notes.md

Linux Basics & Command Line

Started: Aug 29, 2025
Status: Finished
Estimated completion: Sep 7, 2025 Sep 8, 2025

Study Log

Day 1

Learned about Linux history and different distros. Linux has tons of distros, and choosing the right one depends on what you want to do (Ubuntu for beginners, CentOS for servers, RHEL for enterprise, etc).

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Day 2

Practiced shell navigation cmds (pwd, cd, mkdir, touch, cat, cp, mv, rm). Learned I/O redirection:

• > (overwrite): echo hello > file.txt
• >> (append): echo world >> file.txt
• 2> (errors): ls /notfound 2> error.txt
• &> / &>> (stdout+stderr): command &> out.txt or command &>> out.txt
• tee (write + display): echo hello | tee file.txt

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Day 3

Explored Linux variables:

• Local variables (exist only in the current shell)
• Environment variables (passed down to child processes)

Common variable commands:

• echo $VAR to display values
• export VAR=value to create environment variables
• env to list all environment variables

Child shell inheritance test: Variables created in the parent shell are available in bash subshells only when exported. Non-exported variables stay inaccessible to child processes.

The set -o allexport command automatically exports any new variables.

For permanent settings between sessions:

• ~/.bashrc for Bash shell users
• ~/.zshrc for Zsh shell users

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Day 4

Explored essential text-processing commands:

• cut: Extract sections from lines (-c for chars, -f for fields, -d for delimiter)
• uniq: Filter duplicate lines (needs sorted input)
  • -c: Show counts
  • -d: Show only duplicates
• paste: Merge files line by line
• join/split: Combine or break apart files
• head/tail: View beginning/end of files
• expand/unexpand: Convert between tabs and spaces
• sort: Order text lines
• tr: Translate/replace characters
• wc: Count lines/words/characters
• nl: Number lines
• grep: Search text patterns

Started regex. My head hurt. Will try again later.

Completed OverTheWire Bandit levels 0-5.

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Day 5

Learned a bit about text editors like Vim, but I'm sticking with Nano for now since I'm more comfortable with it. That wrapped up Advanced Text-Fu, and I started the User Management lesson.

Practiced the basics of users and groups, including the full user lifecycle: creating accounts, securing them, modifying them, and deleting them. Commands used:

• useradd : add a new user
• useradd -m : add a user and create their home directory
• sudo : run commands as superuser/root
• su user : switch to another user account :loads their shell:
• su - user : switch to another user and load that user's complete environment, including their home directory, PATH variables, and other shell configurations

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Day 6

Continued practicing user administration:

• Managed access with passwd -l / passwd -u (i.e., lock/unlock accounts)
• Managed password aging with chage (controls password expiration policies)
• Modified users with usermod (e.g., usermod -aG group user to add a user to a group)
• Managed groups with groupadd
• Checked identities/memberships with id and groups
• Removed users with:
  • userdel (user only)
  • userdel -r (user + home dir) Note: Use with caution as this deletes all user data

Also, I learned how to:

• Check permissions using the ls -l command
• Change file ownership with chown
• Modify permissions with chmod (using numeric and symbolic ways):
  • 755: owner has full rights (7 → rwx), group has read/execute (5 → r-x), others also get read/execute (5 → r-x)
  • 750: would block "others" completely (0 → ---)
  • u+x: adds execute permission for the user/owner
  • u-x: removes execute permission for the user/owner
  • ug+r: adds read permission for user and group

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Day 7

Since I haven’t written in this journal until now, I decided to catch up and log Day 1 through Day 6 today."

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Day 8

Learned about umask (default permission mask for new files and directories) and special permissions:

• SetUID (SUID): allows a program to run with the file owner's privileges (example: passwd). Applies only to executable files and is ignored for scripts on most modern systems for security reasons. Set with chmod u+s file or chmod 4755 file.
• SetGID (SGID): for files, causes execution with the file's group privileges; for directories, new files inherit the directory's group. Set with chmod g+s file or chmod 2755 file.
• Sticky bit: meaningful only on directories; it ensures that only the file owner or root can remove files inside the directory. Set with chmod +t dir or chmod 1755 dir.

Finished the permissions section and started processes:

• Basic inspection
  • ps shows processes for the current shell
  • ps aux lists all system processes with details (USER, PID, %CPU, %MEM)
  • ps l shows long format
  • top provides a real-time process monitor
• Controlling processes
  • kill <PID> sends the default SIGTERM
  • kill -9 <PID> sends SIGKILL to force termination (cannot be caught or ignored; no cleanup)
  • Signals:
    • SIGHUP (hangup): sent when a controlling terminal closes; daemons often interpret it as "reload config"
    • SIGINT (interrupt, Ctrl+C): interactive interrupt; can be caught to perform cleanup before exiting
    • SIGTERM (terminate): polite termination request; default for kill <PID>; can be caught for graceful shutdown
    • SIGKILL (kill, 9): immediate, uncatchable kill; use only when a process won't respond to SIGTERM
    • SIGSTOP (stop): pause a process (uncatchable); resume with SIGCONT
  • Notes:
    • Prefer SIGTERM for normal shutdowns so processes can clean up; use SIGKILL as last resort.
    • You can send by name: kill -SIGTERM <PID> or by number: kill -15 <PID>.
  • Each process has a PID and PPID; init or systemd adopts orphans
  • _exit ends a process; parent should wait to collect status
  • Orphan: parent dies and process is adopted by init/systemd
  • Zombie: process finished but parent did not wait; the process remains as a zombie until the parent collects its exit status with wait; if the parent never does, init/systemd will eventually reap the zombie.
• Priority and niceness
  • Start a command with adjusted niceness: nice -n <value> command
  • Change a running process priority: renice <value> <PID>
• States and internals
  • Common states: R (running), S (sleeping), D (uninterruptible sleep), Z (zombie), T (stopped)
  • /proc is a virtual filesystem for process and kernel info, e.g. /proc/<PID>/status, /proc/cpuinfo
• Job control in the shell
  • Run in background with &
  • List jobs with jobs
  • Resume in background with bg
  • Bring to foreground with fg
  • Reference jobs with %<job> (example: kill %1)

Practiced examples: setting umask, toggling SUID/SGID/sticky bits, inspecting /proc entries, killing and renicing processes to reinforce concepts.

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Day 9

Completed overthewire Bandit levels 6-10.

Learned about bash scripting basics: shebang (#!/bin/bash), comments (#), variables, and script execution permissions (chmod +x script.sh).

Made my first bash script that lists all users, shows disk usage, and logs with a timestamp. Check the exercises section down below for details.

Exercises & Projects

See Exercises for all hands-on practice.

Resources Used

• Linux Journey - My main Linux learning resource
• LabEx ⭐ – Hands-on labs that reinforced my understanding. (You get 3 sessions a day, each one is a 60-minute session for free. use them wisely)
• OverTheWire Bandit – CTF-style challenges to evaluate my learning
• Learn Shell ⭐ – Shell scripting tutorials with interactive exercises

⭐ I highly recommend

Completion Overview

Total days: : 12 days (9 days of study + 3 days of breaks)

Quick thoughts: : I really enjoyed learning the basics of Linux and command line. My previous experience definitely helped, but I still learned a lot of new things.

Next Topic → Master Markdown & Documentation