Filesystem Layout

# 1) Filesystem Hierarchy Standard (FHS) — the major directories
# /         root of the tree
# /bin      essential user binaries (ls, cp, mv)         — often → /usr/bin
# /sbin     essential system binaries                   — often → /usr/sbin
# /etc      system-wide configuration files
# /home     user home directories (you live here)
# /root     home for the root user
# /var      variable data — logs, mail spools, caches
# /tmp      temporary files (often cleared on reboot)
# /usr      installed software (binaries, libs, headers)
# /opt      large/standalone third-party software
# /proc     virtual filesystem — kernel + process info
# /sys      virtual filesystem — kernel/devices
# /dev      device files (/dev/null, /dev/sda, /dev/tty)
# /mnt /media   mount points for removable drives

# 2) Pwd and home
pwd                          # /home/mara/project
echo "$HOME"                 # /home/mara
cd ~                         # = cd "$HOME"
cd -                         # back to previous directory

# 3) Absolute vs relative paths
ls /etc/nginx/nginx.conf     # absolute — starts with /
ls etc/nginx/nginx.conf      # relative — interpreted from cwd
ls ./script.sh               # explicit 'here'
ls ../sibling/file           # parent then sibling
ls ~/projects                # home-relative

# 4) Show what's in a directory
ls -la                       # long + hidden (dotfiles)
ls -lh                       # human-readable sizes
ls -lt                       # sorted by mtime, newest first
ls -1                        # one entry per line (scripts)
tree -L 2 -a                  # 2-level tree

# Replacements
bat /etc/nginx/nginx.conf     # cat with syntax highlight
exa -la                       # ls with icons + git status
fd 'pattern'                   # find replacement

# 5) Hidden files
ls -d .*                     # all dotfiles in current dir
# Anything starting with a dot is just convention — not protected.

# 6) Permissions — rwx for owner / group / other
ls -l
# -rw-r--r-- 1 mara mara 1234 Aug 15 09:00 notes.md
#  ^^^ ^^^ ^^^
#  user grp other

# Numeric (octal)
#   r=4, w=2, x=1
#   755 = rwxr-xr-x   (typical executable / script)
#   644 = rw-r--r--   (typical file)
#   600 = rw-------   (private — SSH keys)
#   700 = rwx------   (private dir — ~/.ssh)

chmod 755 deploy.sh
chmod u+x deploy.sh           # add execute for owner
chmod -R go-w shared/          # remove write for group + other recursively
chmod 600 ~/.ssh/id_ed25519    # keys MUST be 600 or ssh refuses to use them

# Ownership
chown mara:mara notes.md
chown -R deploy:www /srv/www

# 7) Setuid / setgid / sticky bit
ls -l /usr/bin/passwd
# -rwsr-xr-x  — the 's' in user-execute spot = setuid — runs as the file's owner

# Sticky bit on /tmp — only the FILE'S owner can delete it (or root)
ls -ld /tmp
# drwxrwxrwt — the 't'
chmod +t /shared/upload

# 8) umask — default permissions for newly created files
umask                         # 0022
# new file:  666 - 022 = 644
# new dir:   777 - 022 = 755
umask 077                     # private by default

# 9) Hard links vs symbolic links
# Hard link: another name for the same inode (same data, same permissions, same size)
ln source.txt hardlink.txt
ls -li source.txt hardlink.txt    # SAME inode number — they ARE the same file
# Delete one, the other still exists; only when ALL hard links are gone does the data go.
# Can't span filesystems; can't link directories.

# Symbolic link: a tiny file containing a path
ln -s /var/log/nginx/access.log ./access.log
ls -l access.log               # access.log -> /var/log/nginx/access.log
readlink access.log             # /var/log/nginx/access.log
readlink -f access.log          # canonical absolute path, resolved
# Symlinks can span filesystems, can point to dirs, can dangle (point to nothing).

# 10) Sizes and disk usage
du -sh ~/Downloads             # total size of a dir
du -sh */                       # size of each subdir, sorted
du -h --max-depth=1 / 2>/dev/null | sort -hr | head -20
df -h                           # filesystem usage by mount point
stat notes.md                    # full metadata: inode, size, atime/mtime/ctime

# 11) Find files
find . -type f -name '*.log' -mtime -7        # files modified in last 7 days
find . -type f -size +100M                    # bigger than 100 MB
find . -type d -empty                          # empty directories
find . -name '*.bak' -delete                   # delete matches (use --dry-run alternatives first)
# Modern alternative: fd
fd -e log --changed-within 7d

# 12) Where is something installed?
which python3                   # /usr/bin/python3 — first match on PATH
type -a ls                       # ls is aliased to ... + /usr/bin/ls
command -v node                  # POSIX equivalent of 'which'

# 13) Mounts
mount                            # list active mounts
mount /dev/sdb1 /mnt/usb
umount /mnt/usb
lsblk                            # block device tree
findmnt --target /                # what mounts the path I care about

# 14) /proc and /sys — kernel filesystems
cat /proc/cpuinfo               # CPU details
cat /proc/meminfo               # memory stats
ls /proc/$$                     # everything about the current shell process
cat /proc/$$/status             # state, parent, memory
ls /sys/class/net               # network interfaces

# 15) Common bugs
#   • Editing config in /opt with a non-root user → permission denied; use sudo deliberately
#   • Symlink with relative target moves and breaks → prefer absolute targets for cross-tree links
#   • Forgetting -p in 'mkdir -p path/to/nested' → 'no such file or directory' on parent
#   • SSH keys at 0644 → 'WARNING: UNPROTECTED PRIVATE KEY FILE' → chmod 600
#   • du run from / without permissions → noisy errors; redirect 2>/dev/null
#   • df vs du discrepancy → deleted-but-open files; restart the holder process

# /bin /etc /home /opt /tmp /var /usr — the Filesystem Hierarchy Standard.