Disk System

Disk & System — Quick Reference

df — Disk Space by Filesystem

df -h                         # human-readable sizes (GB, MB)
df -h /                       # space on root partition only
df -hT                        # include filesystem type
df -i                         # inode usage instead of space

du — Directory / File Size

du -sh *                      # size of each item in current dir
du -sh /var/log               # size of a specific directory
du -h --max-depth=1 /var      # one level deep
du -sh * | sort -rh           # sort by size, largest first
du -ah . | sort -rh | head -20  # top 20 largest files/dirs

ls — List Files

ls                            # basic listing
ls -l                         # long format (permissions, size, date)
ls -lh                        # human-readable sizes
ls -la                        # include hidden files
ls -lt                        # sort by modification time
ls -lS                        # sort by file size
ls -R                         # recursive
ls -d */                      # directories only

chmod — File Permissions

chmod 644 file.txt            # rw-r--r--
chmod 755 script.sh           # rwxr-xr-x (executable)
chmod 600 secret.key          # rw------- (private)
chmod +x script.sh            # add execute for all
chmod -x script.sh            # remove execute
chmod -R 755 ./dir            # recursive

Permission Octal Reference:

Octal	Binary	Meaning
`7`	111	read + write + execute
`6`	110	read + write
`5`	101	read + execute
`4`	100	read only
`0`	000	no permissions

Order: [owner][group][others] — e.g. 755 = owner full, group+others read+execute.

chown — Change Ownership

chown user file.txt           # change owner
chown user:group file.txt     # change owner and group
chown -R user:group ./dir     # recursive
chown :group file.txt         # change group only (same as chgrp)

ln — Create Links

# Symbolic link (like a shortcut)
ln -s /path/to/target linkname

# Hard link (same inode, survives target deletion)
ln /path/to/file hardlink

# Update existing symlink
ln -sf /new/target linkname

env / export — Environment Variables

env                           # list all environment variables
echo $PATH                    # print a specific variable
export MY_VAR=value           # set for current shell + child processes
unset MY_VAR                  # remove variable

# Set for a single command only
MY_VAR=value ./script.sh

# Add to PATH
export PATH="$PATH:/new/dir"

which / whereis — Locate Binaries

which python3                 # full path of binary in PATH
which -a python3              # all matches in PATH
whereis python3               # binary + man page + source locations
type ls                       # whether it's a builtin, alias, or binary

mount / umount — Filesystems

mount                         # list all mounted filesystems
mount /dev/sdb1 /mnt/disk     # mount a device
mount -t ext4 /dev/sdb1 /mnt  # specify filesystem type
umount /mnt/disk              # unmount
df -h                         # verify after mounting

free — Memory Usage

free -h                       # human-readable RAM + swap usage
free -s 2                     # refresh every 2 seconds

uname — System Info

uname -a                      # all system info
uname -r                      # kernel version
uname -m                      # architecture (x86_64, arm64, etc.)

uptime — System Load

uptime                        # uptime + load averages (1m, 5m, 15m)

Load average > number of CPU cores = system under pressure.

iostat — Disk I/O Stats

iostat                        # CPU + disk I/O stats
iostat -x 2                   # extended stats, refresh every 2s
iostat -h                     # human-readable

Tips

du -sh * | sort -rh | head -10 is the fastest way to find what’s eating disk space.
chmod 600 for private keys — SSH will refuse to use keys that are too permissive.
Always use -h with df and du — raw bytes are unreadable at a glance.
ln -s symlinks are relative by default — use absolute paths to avoid broken links when moving things around.

← Linux