Introduction to Linux File Systems

/ (root)
├── /bin          → Essential user binaries (ls, cp, mv)
├── /boot         → Kernel, initramfs, GRUB
├── /dev          → Device files (sda, tty, null)
├── /etc          → System configuration files
├── /home         → User home directories
├── /lib          → Shared libraries for /bin, /sbin
├── /media        → Mount points for removable media
├── /mnt          → Temporary mount points
├── /opt          → Optional/add-on software
├── /proc         → Virtual filesystem (kernel info)
├── /root         → Root user home directory
├── /run          → Runtime variable data
├── /sbin         → System binaries (fsck, reboot, ip)
├── /srv          → Data for services (web, ftp)
├── /sys          → Virtual filesystem (kernel devices)
├── /tmp          → Temporary files (cleared on reboot)
├── /usr          → User programs (read-only, shareable)
│   ├── /usr/bin      → Non-essential binaries
│   ├── /usr/lib      → Libraries for /usr/bin
│   ├── /usr/local    → Locally compiled software
│   └── /usr/share    → Architecture-independent data
└── /var          → Variable data (logs, spool, cache)
    ├── /var/log      → System logs
    ├── /var/spool    → Print/mail queues
    └── /var/cache    → Application caches

# Create ext4
mkfs.ext4 /dev/sdX1

# Check/repair
fsck.ext4 -f /dev/sdX1

# Tune
tune2fs -l /dev/sdX1

# Create XFS
mkfs.xfs /dev/sdX1

# Grow (online, mounted)
xfs_growfs /mount/point

# Check
xfs_repair /dev/sdX1

# Create Btrfs
mkfs.btrfs /dev/sdX1

# Snapshot (instant)
btrfs subvolume snapshot /source /snapshots/backup-$(date +%F)

# Compress
mount -o compress=zstd /dev/sdX1 /mnt

lsblk          # Tree view
lsblk -f       # With filesystem info
fdisk -l       # Partition tables
parted -l      # GPT/MBR details

# MBR (legacy BIOS)
fdisk /dev/sdX

# GPT (UEFI)
parted /dev/sdX mklabel gpt
parted /dev/sdX mkpart primary ext4 1MiB 100%

mount /dev/sdX1 /mnt
mount -t ext4 /dev/sdX1 /mnt
mount -o ro /dev/sdX1 /mnt    # Read-only

# <device>    <mount>    <type>    <options>           <dump> <pass>
UUID=xxxx     /          ext4      defaults,noatime    0      1
UUID=yyyy     /home      ext4      defaults,noatime    0      2
UUID=zzzz     /boot/efi  vfat      umask=0077          0      1
UUID=swap     none       swap      sw                  0      0

# /etc/systemd/system/mnt-data.mount
[Unit]
Description=Data Drive

[Mount]
What=/dev/disk/by-uuid/xxxx
Where=/mnt/data
Type=ext4
Options=defaults,noatime

[Install]
WantedBy=multi-user.target

ls -l
# -rw-r--r--  1 user group  1024 Jan 1 12:00 file.txt
#  ^  ^  ^  ^
#  |  |  |  └─ Others: read
#  |  |  └──── Group: read
#  |  └──────── Owner: read, write
#  └─────────── Type: - (file), d (dir), l (link)

chmod 755 file.sh           # Numeric
chmod u+x file.sh           # Symbolic (user + execute)
chmod g-w,o-r file.txt      # Remove group write, other read
chmod -R 755 /dir           # Recursive

chown user:group file       # Change owner:group
chown -R user:group /dir    # Recursive
chown :group file           # Change group only

chmod 4755 /usr/bin/passwd    # SUID
chmod 2775 /shared/dir        # SGID
chmod 1777 /tmp               # Sticky

df -h              # Disk free space (human)
df -i              # Inode usage
du -sh /path       # Directory size
du -sh * | sort -hr # Sort by size
ncdu /             # Interactive (install ncdu)

lsblk -f           # Filesystem details
blkid              # UUID, TYPE, LABEL
findmnt            # Mount tree

# ext4
fsck.ext4 -f /dev/sdX1
e2fsck -y /dev/sdX1

# XFS (unmounted!)
xfs_repair /dev/sdX1

# Btrfs
btrfs check /dev/sdX1
btrfs scrub start /mount  # Background checksum verification

# ext4 (online)
e4defrag /mount/point

# XFS (online)
xfs_fsr /mount/point

# Btrfs (online)
btrfs filesystem defragment -r /mount/point

# Enable discard (TRIM) in fstab
UUID=xxxx  /  ext4  defaults,noatime,discard  0 1

# Or use fstrim timer (preferred)
systemctl enable fstrim.timer
systemctl start fstrim.timer

# Manual trim
fstrim -v /

# Physical Volume
pvcreate /dev/sdX1

# Volume Group
vgcreate vg0 /dev/sdX1

# Logical Volume
lvcreate -L 20G -n root vg0
lvcreate -L 100%FREE -n home vg0

# Format & Mount
mkfs.ext4 /dev/vg0/root
mount /dev/vg0/root /mnt

# Resize (grow)
lvextend -L +10G /dev/vg0/root
resize2fs /dev/vg0/root  # ext4
xfs_growfs /mnt          # XFS

# Create RAID 1 (mirror)
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdX1 /dev/sdY1

# Create RAID 5 (3+ disks)
mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdX1 /dev/sdY1 /dev/sdZ1

# Monitor
cat /proc/mdstat
mdadm --detail /dev/md0

# Add spare
mdadm --add /dev/md0 /dev/sdZ1