Linux Command - Ali Jamshidzadeh

1. Displaying Disk Usage in Megabytes and Gigabytes#

Linux’s df (disk free) command shows mounted filesystem disk space usage. By default it prints in 1K blocks; with -h it uses human-readable units (KB, MB, GB):

# Show sizes with automatic units (e.g. 1K, 234M, 2G)
df -h

# Show sizes in raw 1K blocks
df

# Force output in megabytes (M) or gigabytes (G)
df -BM     # outputs sizes in megabytes
df -BG     # outputs sizes in gigabytes

Alternatively, use du (disk usage) to check a specific directory’s size:

# Recursively summarize sizes, human-readable
du -h /path/to/dir

# Show only the total for that directory
du -sh /path/to/dir

2. Managing File Permissions with `chmod`#

Unix file permissions control read, write, and execute access. To make a script executable for its owner:

chmod u+x example.sh

Symbol	Meaning
`u`	file owner (user)
`g`	owning group
`o`	others
`+`	add permission
`-`	remove permission
`r`	read
`w`	write
`x`	execute

E.g., chmod ug=rw,o=r file.txt → owner & group read/write; others read only.

3. Top 50 Linux Commands You Should Know#

File & Directory Operations#

ls – list files & directories
pwd – print working directory
cd – change directory
mkdir – create directory
rm – remove files/directories
cp – copy files/directories
mv – move or rename files
touch – create empty file
ln – create (symbolic) links
find – search for files by name, size, date, etc.

Viewing & Editing Files#

cat – concatenate & display file content
less – view file content page by page
head – show first N lines
tail – show last N lines
grep – search text via patterns
diff – compare file differences
cmp – byte-by-byte file comparison
sort – sort file lines
wc – count lines/words/bytes
sed – stream editor for text transformation

System Information & Process Management#

uname -a – kernel & system info
whoami – current username
top – real-time process & resource viewer
ps aux – snapshot of processes
kill, killall – terminate processes
df – disk filesystem usage
du – directory usage (see above)
free -h – memory usage
uptime – system uptime & load
htop – interactive process viewer (if installed)

Networking & Remote Access#

ifconfig (or ip addr) – network interfaces
ping – test network connectivity
traceroute – trace packet route
ssh – secure shell access
scp – secure copy over SSH
wget, curl – download files over HTTP/FTP
netstat (or ss) – sockets & network stats
iptables, ufw – firewall rules
route (or ip route) – view/manipulate routing
nslookup, dig – DNS queries

Package Management & Updates#

apt, yum, dnf, pacman – install/remove packages
sudo – execute a command with superuser privileges
systemctl – manage systemd services
service – start/stop SysV services
journalctl – view systemd logs
cron, at – schedule tasks
alias – create command shortcuts
env, export – set environment variables
dd – low-level data copying (e.g. USB images)
whereis, which, whatis – locate commands

4. Terminal Multiplexing with Screen#

GNU Screen lets you run multiple shells inside one terminal, detach, and resume:

# Install on Debian/Ubuntu
sudo apt-get install screen

# Start a new session
screen

# Detach (leave running in background)
Ctrl-a d

# List sessions
screen -ls

# Reattach to a session
screen -r <session_name>

# Create a new window inside screen
Ctrl-a c

# Next/previous window
Ctrl-a n   Ctrl-a p

# List windows
Ctrl-a "

# Split windows (horizontal)
Ctrl-a S

# Close current window
Ctrl-a k

# Rename window
Ctrl-a A

# Kill all screen sessions
killall screen

5. Resolving Default Gateway Conflicts#

Having more than one default gateway (e.g. both eth0 and wlan0) causes routing errors. Choose one interface for your internet traffic; remove the other’s gateway:

# Remove the default route via eth0
sudo ip route del default via 192.168.1.1 dev eth0

Verify with:

ip route show

6. Mounting USB Drives Permanently#

One-off Mount#

# /etc/fstab entry example:
/dev/sda1   /home/pi/usb   ext4   defaults   0   0

# Temporarily mount:
sudo mount /dev/sda1 /home/pi/usb

Bind-Mount (e.g. Nextcloud data folder)#

sudo mount --bind /home/pi/usb /portainer/Files/AppData/Config/Nextcloud/Data/pi/files/usb

Permanent Mount via UUID#

Find the UUID:
```
sudo blkid /dev/sda1
```

Add to /etc/fstab:

UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx  /mnt/external  ext4  defaults,uid=pi,gid=pi,nofail  0  0

Apply:
```
sudo mount -a
```

Adjust ownership for easy access:

sudo chown -R pi:pi /mnt/external

7. Refreshing Arch Linux Mirrors#

On Arch Linux, ensure you have the fastest mirrors:

# Update mirror list with the 5 fastest servers
sudo pacman-mirrors -f 5

# Then update your system
sudo pacman -Syu

8. Starting Docker Service#

If Docker isn’t already running, use:

# Start Docker daemon
sudo systemctl start docker

# Enable it at boot
sudo systemctl enable docker

# Check status
sudo systemctl status docker

9. Working with External Hard Drives#

9.1. Connect, Partition & Format#

Detect drives
```
sudo lsblk
```

Partition (if needed)

sudo fdisk /dev/sda
# Inside fdisk: ‘n’ new, ‘d’ delete, ‘w’ write

Format partition

# ext4 for Linux
sudo mkfs.ext4 /dev/sda1

# exFAT for cross-platform
sudo mkfs.exfat /dev/sda1

Create mount point & mount

sudo mkdir -p /mnt/external
sudo mount /dev/sda1 /mnt/external
sudo chown -R pi:pi /mnt/external

9.2. Permanent Mount#

See section 6 above.

10. Configuring HDD Spin-Down with hd-idle#

Prolong hardware life and save power by spinning down idle drives:

Install
```
sudo apt-get install hd-idle
```

Configure
Edit /etc/default/hd-idle:

# Spin down after 180 seconds idle
HD_IDLE_OPTS="-i 180 -l /var/log/hd-idle.log"

Restart service

sudo systemctl restart hd-idle
# or for SysV:
sudo service hd-idle restart

11. Monitoring Disk Activity & Background Services#

List open files on a mount
```
sudo lsof | grep /home/pi/USB
```
I/O top
```
sudo apt-get install iotop
sudo iotop
```

Running services

systemctl list-units --type=service --state=running

Process snapshot
```
ps aux
# or top for live view
top
```
System logs
```
dmesg | less
journalctl -xe
```

12. Optimizing Samba (SMB) Shares#

If Samba’s smbd keeps accessing your USB mount, disable kernel change notifications:

Edit share config in /etc/samba/smb.conf:

    [USB]
    path = /home/pi/USB
    browseable = yes
    read only = no
    kernel change notify = no

Restart or stop Samba:

sudo systemctl restart smbd
# If you no longer need sharing:
sudo systemctl stop smbd