Fundamentals — Cheat Sheet

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Fundamentals

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Linux Filesystem Hierarchy

Everything in Linux is a file. The Filesystem Hierarchy Standard (FHS) defines a single tree rooted at / — knowing where things live makes you fast on any Linux system.

✓Linux has a single directory tree rooted at / — no drive letters.
✓/etc stores configuration, /var/log stores logs, /home stores user data.
✓/proc and /sys are virtual filesystems — kernel and hardware state as readable files.
✓Everything in Linux is represented as a file: disks (/dev/sda), processes (/proc/PID), sockets.
✓Absolute paths start with /; relative paths start from the current directory.

Linux directory hierarchy

/ (root)
â”œ── bin/      â†’ essential user binaries (ls, cp, mv, bash) — always available
â”œ── sbin/     â†’ system binaries for root (fdisk, ifconfig, reboot)
â”œ── etc/      â†’ system-wide configuration files (nginx.conf, sshd_config, hosts)
â”œ── home/     â†’ user home directories (/home/akshay, /home/alice)
â”œ── root/     â†’ root user's home directory
â”œ── var/      â†’ variable data: logs, caches, spool files
â”‚   â”œ── log/  â†’ system and app logs (syslog, nginx/access.log, auth.log)
â”‚   â””── run/  â†’ runtime PID files and sockets
â”œ── tmp/      â†’ temporary files, cleared on reboot
â”œ── usr/      â†’ user programs (the bulk of installed software)
â”‚   â”œ── bin/  â†’ most user commands (python3, git, vim, curl)
â”‚   â”œ── lib/  â†’ shared libraries (.so files)
â”‚   â””── local/â†’ locally compiled software (not managed by package manager)
â”œ── opt/      â†’ optional third-party software (JetBrains, custom installs)
â”œ── proc/     â†’ virtual FS — live kernel and process info (/proc/cpuinfo)
â”œ── sys/      â†’ virtual FS — hardware and kernel parameters (/sys/class/net)
â”œ── dev/      â†’ device files (disks, terminals, /dev/null, /dev/random)
â”œ── lib/      â†’ shared libraries needed by /bin and /sbin
â”œ── boot/     â†’ bootloader and kernel images (vmlinuz, initrd)
â””── mnt/      â†’ temporary mount points for external filesystems

File Permissions & Ownership

Linux file permissions control who can read, write, or execute every file. Understanding rwx, octal notation, chmod, chown, and setuid is critical for security and system administration.

✓Permissions are three groups of rwx: owner, group, others.
✓Numeric: r=4, w=2, x=1. Common: 755 (binaries), 644 (configs), 600 (private keys).
✓chmod changes permissions; chown changes file ownership.
✓setuid on executables: runs with the file owner's privileges (e.g., passwd runs as root).
✓Sticky bit on directories: only the file owner can delete their own files (used on /tmp).
✓SSH will refuse to use a private key that is not 600 or 400 — "bad permissions" error.

Reading ls -l permission output

ls -la /etc/nginx/nginx.conf
# -rw-r--r-- 1 root root 2691 Jan 15 10:23 nginx.conf
# â”‚â””─â”¬──â”˜â””─â”¬──â”˜â””─â”¬──â”˜
# â”‚  owner  group  others
# â”‚
# â””── file type:
#     - = regular file
#     d = directory
#     l = symbolic link
#     b = block device
#     c = character device

# Permission bits: r=read w=write x=execute
# rw-  r--  r--
# 110  100  100   â† binary
#  6    4    4    â† octal = 644

# Common permission patterns:
# 755  rwxr-xr-x  binaries, directories (owner full, others read+execute)
# 644  rw-r--r--  config files (owner read-write, others read-only)
# 600  rw-------  private keys, secrets (owner only)
# 777  rwxrwxrwx  âš ï¸ world-writable — security risk, avoid in production
# 400  r--------  read-only secrets (SSH private keys)

ls -la ~/.ssh/
# -rw------- 1 akshay akshay 1823 Jan 10 id_rsa       â† 600: good
# -rw-r--r-- 1 akshay akshay  399 Jan 10 id_rsa.pub   â† 644: good

Users, Groups & sudo

Linux multi-user system — every process runs as a user, every file has an owner. Understanding users, groups, /etc/passwd, /etc/shadow, and sudo is essential for system administration and security.

✓Every process runs as a UID; every file has an owner UID and group GID.
✓System users (UID < 1000) run services — they have /sbin/nologin shell (no interactive login).
✓/etc/passwd is world-readable; /etc/shadow (password hashes) is root-only.
✓usermod -aG adds a user to a group without removing existing groups (-a = append).
✓Always edit /etc/sudoers with visudo — it validates syntax before saving.
✓Prefer /etc/sudoers.d/ drop-in files to reduce risk of breaking the sudoers file.

bash — user and group management

# Create users
useradd -m -s /bin/bash akshay        # -m: create home, -s: shell
useradd -r -s /sbin/nologin www-data  # system user (no login shell)
adduser akshay                        # interactive (Ubuntu/Debian)

# Set/change password
passwd akshay            # interactive
echo "newpass" | passwd --stdin akshay  # non-interactive (scripting)

# Modify a user
usermod -aG docker akshay    # add akshay to docker group (append, don't replace)
usermod -aG sudo akshay      # grant sudo access on Debian/Ubuntu
usermod -s /bin/zsh akshay   # change shell
usermod -L akshay            # lock account (disable password login)
usermod -U akshay            # unlock account

# Delete user
userdel akshay               # keep home directory
userdel -r akshay            # delete home and mail spool too

# Groups
groupadd developers
groupdel developers
groups akshay                # list groups for user
id akshay                    # uid, gid, and all groups

# Who is logged in?
who                  # current sessions
w                    # with what they're doing
last                 # login history

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Cheat Sheet

Fundamentals

Linux

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Fundamentals

Linux Filesystem Hierarchy

✓Linux has a single directory tree rooted at / — no drive letters.
✓/etc stores configuration, /var/log stores logs, /home stores user data.
✓/proc and /sys are virtual filesystems — kernel and hardware state as readable files.
✓Everything in Linux is represented as a file: disks (/dev/sda), processes (/proc/PID), sockets.
✓Absolute paths start with /; relative paths start from the current directory.

/ (root)
â”œ── bin/      â†’ essential user binaries (ls, cp, mv, bash) — always available
â”œ── sbin/     â†’ system binaries for root (fdisk, ifconfig, reboot)
â”œ── etc/      â†’ system-wide configuration files (nginx.conf, sshd_config, hosts)
â”œ── home/     â†’ user home directories (/home/akshay, /home/alice)
â”œ── root/     â†’ root user's home directory
â”œ── var/      â†’ variable data: logs, caches, spool files
â”‚   â”œ── log/  â†’ system and app logs (syslog, nginx/access.log, auth.log)
â”‚   â””── run/  â†’ runtime PID files and sockets
…

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Fundamentals

File Permissions & Ownership

✓Permissions are three groups of rwx: owner, group, others.
✓Numeric: r=4, w=2, x=1. Common: 755 (binaries), 644 (configs), 600 (private keys).
✓chmod changes permissions; chown changes file ownership.
✓setuid on executables: runs with the file owner's privileges (e.g., passwd runs as root).
✓Sticky bit on directories: only the file owner can delete their own files (used on /tmp).

ls -la /etc/nginx/nginx.conf
# -rw-r--r-- 1 root root 2691 Jan 15 10:23 nginx.conf
# â”‚â””─â”¬──â”˜â””─â”¬──â”˜â””─â”¬──â”˜
# â”‚  owner  group  others
# â”‚
# â””── file type:
#     - = regular file
#     d = directory
#     l = symbolic link
…

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Fundamentals

Users, Groups & sudo

✓Every process runs as a UID; every file has an owner UID and group GID.
✓System users (UID < 1000) run services — they have /sbin/nologin shell (no interactive login).
✓/etc/passwd is world-readable; /etc/shadow (password hashes) is root-only.
✓usermod -aG adds a user to a group without removing existing groups (-a = append).
✓Always edit /etc/sudoers with visudo — it validates syntax before saving.

# Create users
useradd -m -s /bin/bash akshay        # -m: create home, -s: shell
useradd -r -s /sbin/nologin www-data  # system user (no login shell)
adduser akshay                        # interactive (Ubuntu/Debian)

# Set/change password
passwd akshay            # interactive
echo "newpass" | passwd --stdin akshay  # non-interactive (scripting)

…

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