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📜Linux Essentials

Quick reference of linux essentials and utilities to the beginner linux user. These notes were written quite a whilte ago, so some inaccurasies might be expected.

Filesystem Hierarchy Standard

Is a Linux filesystem standard, presented in the following screen

/

Primary hierarchy root and root directory of the entire file system hierarchy.

/bin

Essential command binaries that need to be available in single user mode; for all users, e.g., cat, ls, cp.

/boot

Boot loader files, e.g., kernels, initrd.

/dev

Device files, e.g., /dev/null, /dev/disk0, /dev/sda1, /dev/tty, /dev/random.

/etc

Host-specific system-wide configuration files

There has been controversy over the meaning of the name itself. In early versions of the UNIX Implementation Document from Bell labs, /etc is referred to as the etcetera directory,[3] as this directory historically held everything that did not belong elsewhere (however, the FHS restricts /etc to static configuration files and may not contain binaries).[4] Since the publication of early documentation, the directory name has been re-explained in various ways. Recent interpretations include backronyms such as "Editable Text Configuration" or "Extended Tool Chest".[5]

/etc/opt

Configuration files for add-on packages that are stored in /opt.

/etc/sgml

Configuration files, such as catalogs, for software that processes SGML.

/etc/X11

Configuration files for the X Window System, version 11.

/etc/xml

Configuration files, such as catalogs, for software that processes XML.

/home

Users' home directories, containing saved files, personal settings, etc.

/lib

Libraries essential for the binaries in /bin and /sbin.

/lib<qual>

Alternative format essential libraries. Such directories are optional, but if they exist, they have some requirements.

/media

Mount points for removable media such as CD-ROMs (appeared in FHS-2.3 in 2004).

/mnt

Temporarily mounted filesystems.

/opt

Optional application software packages.[6]

/proc

Virtual filesystem providing process and kernel information as files. In Linux, corresponds to a procfs mount. Generally automatically generated and populated by the system, on the fly.

/root

Home directory for the root user.

/run

Run-time variable data: Information about the running system since last boot, e.g., currently logged-in users and running daemons. Files under this directory must be either removed or truncated at the beginning of the boot process; but this is not necessary on systems that provide this directory as a temporary filesystem (tmpfs).

/sbin

Essential system binaries, e.g., fsck, init, route.

/srv

Site-specific data served by this system, such as data and scripts for web servers, data offered by FTP servers, and repositories for version control systems (appeared in FHS-2.3 in 2004).

/sys

Contains information about devices, drivers, and some kernel features.[7]

/tmp

Temporary files (see also /var/tmp). Often not preserved between system reboots, and may be severely size restricted.

/usr

Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications.[8]

/usr/bin

Non-essential command binaries (not needed in single user mode); for all users.

/usr/include

Standard include files.

/usr/lib

Libraries for the binaries in /usr/bin and /usr/sbin.

/usr/lib<qual>

Alternative format libraries, e.g. /usr/lib32 for 32-bit libraries on a 64-bit machine (optional).

/usr/local

Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g., bin, lib, share.[9]

/usr/sbin

Non-essential system binaries, e.g., daemons for various network-services.

/usr/share

Architecture-independent (shared) data.

/usr/src

Source code, e.g., the kernel source code with its header files.

/usr/X11R6

X Window System, Version 11, Release 6 (up to FHS-2.3, optional).

/var

Variable files—files whose content is expected to continually change during normal operation of the system—such as logs, spool files, and temporary e-mail files.

/var/cache

Application cache data. Such data are locally generated as a result of time-consuming I/O or calculation. The application must be able to regenerate or restore the data. The cached files can be deleted without loss of data.

/var/lib

State information. Persistent data modified by programs as they run, e.g., databases, packaging system metadata, etc.

/var/lock

Lock files. Files keeping track of resources currently in use.

/var/log

Log files. Various logs.

/var/mail

Mailbox files. In some distributions, these files may be located in the deprecated /var/spool/mail.

/var/opt

Variable data from add-on packages that are stored in /opt.

/var/run

Run-time variable data. This directory contains system information data describing the system since it was booted.[10]

In FHS 3.0, /var/run is replaced by /run; a system should either continue to provide a /var/run directory, or provide a symbolic link from /var/run to /run, for backwards compatibility.[11]

/var/spool

Spool for tasks waiting to be processed, e.g., print queues and outgoing mail queue.

/var/spool/mail

Deprecated location for users' mailboxes.[12]

/var/tmp

Temporary files to be preserved between reboots.

Linux Permission & Groups

Each file in Linux has Three(3) modes of access

  • Owner (section 2)

  • Group (section 3)

  • Others (section 4)

Each "mode of access" or user can possess a combination of Three(3) available permissions

  • Read - R - 4 (octal representation of permission)

  • Write - W - 2

  • Execute - X - 1

The Octal representation can benefit us when using tools like 'chmod' to modify the permissions

  • Section 1 reveals what type the object is:

    • d : directory

      • : regular file

    • l : soft link

  • Section 2,3,4 are detailed permissions for the respective user

  • Section 5 details any alternative access methods

    • '-' : No other method

    • '.' : SELinux

    • '+' : FACLs

    • '+' : Any combination of methods

  • Section 6,7 are names of the Owner and Group respectively

Programs manual - "man" command

The manuals also document and present the type of "command" or tool as the following table displays:

  • 1 - User Commands

  • 2 - Programming interfaces for kernel system calls

  • 3 - Programming interfaces to the C library

  • 4 - Special files such as device nodes and drivers

  • 5 - File formats

  • 6 - Games and amusements such as screen-savers

  • 7 - Miscellaneous

  • 8 - System administration commands

Note:

Can search using keywords with the "-k" flag and also with Regular Expressions, Just apply it inline

Example:

Output:

Descriptive tools search - "apropos" command

A way to search for a given tool by searching keywords from it's description.

Example:

Output:

Find files in Linux

System Services management

  • systemctl

  • ss (viewing)

Advanced Package Tools

Kali linux is a Debian distribution that utilizes the Advanced package tool (APT) software to provide a package management software services in kali linux.

Two main application exists as an interface to managing software packages:

  • apt

    apt is a simplified and user-friendly version of apt-get

    Common command:

  • apt-get

Note:

apt installs software package and fulfils those software's dependencies recursively

dpkg

dpkg is a core tool used to install packages on linux directly or indirectly through "apt".

dpkg those not resolve and install dependencies recursively according to the software's needs so that's something to take into account

  • dpkg can also install packages offline from a .deb file by using the -i or —install flag

Bash - Tips & Tricks

Environment Variables

Example:

View environment variable:

  • env Add environment variable:

  • export

Piping and Redirection

The are 3 streams of information in linux CLI

  • STDIN(0) - program input

  • STDOUT(1) - program output / result output

  • STRERR(2) - error messages stream

the information present in those streams can be redirected in multiple ways:

  • redirect information to be written as a file ( >, >> )

  • redirect information and input to a program ( | )

  • reverse redirection, etc. ( < )

Example:

Bash history & CLI tricks

Bash have a history file named ".bash_history" that can be displayed by the command "history"

Shortened CLI command execution:

  • ! - will execute the command in the specified numerical row in the history file

  • !! - will execute the last executed command

  • CTRL + R - reverse-i-search will prompt a search bar for search commands in the history file and then executing them.

Text Searching and Manipulation

Common and powerful tools are:

  • grep - searches for regex occurrences in the files

  • awk - like cut but more complex and verbose

  • cut - used to cut text from a line according to specific parameters

  • sed - edits text from a stream

File Editing, Manipulation & Analysis

Text Editors

There are many text editor, some more capable and complicated than others

  • nano

  • vi

    • Good to know flags

    • insert - to begin typing

    • ~ - to enter command mode

    • :q! - Quit without saving changes

    • :w - Write file to disk

    • :wq! - Save changes and quit

Comparing Files

  • comm - creates output of 3 columns in this order [ A_UNIQUE_LINES(1), B_UNIQUE_LINES(2), AB_SIMILLAR_LINES]

    • -n <column_num> - a flag to control visibility of columns

    -n 12 (column 1 and 2)

  • diff - ' - ' precedes occurrence in 1st file where not in 2nd file ' + ' precedes occurrence in 2nd file where not in 1st file

    Note: When using the -c format flag, similar lines will be present in the output, when using the -u format flag, only different lines will be printed

  • vimdiff - compares two files against each other visually with color coding

Managing Process

Processes can work in the "background" or in the "foreground", the difference between the two is the terminal focus position.

Also, there is a mechanism that combines multiple processes that work together as a "job" which can also be managed.

  • bg - sending a process / job to the background allowing you to continue working in the terminal

    • Another easy way to background a process is by appending "&" to the end of the execution command.

  • fg - Sending a process or a job to foreground

  • jobs - shows the current active jobs with an ID number for each, you can use bg and fg in combination with that number.

  • ps - shows active process on the machine

    • -ef - select all process with full format listing

    • -C - searches for the process execution command

  • kill - kills a process by getting a PID

File Monitoring

tail

"tail" display the last lines of a given text file

"-n" enables you to choose number of lines to display

watch

"watch" is a utility that executes a command in a given (or default) time interval

"-n" will determine the interval

File Download

wget

download webpages and files.

-O to choose a new name for the downloaded file

curl

download webpages and files.

axel

A download accelerator, downs a file in multiple streams

Bash Environment Customazation

.bashrc

~/.bashrc is the bash configuration file.

export

a utility to export environment variable for global terminal usage

export HISTTIMEFORMAT='%F %T '

alias

A utility for creating aliases

alias .. ="cd .."

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