about the system, including the name, version, release, and machine architecture.

To display the machine hardware name, e.g. x86_64 for a 64-bit system.

To display the hostname of the system.

lsb_release

The lsb_release command is used to display information about the Linux distribution and its version.

Here's the basic syntax of the lsb_release command:

Where [OPTION] is one or more of the following options:

• -a: Display all information about the distribution, including the name, description, and release version.
• -d: Display the description of the distribution.
• -i: Display the distribution ID.
• -r: Display the release version of the distribution.
• -s: Display a short description of the distribution.

Example usage:

To display all information about the distribution, including the name, description, and release version.

This command displays a short description of the distribution:

cat /proc/version

The /proc/version file is a virtual file in the /proc file system that provides information about the Linux kernel version that is running on the system. The cat command can be used to display the contents of this file.

The output of the cat /proc/version displays the following information:

• The Linux version string, which includes the Linux kernel version and the name of the distribution that is running the kernel.
• The build time and date of the Linux kernel.
• The name and email address of the person who built the Linux kernel.

Example usage:

To display the information about the Linux kernel version and builds information use the following command:

In this example, the Linux kernel version is 4.15.0-88-generic, and the distribution that is running the kernel is Ubuntu 18.04.1.

cat /proc/cpuinfo 

• Processor vendor and model name.
• Processor clock speed.
• Cache size.
• Processor flags, which represent features that the processor supports.

Example usage:

This command displays information about the central processing unit (CPU) in the system.

Here's a sample output of cat /proc/cpuinfo:

cat /proc/meminfo 

The /proc/meminfo file is a virtual file in the /proc file system that provides information about the memory usage and statistics of the system. The cat command can be used to display the contents of this file.

The output of cat /proc/meminfo displays information about the system's memory usage, including:

• Total installed memory (MemTotal).
• Free memory is available (MemFree).
• Buffered memory is used by the kernel (Buffers).
• Cached memory is used by the kernel (Cached).
• Total swap space (SwapTotal).
• Free swap space (SwapFree).

Example usage:

To display the information about the memory usage and statistics of the system. Here's a sample output of cat /proc/meminfo:

In this example, the system has a total of  1511588 KB of memory, with 439024 KB of free memory available. The system also has a total of 1139848 KB of swap space, with 699344 KB of cache space.

df 

• Filesystem: the type of file system.
• 1K-blocks: the total size of the file system in 1KB blocks.
• Used: the amount of disk space used by the file system.
• Available: the amount of disk space available on the file system.
• Use%: the percentage of disk space used.
• Mounted on: the mount point of the file system.

Example usage:

This command displays the disk space usage information for all mounted file systems in the system. Here's a sample output of df:

In this example, the system has two file systems, /dev/sda1 and /dev/sdb1, mounted at the root directory (/) and the /data directory, respectively.

free

The free command is used to display information about the total amount of physical memory and swap memory available, as well as the amount of memory being used and the amount of memory available for processes to allocate.

By default, free displays information in a table format, with columns indicating the following information:

• total: the total amount of physical memory or swap memory available.
• used: the amount of physical memory or swap memory being used.
• free: the amount of physical memory or swap memory available for processes to allocate.
• shared: the amount of physical memory shared between processes.
• buffers: the amount of physical memory used as buffers.
• cache: the amount of physical memory used as a cache.
• available: the amount of physical memory available to allocate to running processes, taking into account cache and buffers.

Example usage:

This command displays the information about the total amount of physical memory and swap memory available, as well as the amount of memory being used and the amount of memory available for processes to allocate. Here's a sample output for free:

In this example, the system has 1511588 KB of physical memory, and 1535360 KB of swap memory. Of the physical memory, 197620 KB is being used, 0 is free, 3368 KB is shared, and 876660 KB is used as buffers/cache. The available column shows 1138800 KB of physical memory available to allocate to running processes, taking into account cache and buffers.

top

The top command is used to display the processes currently running on a Linux system, and information about the system resources being used. It is a dynamic, real-time view of the system's processes and resource usage.

By default, the top displays information about the following:

• Current system uptime.
• The number of users logged into the system.
• Load average: an indicator of system activity, represented by three values representing the average number of system processes over the past 1, 5, and 15 minutes.
• The total number of tasks (processes) running and the number of tasks in each process state (running, sleeping, stopped, etc.).
• Physical memory usage and swap memory usage.
• The list of processes is sorted by CPU usage. For each process, the top displays the following information:
• Process ID (PID).
• The user running the process.
• CPU usage percentage.
• Memory usage.
• Command name.

Example usage:

To display the real-time view of the processes running on the system and information about the system resources being used.

Here's a sample output of the top:

In this example, the system has been up for 3:25 minutes. The load average is 0.10, 0.05, and 0.01 for the past 1, 5, and 15 minutes, respectively. There are 110 tasks running, with 1 task running and 77 tasks sleeping. 99.5% of the CPU is idle. The system has 1511588 KB of physical memory and 1535360 KB of swap memory. The first process listed, with PID 819, is using 0.3% of the CPU and 3.2% of the memory, and has been running for 0:24:15. The second process listed, with PID 1, is using 0.0% of the CPU and 0.6% of the memory, and has been running for 0:01:27.

htop

'htop' is a system monitoring tool for Linux that provides a real-time, interactive overview of the processes running on a system. It provides information on the system's performance, including CPU usage, memory usage, and other statistics. Here are some of the key features of 'htop':

• User-friendly interface: 'htop' provides a visually appealing and easy-to-use interface compared to the standard 'top' command.
• Color coding: Different process states, such as running, sleeping, or stopping, are color-coded to make it easier to distinguish between processes.
• Sortable columns: 'htop' allows you to sort the process list by multiple criteria, such as CPU usage, memory usage, and process ID.
• Detailed process information: 'htop' provides a detailed process information screen that can be accessed by selecting a process with the cursor. This screen provides information on the process's status, memory usage, and other statistics.
• Process tree view: 'htop' displays a process tree view that shows the relationships between processes and the parent-child relationships between processes.
• Real-time updates: 'htop' updates its display in real-time, allowing you to see changes in system performance as they occur.
• Interactive controls: 'htop' provides interactive controls that allow you to perform actions, such as killing a process, directly from the 'htop' interface.

In summary, 'htop' is a powerful and user-friendly system monitoring tool that provides a real-time, interactive overview of the performance of a Linux system.

lscpu 

The command 'iscpu' is not a standard Linux command. It appears to be a typo or a custom command that may not exist on your system.

If you were looking for information about the 'lscpu' command, it is a Linux utility that provides information about the system's CPU architecture. The information provided by 'lscpu' includes the processor type, number of cores, clock speed, and other information. 

To run 'lscpu', open a terminal window and type 'lscpu', and press enter.

The 'ps' command is used to display information about the currently running processes on a Linux system.

The basic syntax for using 'ps' is:

Some common options for the 'ps' command include:

• '-e' or '--every': display information about all processes, not just those started by the current user.
• '-f' or '--forest': display the process hierarchy as a tree.
• '-u' or '--user': display information about processes for a specific user.
• '-A' or '--all': display information about all processes, not just those that are running.

By default, 'ps' displays information about the process ID (PID), the terminal associated with the process, the process owner, the process status, and the command used to start the process.

The 'ps' command is a basic and powerful tool for investigating and managing processes on a Linux system and is commonly used in combination with other commands like 'kill' or 'top' to control the behavior of processes and the allocation of system resources.

top

The 'top' command is a real-time system monitoring utility in Linux that displays information about the system's running processes.

The basic syntax for using 'top' is:

When run, 'top' displays a continuously updated list of processes, showing their current status, the amount of CPU and memory resources they are using, and other information such as the user running the process and the process ID (PID).

Some common options for the 'top' command include:

• '-d' or '--delay': specify the delay between updates in seconds.
• '-p' or '--pid': display information about a specific process ID.
• '-u' or '--user': display information about processes for a specific user.

By default, 'top' sorts the process list by the number of CPU resources being used, but you can sort by other criteria, such as memory usage or process ID, by pressing certain keys while 'top' is running.

Its usage is simple, you just typetop, and the terminal will be fully immersed in this new view:

The process is long-running. To quit, you can type the q letter orCtrl + C. 

There's a lot of information being given to us: the number of processes, how many are running or sleeping, the system load, the CPU usage, and a lot more.

Below, the list of processes taking the most memory and CPU is constantly updated. By default, as you can see from the%CPUcolumn highlighted, they are sorted by the CPU used. 

The 'top' command is a powerful tool for understanding how system resources are being used and for identifying processes that may be causing performance issues. It is often used by system administrators and developers to diagnose and resolve performance problems on Linux systems.

htop

'htop' is an alternative to the 'top' command in Linux. It is a real-time system monitoring utility that provides a more user-friendly interface and additional features compared to 'top'.

The basic syntax for using 'htop' is:

When run, 'htop' displays a continuously updated list of processes, showing their current status, the amount of CPU and memory resources they are using, and other information such as the process ID (PID), the user running the process, and the command used to start the process.

Some of the key features of 'htop' include:

• A color-coded interface that makes it easier to quickly identify processes and system resources.
• The ability to sort the process list by multiple criteria, such as CPU usage, memory usage, process ID, and more.
• A detailed process view that shows information about a specific process, including its full command line, environment variables, and more.
• A system status bar that provides information about system load, memory usage, and other key metrics.
• Support for mouse-based navigation and process management, making it easier to interact with the utility.

'htop' is a useful tool for system administrators and developers who need to monitor the performance of Linux systems and manage processes in real time. It provides a more user-friendly interface and additional features compared to 'top', making it a popular alternative for system monitoring on Linux.

kill

The 'kill' command is a Linux utility that is used to send a signal to a process. It can be used to terminate a process, stop a process, or resume a stopped process. The syntax for using the 'kill' command is as follows:

Some common signals that can be sent using the 'kill' command include:

• SIGTERM (terminate): This is the default signal sent by 'kill' and is used to request that a process terminate gracefully.
• SIGKILL (kill): This signal cannot be ignored or caught by the process, and is used to force a process to terminate immediately.
• SIGSTOP (stop): This signal stops a process, and it cannot be resumed until a SIGCONT signal is sent.
• SIGCONT (continue): This signal resumes a stopped process.

It's not just used to terminate a program like a name would suggest, but that's its main job. We use it in this way:

By default, this sends theTERMsignal to the process id specified. We can use flags to send other signals, including:

• HUPmeans to hang up. It's sent automatically when a terminal window that started a process is closed before terminating the process.
• INTmeans to interrupt, and it sends the same signal used when we pressctrl-Cin the terminal, which usually terminates the process.
• KILLis not sent to the process, but to the operating system kernel, which immediately stops and
  terminates the process.
• TERMmeansterminate. The process will receive it and terminate itself. It's the default signal sent by kill.
• CONTmeanscontinue. It can be used to resume a stopped process.
• STOP is not sent to the process, but to the operating system kernel, which immediately stops (but does not terminate) the process.

You might see numbers used instead, likekill -1 <PID>. 

In this case, 

• 1corresponds toHUP
• 2corresponds toINT
• 9 corresponds toKILL
• 15corresponds toTERM
• 18corresponds toCONT
• 15corresponds to STOP

It's important to note that the 'kill' command should be used with caution, as it can cause data loss or corruption if used improperly. Before using the 'kill' command to terminate a process, it's a good idea to try and determine what the process is doing, and if possible, to try and terminate it gracefully using SIGTERM first.

pkill

The 'pkill' command is a Linux utility that is used to send a signal to a process based on its name, rather than its process ID. It is similar to the 'killall' command, but with some key differences. 

The syntax for using the 'pkill' command is as follows:

Where "pattern" is a string that matches the name of the process you want to signal, and options are optional arguments that specify the type of signal to be sent and other behavior of the 'pkill' command.

Some common options that can be used with 'pkill' include:

• -signal: Specifies the type of signal to be sent. If no signal is specified, the default signal SIGTERM (terminate) is sent.
• -f: Matches the process name against the full command line, rather than just the process name.
• -u: username: Limits the search for processes to those owned by the specified user.
• -n: Matches the most recently started process.

killall

The 'killall' command is a Linux utility that is used to send a signal to multiple processes based on their names. It is similar to the 'pkill' command, but with some key differences. The syntax for using the 'killall' command is as follows:

Where name is the name of the process you want to signal, and options are optional arguments that specify the type of signal to be sent and other behavior of the 'killall' command.

• -signal: Specifies the type of signal to be sent. If no signal is specified, the default signal SIGTERM (terminate) is sent.
• -e: Matches the exact process name, rather than a partial match.
• -I: Matches the process name exactly, ignoring the case.
• -u username: Limits the search for processes to those owned by the specified user.

The 'nice' command is a Linux utility used to set the priority of a process. By default, processes run with a priority of 0, but the 'nice' command allows you to specify a different priority level, which can range from -20 (highest priority) to 19 (lowest priority). The higher the priority of a process, the more CPU time it will receive.

The syntax for using the 'nice' command is as follows:

Where options are optional arguments that specify the priority level and other behavior of the 'nice' command, and command [arguments] is the command you want to run with the specified priority.

If no priority level is specified, the default priority level of 10 is used. To specify a different priority level, use the -n option followed by the priority level:

For example, to run the 'ls' command with a priority level of 5, you would use the following command:

Note that the 'nice' command only affects the priority of a process, not the amount of memory or other resources it can access. Additionally, only processes run by the superuser can increase their priority, while all users can decrease the priority of their own processes.

renice

The 'renice' command is a Linux utility used to change the priority of an already running process. It allows you to adjust the priority of a process, which determines how much CPU time the process will receive. By default, processes run with a priority of 0, but the 'renice' command allows you to specify a different priority level, which can range from -20 (highest priority) to 19 (lowest priority).

The syntax for using the 'renice' command is as follows:

Where priority is the new priority level you want to set for the process, processID is the ID of the process you want to change the priority of, and -p is an optional argument that specifies that you want to change the priority of a process by its process ID, rather than by its name.

For example, to change the priority of the process with ID 12345 to a priority level of 5, you would use the following command:

Note that the 'renice' command only affects the priority of a process, not the amount of memory or other resources it can access. Additionally, only processes run by the superuser can increase the priority of other processes, while all users can decrease the priority of their own processes.

The created user can be further configured using other user management commands such as usermod and userdel.

Note that the exact usage and options of ifconfig may vary depending on the distribution of Linux you are using.

The basic syntax for using ping is 

• -a: Display all information about the host. 
• -d: Print debugging information. 
• -r: Disable recursion. 
• -v: Display verbose output. 
• -t type: Specify the query type (e.g. -t MX, -t A, -t NS, etc.). 
• -x address: Reverse lookup of an IP address.

Examples:

To resolve a domain name to an IP address: nslookup google.com

Options: 

• -a, --alias: Display the alias name (if used) 
• -d, --domain: Display the name of the DNS domain 
• -f, --fqdn, --long: Display the fully qualified domain name 
• -A, --all-fqdns: Display all fully qualified domain names 
• -i, --ip-address: Display the IP address(es) associated with the hostname 
• -s, --short: Display the short hostname