Linux_Commands

linux's most used commands for quick references

LINUX_BASICS

Absolute Basics - Bash key Features:

Key or key combination	Function
Ctrl+A	Move cursor to the beginning of the command line.
Ctrl+C	End a running program and return the prompt.
Ctrl+D	Log out of the current shell session, equal to typing exit or logout.
Ctrl+E	Move cursor to the end of the command line.
Ctrl+H	Generate backspace character.
Ctrl+L	Clear this terminal.
Ctrl+R	Search command history.
Ctrl+Z	Suspend a program.
ArrowLeft and ArrowRight	Move the cursor one place to the left or right on the command line, allowing insertion of characters at other positions.
ArrowUp and ArrowDown	Browse command history. Go to the line that you want to repeat, edit details if necessary, and press Enter to save time.
Shift+PageUp and Shift+PageDown	Browse terminal buffer (to see text that has "scrolled off" the screen).
Tab	Command or filename completion; when multiple choices are possible, the system will provide options.
Tab Tab	Shows file or command completion possibilities.

Commands

man

The man command is short for "manual." It is used to display the manual pages for various commands, providing detailed information about their usage, options, and examples. The manual pages serve as a valuable resource for understanding and utilizing different commands effectively.

info

The info command is used to access Info pages, which are an alternative form of documentation commonly found in GNU software. Info pages provide detailed information, similar to manual pages, but often offer more extensive and hierarchical content. They allow for navigating through different sections and subtopics to explore a command or topic in-depth.

whatis

The whatis command is used to search the "whatis" database, which contains brief descriptions of various commands, files, or other entities on the system. It provides a concise summary of the queried item, allowing users to quickly identify its purpose or functionality.

apropos

The apropos command is used to search the "whatis" database for strings or keywords related to specific topics or commands. It helps users find relevant commands, files, or other entities based on their descriptions or functionalities. This command is particularly useful when trying to discover new tools or when unsure about the exact command name.

Here are proper examples of using the commands man, info, whatis, and apropos:

• To view the manual page for the ls command, use: man ls
• To access the Info page for the topic "gcc" (GNU Compiler Collection), use: info gcc
• To search the "whatis" database for the term "grep" (search tool), use: whatis grep
• To search for commands or entities related to the topic "network," use: apropos network

These examples demonstrate how to use each command with the appropriate arguments to access the desired information. Remember to replace command-name and search-term with the actual command or term you want to explore or search for.

NOTE: Some commands don't have separate documentation because they are part of another command. cd, exit, logout, and pwd are such exceptions.

FILE AND FILE-SYSTEMS ON LINUX

On a UNIX system, everything is a file; if something is not a file, it is a process. In simple terms, this means that in a UNIX system, everything is treated as a file. Although there are special files like named pipes and sockets that are more than just regular files, it's easier to think of everything as a file. This includes directories, programs, services, texts, images, and even devices like input and output devices. The system considers them all as files, making it simpler to work with and manage them.

Here's the information formatted in a table with descriptions:

Symbol	Meaning	Description
-	Regular file	Most files are regular files that contain normal data, such as text files, executable files or programs, and input/output data.
d	Directory	Directories are special files that serve as containers for organizing and storing other files and directories.
l	Link	Links create connections between different locations in the file system, allowing a file or directory to be visible in multiple places.
c	Special file	Special files are used for input and output mechanisms, often located in the /dev directory.
s	Socket	Sockets are special files that provide inter-process networking, similar to TCP/IP sockets, protected by file system access control.
p	Named pipe	Named pipes act as a way for processes to communicate with each other without using network socket semantics.
b	Block device	Block devices represent physical or virtual devices that transfer data in fixed-size blocks, such as hard drives or SSDs.

The -l option to ls displays the file type, using the first character of each input line:

The file system tree in a UNIX-based system starts at the root directory, represented by a forward slash (/). This directory is the top-level directory that contains all other directories and files. It is commonly referred to as the root directory.

Directories located directly below the root directory are often prefixed with a slash (/) to indicate their position and avoid confusion with directories that may have the same name. It is recommended to explore the root directory when starting with a new system to get an overview of its contents.

Here is an overview of some important directories you may encounter:

Directory Content

Directory	Description
/bin	Common programs shared by the system, system administrators, and users.
/boot	Startup files and the kernel (e.g., vmlinuz) for booting the system.
/dev	Contains references to CPU peripheral hardware represented as special files.
/etc	Stores important system configuration files, similar to the Control Panel in Windows.
/home	Home directories of common users.
/initrd	(On some distributions) Information required for booting.
/lib	Library files required by the system and users' programs.
/lost+found	Every partition has a lost+found directory for recovering files saved during failures.
/misc	Used for miscellaneous purposes.
/mnt	Standard mount point for external file systems, such as CD-ROMs or digital cameras.
/net	Standard mount point for remote file systems.
/opt	Typically contains additional and third-party software.
/proc	A virtual file system providing information about system resources.
/root	The home directory for the administrative user (root).
/sbin	Programs for system use and system administration tasks.
/tmp	Temporary space for the system, cleaned upon reboot. Avoid saving important work here.
/usr	Programs, libraries, and documentation for user-related programs.
/var	Storage for variable and temporary files created by users, log files, mail queue, etc.

Please note that this table provides a brief description of each directory, and there may be additional subdirectories and files within each directory.

To find out the partition on which a directory is located, you can use the df command with a dot (.) as an option. It displays the partition information for the current directory, including the used and available space.

Example: $ df -h . Filesystem Size Used Avail Use% Mounted on /dev/hda7 980M 163M 767M 18% / By running this command, you can determine the partition and the amount of space utilized on that partition.

In a file system, each file is represented by an inode, which acts as a unique identifier containing essential information about the file. This information includes details about the file's ownership, as well as its physical location on the hard disk.

Every partition in the system has its own set of inodes, allowing files with the same inode number to exist across different partitions.

Each inode corresponds to a data structure on the hard disk, storing various properties of a file, including the actual location of the file's data. During the initialization of a hard disk for data storage, such as during system installation or when adding additional disks, a specific number of inodes per partition is created. This number determines the maximum number of files, including directories, special files, and links, that can coexist on that partition at any given time. Typically, it is expected to have approximately 1 inode for every 2 to 8 kilobytes of storage. It is important to note that the inode does not contain the file name or directory information, as these are stored in separate directory files. By correlating file names with inode numbers, the system can construct a tree-like structure that users can comprehend. If desired, users can display inode numbers using the -i option with the ls command. The inodes have their dedicated space on the disk, separate from the actual file data.

The Path

When executing commands in the system, it is usually unnecessary to provide the full path to the command. For example, even though we know that the ls command is located in the /bin directory (verified with which -a ls), we don't need to enter the command /bin/ls to list the contents of the current directory.

This convenience is made possible by the PATH environment variable. The PATH variable contains a list of directories where executable files can be found, eliminating the need for users to type out or remember specific command locations. The PATH variable typically includes directories with "bin" in their names. You can use the echo command to display the contents of the PATH variable:

rogier:> echo $PATH /opt/local/bin:/usr/X11R6/bin:/usr/bin:/usr/sbin/:/bin

Absolute and Relative Paths

Paths in a file system can be categorized as either absolute paths or relative paths. Understanding the difference between these two types of paths is crucial for navigating and accessing files and directories effectively.

Absolute Paths:

An absolute path refers to the complete and exact location of a file or directory within the file system hierarchy. It starts from the root directory, denoted by a forward slash ("/"), and includes all the intermediate directories necessary to reach the desired file or directory.

For example:

/home/user/Documents/file.txt: This is an absolute path that specifies the exact location of the file "file.txt" within the "Documents" directory of the "user" directory in the "home" directory.

/var/log/syslog: This absolute path points to the "syslog" file located in the "log" directory of the "var" directory.

Absolute paths provide an unambiguous and fixed reference to a file or directory regardless of the current working directory. They always start with a forward slash and can be used to access files and directories from anywhere in the file system.

Relative Paths:

A relative path specifies the location of a file or directory relative to the current working directory. It does not begin with a forward slash and relies on the current context to determine the complete path.

Relative paths are typically shorter and more concise than absolute paths. They are commonly used when referring to files and directories within the same directory or in related directories.

For example:

Suppose the current working directory is /home/user/, and there is a file named "data.txt" located in the same directory. The relative path to access this file would be data.txt. This path assumes that the file is in the current working directory.

If there is a subdirectory named "images" within the current working directory, and a file named "pic.jpg" resides inside that subdirectory, the relative path to access the file would be images/pic.jpg. This path indicates the location of the file relative to the current working directory.

Relative paths provide flexibility and convenience by allowing you to navigate and access files and directories relative to your current position in the file system.

It's important to note that the interpretation of relative paths can vary depending on the context. When changing the current working directory, the meaning of a relative path may change accordingly.

Important Files and Directories

1. The Kernel The kernel is like the heart of the Linux system. It manages communication between the hardware and peripherals, starts and stops processes and server processes, and performs many other important tasks. In short, the kernel is the most crucial file in the system.

2. The Shell 2.1 What is a Shell? The shell is a way to communicate with the computer. It can be compared to a language that allows users to interact with the system. Unlike graphical interfaces where the computer leads the conversation, the shell allows users to take the initiative and have a two-way conversation with the system. It provides flexibility, automation, and a powerful tool for both beginners and advanced users.

2.2 Shell Types There are different shell types, just like there are different languages and dialects spoken by people. Some common shell types include:

sh or Bourne Shell: The original shell used on UNIX systems. bash or Bourne Again SHell: The standard GNU shell, widely used and recommended for beginners. csh or C Shell: Resembles the syntax of the C programming language. tcsh or Turbo C Shell: Enhanced version of C Shell with improved user-friendliness and speed. ksh or Korn shell: Popular among UNIX users, but can be challenging for beginners. To switch between shells, simply enter the name of the desired shell in the active terminal. Each shell has its own appearance and functionality.

3. Your Home Directory Your home directory is the default destination when you connect to the system. It usually resides in the /home directory but can vary depending on the system configuration. The path to your home directory is stored in the HOME environment variable, so you don't need to worry about it. You can store files, create directories, and organize your data within your home directory. However, there may be limits on the amount of data you can store, depending on system settings.

4. Configuration Files Most configuration files are stored in the /etc directory. These files control various aspects of the system's behavior and can be viewed using the cat command. Common configuration files include those for mail servers, web servers, cron jobs (scheduled tasks), system defaults, network settings, authentication modules, and more. These files are usually well-commented and self-explanatory.

5. Devices Devices, such as peripherals connected to your computer, are represented as entries in the /dev directory. Linux handles devices in a unified way, making it easier for users and the system to interact with them. Some common device names include CD drives, serial ports, printers, sound cards, USB devices, and more.

6. Variable Files Variable files are stored in the /var directory and hold data that frequently changes, such as log files, mailboxes, spoolers, and temporary files. These files are typically kept separate from system files for security and organization purposes. It's common for the /var directory to be on a separate partition to prevent issues like disk space filling up due to excessive log files or user activity.

Commands related to removing files:

To remove a single file: rm <filename>

To remove an empty directory: rmdir <directory>

To remove a non-empty directory (including all its files and subdirectories): rm -r <directory>

To remove a directory and its contents without prompting for confirmation: rm -rf <directory>

To remove multiple files at once: rm <file1> <file2> <file3> ...

To remove files matching a specific pattern using wildcards: rm <pattern>

To prompt for confirmation before removing each file: rm -i <filename>

Note: Be cautious when using the rm command, especially with the -r and -f options, as they can permanently delete files and directories without further confirmation. It's always a good practice to double-check the files you intend to remove before executing the command.

Commands related to using the ls command:

List files and directories in the current directory: ls

List all files and directories, including hidden files: ls -a

List files and directories in long format (including permissions, size, owner, etc.): ls -l

List a specific directory in long format: ls -l <directory>

List only the directory itself (not its contents) in long format: ls -ld <directory>

List files and directories in reverse order of the last change: ls -ltr

List files with their corresponding file types indicated by colors (if enabled): ls --color=auto

List files using the default suffix scheme for file types: ls --classify

Read the full documentation and functionality of the ls command: info coreutils ls

List of file/folders along with their inode numbers: ls -i

Here's the table with the color scheme:

Color	File Type
blue	directories
red	compressed archives
white	text files
pink	images
cyan	links
yellow	devices
green	executables
flashing red	broken links

Default suffix scheme for ls:

Character	File Type
nothing	regular file
/	directory
*	executable file
@	link
=	socket
|	named pipe

Type ls /dev -F and check out suffix scheme

This table provides the mapping between colors, characters, and file types used in the ls command's output.

Note: The ls command is highly customizable and has many more options and features. You can refer to the man page (man ls) or the Info pages for more detailed information on how to utilize the command effectively.

The file command is used to determine the type of a file. It applies tests to check properties, magic numbers, and language tests to make an educated guess about the file's format. mike:> file vi.gif vi.gif: GIF image data, version 89a, 88 x 31
mike:> file /dev/log /dev/log: socket

Commands related to creating directories, moving files, and copying files:

Making a directory/folder: mkdir <name>

Creating directories and subdirectories in one step: mkdir -p <dir>/<sub-dir>/<sub-dir>/<sub-dir>

The mv command is used to move or rename files: mv <source file/files> <destination directory>

Coping from old to new file/directory(empty): cp <old> <new>

The -R option allows recursive copying, which means copying all files and subdirectories within a directory: cp -R <old> <new>

Finding Files

find <path> -name <searchstring>: This can be interpreted as "Look in all files and subdirectories contained in a given path, and print the names of the files containing the search string in their name" (not in their content).

locate can be used as well for finding files, it's better for machine performance.

NOTES: The "which" command is used to search the directories specified in the user's search path for a specific executable file. It is particularly useful for troubleshooting "Command not Found" issues.

The "grep" command is a powerful text-searching tool used in Unix-like operating systems.

Searching for a specific word in a file: grep "apple" fruits.txt

Searching for a pattern using regular expressions: grep "[0-9]{3}-[0-9]{4}" contacts.txt

Searching recursively in a directory: grep "error" -r /var/log/

Viewing Files cat <filename> , head or tail -n <filename> head command helps view the first "n" lines on the file and tail command helps view the last "n" lines on the file.

Creating symbolic links The command to make links is ln. In order to create symlinks, you need to use the -s option:

ln -s targetfile linkname

File security

Access Mode Codes

Code	Meaning
0 or -	The access right that is supposed to be on this place is not granted.
4 or r	Read access is granted to the user category defined in this place.
2 or w	Write permission is granted to the user category defined in this place.
1 or x	Execute permission is granted to the user category defined in this place.

User Group Codes

Code	Meaning
u	User permissions
g	Group permissions
o	Permissions for others

There are also operators to manipulate the permissions:

Task	Operator
Grant a level of access	+
Remove a level of access	-
Set a level of access	=

Using chmod

chmod u+rwx,go-rwx <file> is used to modify the permissions of the file using the chmod command.

Here's the breakdown of the command:

u+rwx: Grants read (r), write (w), and execute (x) permissions to the user (owner) of the file. ,: Separates the permission specification for the user from the permission specification for the group and others. go-rwx: Removes(-)all permissions (rwx) for both the group and others on the file hello.

Using chmod with numeric arguments:

Command	Meaning
chmod 400 file	To protect a file against accidental overwriting.
chmod 500 directory	To protect yourself from accidentally removing, renaming or moving files from this directory.
chmod 600 file	A private file only changeable by the user who entered this command.
chmod 644 file	A publicly readable file that can only be changed by the issuing user.
chmod 660 file	Users belonging to your group can change this file, others don't have any access to it at all.
chmod 700 file	Protects a file against any access from other users, while the issuing user still has full access.
chmod 755 directory	For files that should be readable and executable by others, but only changeable by the issuing user.
chmod 775 file	Standard file sharing mode for a group.
chmod 777 file	Everybody can do everything to this file.

Sticky bit

In Linux, there is a special feature called the sticky bit within the permission system. The sticky bit can be applied to directories, and its purpose is to restrict the deletion or renaming of files within that directory to only the owner. Other users may or may not have the ability to edit the files, but only the owner has the privilege to delete or rename them. You can set the sticky bit on a folder using the following command: chmod +t someDirectory/

When the sticky bit is set, the permission string for the directory will end with a t, such as drwxrwxr-t. To remove the sticky bit, you can use the command:

chmod -t someDirectory/

By utilizing the sticky bit, you can ensure that important files within a directory remain protected from accidental deletion or renaming by users other than the owner.

Here's the table for the new commands in Files and the file system:

Command	Meaning
bash	GNU shell program.
cat file(s)	Send content of file(s) to standard output.
cd directory	Enter directory. cd is a bash built-in command.
chgrp newgroup file(s)	Change the group ownership of file(s) to newgroup.
chmod mode file(s)	Change access permissions on file(s).
chown newowner[:newgroup] file(s)	Change file owner and group ownership.
cp sourcefile targetfile	Copy sourcefile to targetfile.
df file	Reports on used disk space on the partition containing file.
echo string	Display a line of text.
export	Part of bash that announces variables and their values to the system.
file filename	Determine file type of filename.
find path expression	Find files in the file system hierarchy.
grep PATTERN file	Print lines in file containing the search pattern.
head file	Send the first part of file to standard output.
id	Prints real and effective user name and groups.
info command	Read documentation about command.
less file	View file with a powerful viewer.
ln targetfile linkname	Make a link with name linkname to targetfile.
locate searchstring	Print all accessible files matching the search pattern.
ls file(s)	Prints directory content.
man command	Format and display online (system) manual pages for command.
mkdir newdir	Make a new empty directory.
mv oldfile newfile	Rename or move oldfile.
newgrp groupname	Log in to a new group.
pwd	Print the present or current working directory.
quota	Show disk usage and limits.
rm file	Removes files and directories.
rmdir file	Removes directories.
tail file	Print the last part of file.
umask [value]	Show or change new file creation mode.
wc file	Counts lines, words, and characters in file.
which command	Shows the full path to command.

And remember, it is highly recommended to read the manual pages for detailed information about each command.

Processes

(Part of) Command	Meaning
regular_command	Runs this command in the foreground.
command &	Run this command in the background (release the terminal).
jobs	Show commands running in the background.
Ctrl+Z	Suspend (stop, but not quit) a process running in the foreground (suspend).
Ctrl+C	Interrupt (terminate and quit) a process running in the foreground.
%n	Every process running in the background gets a number assigned to it. Use %n to refer to a job by its number, for example, fg %2.
bg	Reactivate a suspended program in the background.
fg	Puts the job back in the foreground.
kill	End a process (also see Shell Builtin Commands in the Info pages of bash).

The ps command is one of the tools for visualizing processes. This command has several options which can be combined to display different process attributes.

The 'ps' command provides a snapshot of the current state of active processes, capturing their information at a specific moment. On the other hand, the 'top' program offers a more detailed and up-to-date view. It continuously updates the results obtained from 'ps' (with various options) every five seconds, generating a fresh list of processes that are putting the most strain on the system. Additionally, 'top' incorporates additional details such as swap space usage and CPU status, gathering this information from the proc file system.

NOTE: The relations between processes can be visualized using the pstree command.

**kill -l** command displays a list of signal names and their corresponding numbers. Signals are used to communicate with processes and manage their behavior. OUTPUT: 1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM 16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR 31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1 64) SIGRTMAX

Signal Name	Signal Number	Meaning
SIGTERM	15	Terminate the process in an orderly way.
SIGINT	2	Interrupt the process. A process can ignore this signal.
SIGKILL	9	Interrupt the process. A process cannot ignore this signal.
SIGHUP	1	For daemons: reread the configuration file.

States of process

State of process	Description
R	The process is running
S	Interruptable sleep
D	Uninterruptable sleep
Z	Zombie
T	Stopped

Command	Meaning	Usage
at	Queue jobs for later execution.	at [-f file] time
atq	Lists the user's pending jobs.	atq [-q queue]
atrm	Deletes jobs, determined by their job number.	atrm <job>
batch	Executes commands when system load level permits.	batch
crontab	Maintain crontab files for individual users.	crontab [-u user] file
halt	Stop the system.	halt
init run level	Process control initialization.	init <runlevel>
jobs	Lists currently executing jobs.	jobs [-l]
kill	Terminate a process.	kill [-signal] pid
mesg	Control write access to your terminal.	mesg [y/n]
netstat	Display network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.	netstat [-options]
nice	Run a program with modified scheduling priority.	nice [-n increment] command
pgrep	Display processes.	pgrep [-signal] pattern
ps	Report process status.	ps [-options]
pstree	Display a tree of processes.	pstree [-options]
reboot	Stop the system.	reboot
renice	Alter the priority of running processes.	renice [-n increment] [-p pid]
shutdown	Bring the system down.	shutdown [-options]
sleep	Delay for a specified time.	sleep <duration>
time	Time a command or report resource usage.	time command
top	Display top CPU processes.	top
uptime	Show how long the system has been running.	uptime
vmstat	Report virtual memory statistics.	vmstat [-options]
w	Show who is logged on and what they are doing.	w [-husf]
wall	Send a message to everybody's terminals.	wall [-n] message
who	Show who is logged on.	who [-Hu]
write	Send a message to another user.	write user [terminal]

I/O redirection

There are three types of I/O, which each have their own identifier, called a file descriptor:

standard input: 0 : <

standard output: 1 : >

standard error: 2

Output redirection with > and | e.g. cat test1 test2 > test3 this will redirect the contents of test1 and test2 files to test3 as well as ovewrite the content of test3. Use >> operator to append content.

Redirecting "nothing" to an existing file is equal to emptying the file: > list This process is called truncating.

Create a new empty file : > newlist cat file | mail someone or mail [email protected] < file

tee command to copy input to standard output and one or more output files in one move. date | tee file1 file2 tee called on through a pipe (|). uptime | tee -a file2 -a option to tee results in appending input to the file(s).

A program that takes input, performs operations on it, and produces the output on the standard output is known as a filter. Filters are frequently used to modify the structure of output.

1. https://www.gnu.org/software/grep/manual/grep.html grep
2. https://cheat.sh/sort sort

Text Editor: enter the vimtutor command in your shell or command line after installing vim.

You can view a comprehensive list of all variables currently defined for your session by using the "printenv" command.

Common environment variables:

Variable name	Stored information
DISPLAY	Used by the X Window system to identify the display server
DOMAIN	Domain name
EDITOR	Stores your favorite line editor
HISTSIZE	Size of the shell history file in number of lines
HOME	Path to your home directory
HOSTNAME	Local host name
INPUTRC	Location of definition file for input devices
LANG	Preferred language
LD_LIBRARY_PATH	Paths to search for libraries
LOGNAME	Login name
MAIL	Location of your incoming mail folder
MANPATH	Paths to search for man pages
OS	String describing the operating system
OSTYPE	More information about version, etc.
PAGER	Used by programs like man for handling large outputs
PATH	Search paths for commands
PS1	Primary prompt
PS2	Secondary prompt
PWD	Present working directory
SHELL	Current shell
TERM	Terminal type
UID	User ID
USER(NAME)	User name
VISUAL	Your favorite full-screen editor
XENVIRONMENT	Location of your personal settings for X behavior
XFILESEARCHPATH	Paths to search for graphical libraries

echo $VARIBLE_NAME to check out a specific variable. In order to change the content of a variable use export VARIABLE = value e.g. export PATH=$PATH:/path to directory

NOTE:When you export a variable in a session, it is valid only for that specific session and its child processes. Once you restart the computer or start a new session, the exported variables are no longer available. To make the changes to environment variables persistent across system reboots or new sessions, you would need to set them in configuration files that are read during the system startup or login process. The specific files and methods for setting persistent environment variables may vary depending on the operating system and shell you are using.

https://tldp.org/HOWTO/Bash-Prompt-HOWTO/ Bash Prompt HOWTO https://tldp.org/ The Linux Documentation Project

Command	Meaning	Usage
aptitude	Manage packages Debian-style.	aptitude install <package>
automount	Automatically include newly inserted file systems.	automount -m <mountpoint>
dpkg	Debian package manager.	dpkg -i <package.deb>
dselect	Manage packages Debian-style.	dselect
loadkeys	Load keyboard configuration.	loadkeys <keymap>
lsof	Identify processes.	lsof -i :<port>
mount	Include a new file system into the existing file system tree.	mount <device> <mountpoint>
ntpdate	Set the system time and date using a time server.	ntpdate <time_server>
quota	Display information about allowed disk space usage.	quota -u <username>
recode	Convert files to another character set.	recode <encoding> <inputfile> > <outputfile>
rpm	Manage RPM packages.	rpm -i <package.rpm>
setfont	Choose a font.	setfont <fontfile>
timezone	Set the timezone.	timezone <timezone>
tzconfig	Set the timezone.	tzconfig
ulimit	Set or display resource limits.	ulimit -n <limit>
up2date	Manage RPM packages.	up2date -u
urpmi	Manage RPM packages.	urpmi <package>
yum	Manage RPM packages.	yum install <package>

Please note that the above usage examples are simplified and may require additional options or arguments depending on the specific use case. Make sure to consult the command's documentation or man page for more detailed information on how to use each command.

Commands related to printing:

Command	Meaning	Usage
lpr or lp	Print file	lpr <file> or lp <file>
lpq or lpstat	Query print queue	lpq or lpstat
lprm or cancel	Remove print job	lprm <jobID> or cancel <jobID>
acroread	PDF viewer	acroread <file.pdf>
groff	Formatting tool	groff -ms <file>
gv	PostScript viewer	gv <file.ps>
printconf	Configure printers	printconf
xdvi	DVI viewer	xdvi <file.dvi>
xpdf	PDF viewer	xpdf <file.pdf>
*2ps	Convert file to PostScript	<command> <file> > output.ps

Please note that the above usage examples are simplified and may require additional options or arguments depending on the specific use case. Make sure to consult the command's documentation or man page for more detailed information on how to use each command.

Archiving data using the tar command, as well as compressing and unpacking the archives using gzip and bzip2.

1. Preparing Your Data: Archiving with tar: -The tar command is used to create archive files by concatenating and compressing listed files. -Use the -c option to create an archive and the -f option to specify the archive file name. -Example: tar cvf archive.tar files/ creates an archive named archive.tar containing the files/ directory. -Example: tar cvf archive.tar file1 file2 creates an archive named archive.tar containing file1 and file2.

2. Incremental Backups with tar:

    -tar supports creating incremental backups using the `-N` option.
    -With -N, you can specify a date, and tar will include files modified more recently than the specified date in the backup.
    -Example: `tar -N backup.tar -cvp files/` creates an incremental backup of files modified since the backup.tar archive.
    -Example: `tar -N backup.tar -cvp newfile` creates an incremental backup including only the newfile that is more recent than the backup.tar archive.
   

3. Compressing and Unpacking with gzip or bzip2:

    -Data, including tarballs, can be compressed using `gzip` or `bzip2`.
    -Use gzip to compress files, adding the `.gz` extension to the file name.
    -Example: `gzip file.txt` compresses file.txt and creates `file.txt.gz`.
    -Use `gzip -d` or `gunzip` to uncompress .gz files.
    -Example: `gzip -d file.txt.gz` uncompresses file.txt.gz.
    -Use `bzip2` for improved compression, creating smaller files than gzip.
    -Example: `bzip2 file.txt` compresses file.txt and creates file.txt.bz2.
    -Use `bzip2 -d` or `bunzip2` to uncompress .bz2 files.
    -Example: `bzip2 -d file.txt.bz2` uncompresses file.txt.bz2.
   

4. Unpacking Compressed Archives with tar:

    -tar can directly handle compressed archives using the appropriate options.
    -Use `tar zxvf file.tar.gz` to extract and uncompress a .tar.gz or .tgz archive.
    -Use `tar jxvf file.tar.bz2` to extract and uncompress a .tar.bz2 archive.
   

Data Encryption with GNU Privacy Guard(gpg) generate a key: gpg --key-gen get information about your key: gpg --list-keys encrypt a .tar archive: gpg -e -r (part of) uid archive , -e option tells gpg to encrypt, the -r option indicates who to encrypt for.

Note: Manual page for gpg

*NOTE: GIVING SPACE BEFORE TYPING A COMMAND REMOVES IT FROM HISTORY. USE !NUM ASSOCIATED TO THE COMMAND IN ORDER TO DIRECTLY GET THE COMMAND