Learning Linux: Week 1 – Mastering the Basics

Here is a comprehensive guide to the basics of Linux that you need to learn

What is OS?

• An OS is a software that acts as a middleman or a bridge between computer hardware and the computer user.

• It provides a user interface and controls the computer hardware so that software can function

What is Shell?

• a program that takes your input and sends it to the OS to perform

• Terminal / Console → programs that launch a shell for you

• The default shell for almost all Linux distributions is bash (AL 2022 or AL 2023, etc.)

Basic Commands

• pwd → used to see where you are currently ( i.e., print working directory)

• cd → used to change directory

  • eg: cd /home/pete/Pictures → Change your working directory to /home/pete/Pictures

  • Few shortcuts:

    • . → Current Directory

    • .. → Parent Directory

    • ~ → Home Directory

    • - → This will take you to previous directory You were just at

• ls → used to list or display the content of a directory

  • -a → lets you view all files (even the hidden ones)

  • -l → long format with detailed info ( file permissions, links , owner , owner group , file size , timestamp of last modification )

• touch → allows you to create new empty files

  • Syntax: touch <filename>

• cat → It is used to display the content of a file on the terminal itself, but also allows you to combine multiple files and show you the output of them

  • Syntax: cat <filename>

• file → to display the kind or type of file

  • Syntax : file <filename>

• whoami → tells you the username of the current user

• clear → clears the terminal

• echo → print anything on the terminal

  • Syntax: echo “HEllo World!”

• history → used to display command history, it stores the history of all commands used in the current session

• exit → exit the terminal with the current user

• whatis → displays a one-line description of the command

• type → displays the type of every Linux command that you want

• w ⇒ to see who is logged into the system and who is using it

  • -s ⇒ short notation (remove LOGGING, JCPU,, PCPU

  • -V ⇒ version

  • -o ⇒ old-style output

  • <user> ⇒ for a particular user

  • -h ⇒ no-header

• last ⇒ listing of last logged-in users

  • -x ⇒ to display system shutdown information and run level changes

  • -a ⇒ display host names in the last columns

  • -w ⇒ display full user name and domain names

• diff → compares files line by line

  • -y → display output in two columns side by side

  • -q → report only if the file differs

  • -c → to display the output and contents of the file

    • - → to delete

    • ! → change

• There are multiple options you can use with each command, to check use man <command> or <command> —help

Directory Tree

• The directory tree in Linux refers to the hierarchical structure used to organize files and directories within the file system. It is a fundamental concept in Linux and Unix-like operating systems.

• All directories are installed under / → Root Directory or ROOT file system (parent of all directories)

• /bin → contains executable files of many basic shell commands

• /opt → used for storing or installing the files of 3rd party applications not available from the distribution’s repo

  • command practice → store software code in /opt and binary files in /bin

• /home → each user has a home directory where they store user-specific config files and user data

  • One user doesn’t have access to another user’s home directory

• /usr → stores user bin and all executable files, libraries.

  • /usr/bin → contains basic user commands

  • /usr/sbin → contains additional commands for the admin

  • /usr/share → contains documentation common to all libraries

• /root → home directory for root user (separate from /home directory)

• /lib → codes that can be used by executable binaries

• /dev → details about system devices or hardware ( virtual files not physically on disk )

  • /dev/null → path to which anything sent will be destroyed

    • Use: When you want to discard output or prevent clutter.

  • /dev/zero → contains an infinite sequence of 0

    • Use: When you need to create empty files or initialize storage with zero bytes.

  • /dev/random → contains an infinite sequence of random numbers

    • Use: When you need secure random numbers, like for encryption keys.

• /tmp → stores temporary files, which get removed when the system reboots

• /etc → contains config files for scripts, programs,etc; contains core config files for the system

• /boot → has boot files, config, and info about boot files and details of the kernel of Linux distribution

• /sbin → system binaries, similar to /bin Except it contains binaries that are run by root or the sudo user

• /var/ → contains data and info that changes / updates most of the time

  • contains directors for log , mail , spok

• /media → mount point for removable media like USB, DVD, and SD Card. When connected, a directory is automatically created under this

• /mnt → Mount directory, similar to /media But not automatically, system admins use this to manually mount a filesystem to view data

• /srv → Service data , contains data for services provided by the system

rm -rf / cleans the entire linux system but won’t work unless you provide -no-preserve-root with it

Absolute vs Relative Paths

Searching

• Using Find

  • a built-in command in Linux (use man find for details)

  • Finding By Name

    • using -name option we can find by the filename

    • Syntax: find -name “query”

    • for ignoring case-sensitivity in filenames using -i (Eg: find -iname “query”)

    • Invert the search using ! → find \\! -name “query_to_avoid”

    • use -not → to match with not equal

  • Find by Type

    • using -type we can search by type of file

      • f → regular file

      • d → directory

      • l → symbolic link

      • c → character device

      • b → block device

    • Syntax: find <path> -type c

  • Find by Time & Size

    • using parameter -size to filter by the size of file

    • commonly used size suffix

      • c → bytes

      • k → kilobytes

      • M → megabytes

      • G → gigabytes

      • b → 512-byte blocks

            find /usr -size 50c # less than 50 bytes

            find /usr -size +700M # more than 700 egabytes

• Find by Owner and Permissions

  • search via username using -user option

    • find /var -user syslog

  • search via groupname using -group option

    • find /etc -group shadow

• Find by specific permission

  • using -perm option

    • find / -perm 644

  • search for files with at least these permissions using - before permission

    • find / -perm -644

• Find by depth

  • There can be multiple recursive folders or files in the specified file, so you can set the max and min depth to search up to using -maxdepth and -mindepth options

    find -maxdepth 2 -name file1 find -mindepth num -name query

• Using Locate

  • A third-party package mlocate It is used to search for files throughout your system

    • Install via yum install mlocate or apt-get install mlocate

  • This is considerably faster than find because it uses a database that lists all files on the filesystem

  • This database needs to be updated either once everyday with a cron or manually via updatedb command

  • the locate database must always be up-to-date to find new files

      locate query
    
      locate -b query # basename should be equal to the query
    
      locate -e query # only return results that still exist in the filesystem
    
      locate -S # statistics about the information
    

Wildcards ( *, ? , ^ , [ ] )

• Asterisk ( * )

  • matches any no. of characters, including zero characters

    • (In Linux, the term "zero characters" primarily refers to null characters (ASCII NUL, 0x00). These are control characters that do not have a visual representation and are often used in programming contexts)

  • used to match any file or directory that fit a certain pattern

  • Eg : *.txt or file*

• Question Mark ( ? )

  • matches exactly one character

  • used to match only a specific number of characters

  • Eg: file?.txt → will match for file1.txt, file2.txt

• Caret ( ^ )

  • used in regular expressions, not as wildcards in simple file matching

  • Eg: ^hello → would match lines that start with the word “hello” in a text file

• Square bracket ( [ ] )

  • used to match a single character from a set or range of characters

  • Usage → characters inside the square brackets define a set or range that the character must match

  • Eg: file[a-z].txt → match for files like filea.txt, fileb.txt, etc.

Hard Links vs Soft Links