Linux

Personal reference notes on Linux: core concepts, the filesystem, permissions, and the commands I actually reach for day to day.

Related

Working on Windows instead? See PowerShell for the shell fundamentals, or Terminal Commands for ready-to-use PowerShell / Node.js scripts (project linting, file management, duplicate detection, etc.).

Core Concepts

• Everything is a file — devices, sockets, and pipes are all represented as files under /dev, /proc, etc.
• Kernel vs. distro — the kernel is the core (process/memory/device management); a distribution (Ubuntu, Debian, Fedora, Arch, Alpine...) bundles the kernel with a package manager, init system, and userland tools.
• Shell — the command interpreter. Most distros default to bash; zsh and fish are popular alternatives. Check with:

echo $SHELL

• Init system — most modern distros use systemd to manage services/boot. Older or minimal systems may use SysVinit or OpenRC.

Filesystem Hierarchy Standard (FHS)

Path	Purpose
/	Root of the filesystem
/bin, /usr/bin	Essential user binaries
/sbin, /usr/sbin	System admin binaries
/etc	System-wide configuration files
/home	User home directories
/root	Root user's home directory
/var	Variable data — logs, caches, spool files
/var/log	Log files
/tmp	Temporary files (often cleared on reboot)
/opt	Optional/third-party software
/proc	Virtual filesystem exposing kernel/process info
/dev	Device files
/mnt, /media	Mount points for filesystems/removable media
/lib, /usr/lib	Shared libraries
/boot	Bootloader files, kernel images

File Permissions

Permissions shown by ls -l look like -rwxr-xr--:

• Position 1: file type (- file, d directory, l symlink)
• Next 3: owner permissions (rwx)
• Next 3: group permissions (rwx)
• Next 3: other permissions (rwx)

Numeric (octal) shorthand: r=4, w=2, x=1, summed per group.

chmod 755 script.sh      # rwxr-xr-x
chmod u+x script.sh      # add execute for owner
chown user:group file    # change owner and group
chgrp group file         # change group only

Special bits:

• setuid (4000) — run as file owner
• setgid (2000) — run as group owner / inherit group on new files in a dir
• sticky bit (1000) — only owner can delete files in a shared dir (e.g. /tmp)

chmod 4755 binary   # setuid example
chmod 1777 /tmp      # sticky bit example

Popular Commands

Navigation & File Management

pwd                     # print working directory
ls -la                  # list all files, long format, incl. hidden
cd /path/to/dir         # change directory
cd -                    # jump to previous directory
cp -r src/ dest/        # copy recursively
mv old new               # move/rename
rm -rf dir/             # remove recursively, force (careful!)
mkdir -p a/b/c           # create nested dirs
touch file.txt           # create empty file / update timestamp
find . -name "*.log"     # find files by name
find . -mtime -7         # files modified in last 7 days
locate filename           # fast file search (needs updatedb / mlocate)
tree -L 2                 # show directory tree, 2 levels deep

Viewing & Editing Files

cat file.txt              # print whole file
less file.txt              # paginated viewer (q to quit)
head -n 20 file.txt          # first 20 lines
tail -n 20 file.txt           # last 20 lines
tail -f app.log                # follow file live (logs)
nano file.txt                   # simple terminal editor
vim file.txt                     # modal editor

Text Processing

grep -rn "pattern" .        # recursive, show line numbers
grep -i "pattern" file       # case-insensitive
grep -v "pattern" file        # invert match (exclude)
sed 's/foo/bar/g' file         # find & replace
awk '{print $1}' file           # print first column
sort file | uniq -c              # sort + count duplicates
wc -l file                        # count lines
cut -d',' -f2 file.csv             # extract 2nd CSV column
diff file1 file2                    # compare files
xargs                                # build/execute commands from stdin

Permissions & Ownership

chmod 644 file
chown -R user:group dir/
umask                       # view default permission mask

Process Management

ps aux                     # list all running processes
ps aux | grep nginx         # find a specific process
top                          # live process viewer
htop                          # nicer live process viewer (if installed)
kill PID                        # terminate a process (SIGTERM)
kill -9 PID                      # force kill (SIGKILL)
killall processname               # kill by name
pkill -f pattern                    # kill by matched command line
bg / fg                              # background/foreground a job
jobs                                  # list background jobs
nohup command &                        # run immune to hangup, in background
disown                                  # detach job from shell
nice -n 10 command                       # run with lower priority
renice 10 -p PID                          # change priority of running process

Disk & Storage

df -h                     # disk free space, human-readable
du -sh *                   # size of each item in current dir
du -sh dir/                 # total size of a directory
lsblk                        # list block devices
mount /dev/sdb1 /mnt          # mount a device
umount /mnt                    # unmount
fdisk -l                        # list partitions (needs sudo)
mkfs.ext4 /dev/sdb1               # format a partition

Networking

ip a                       # show IP addresses / interfaces
ip r                        # show routing table
ping host                    # test connectivity
curl -I https://example.com   # fetch headers
curl -o file.zip URL           # download a file
wget URL                        # download a file
ss -tulpn                        # show listening ports (modern netstat)
netstat -tulpn                    # legacy version of the above
dig domain.com                     # DNS lookup
nslookup domain.com                 # DNS lookup, simpler output
traceroute host                      # trace network path
scp file user@host:/path              # copy file over SSH
rsync -avz src/ user@host:/dest/       # efficient sync (local or remote)
ssh user@host                           # remote shell
ssh -i key.pem user@host                 # remote shell with key

Package Management

# Debian/Ubuntu (APT)
apt update && apt upgrade
apt install package
apt remove package
apt search keyword

# RHEL/Fedora (DNF/YUM)
dnf install package
dnf update

# Arch (Pacman)
pacman -S package
pacman -Syu

# Universal / language-agnostic
snap install package
flatpak install package

Users & Groups

whoami                    # current user
id                          # current user's UID/GID/groups
sudo command                 # run as root
su - username                 # switch user
adduser username               # create a user (interactive, Debian-style)
useradd -m username              # create a user with home dir
passwd username                    # set/change password
usermod -aG groupname username      # add user to a group
groups username                       # list groups a user belongs to

System Info & Monitoring

uname -a                  # kernel/system info
hostnamectl                 # hostname & OS info
uptime                        # how long system has been running + load avg
free -h                        # memory usage
lscpu                            # CPU info
dmesg | tail                       # recent kernel messages
journalctl -xe                       # recent systemd logs
journalctl -u service_name             # logs for a specific service
history                                  # command history
history | grep ssh                        # search command history

systemd Services

systemctl status service_name
systemctl start service_name
systemctl stop service_name
systemctl restart service_name
systemctl enable service_name    # start on boot
systemctl disable service_name
systemctl list-units --type=service

Archiving & Compression

tar -czvf archive.tar.gz dir/    # create gzip tarball
tar -xzvf archive.tar.gz          # extract gzip tarball
tar -tzvf archive.tar.gz           # list contents without extracting
zip -r archive.zip dir/             # create zip
unzip archive.zip                     # extract zip

Environment & Shell

export VAR=value           # set env var for session
echo $VAR                    # print env var
env                           # list all env vars
alias ll='ls -la'              # create a shortcut
which command                    # locate a command's binary
type command                      # show how shell resolves a command
man command                        # manual page
command --help                      # quick usage help

Useful Shortcuts (Bash)

Shortcut	Action
Ctrl+C	Kill current foreground process
Ctrl+Z	Suspend current process (resume with fg)
Ctrl+R	Reverse search command history
Ctrl+A / Ctrl+E	Jump to start / end of line
Ctrl+L	Clear screen
Ctrl+D	Exit shell / EOF
!!	Repeat last command
!$	Last argument of previous command
sudo !!	Re-run last command with sudo

Redirection & Pipes

command > file        # redirect stdout, overwrite
command >> file         # redirect stdout, append
command 2> file           # redirect stderr
command &> file             # redirect both stdout and stderr
command < file                # use file as stdin
cmd1 | cmd2                     # pipe stdout of cmd1 into cmd2
command &                        # run in background

Things I Keep Forgetting

• chmod -R and chown -R apply recursively — double check the target directory before running.
• rm -rf has no undo. Consider rm -i for confirmation prompts, or trash-cli as a safer alternative.
• sudo !! re-runs the last command with sudo prepended — handy after a Permission denied.
• Symlinks: ln -s target linkname (target first, link name second — easy to mix up).
• du -sh * doesn't include hidden files; add .[!.]* or use du -sh .[!.]* * to catch dotfiles too.
• ss is the modern replacement for netstat on most current distros.
• Exit status of the last command: echo $? (0 = success).

Further Reading

• Linux man pages online
• Arch Wiki — excellent even if not using Arch
• explainshell.com — breaks down what a shell command does, piece by piece