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):

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# Show sizes with automatic units (e.g. 1K, 234M, 2G)
2
df -h
3

4
# Show sizes in raw 1K blocks
5
df
6

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# Force output in megabytes (M) or gigabytes (G)
8
df -BM     # outputs sizes in megabytes
9
df -BG     # outputs sizes in gigabytes

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

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# Recursively summarize sizes, human-readable
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du -h /path/to/dir
3

4
# Show only the total for that directory
5
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:

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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:

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# Install on Debian/Ubuntu
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sudo apt-get install screen
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# Start a new session
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screen
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# Detach (leave running in background)
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Ctrl-a d
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# List sessions
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screen -ls
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# Reattach to a session
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screen -r <session_name>
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# Create a new window inside screen
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Ctrl-a c
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# Next/previous window
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Ctrl-a n   Ctrl-a p
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# List windows
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Ctrl-a "
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# Split windows (horizontal)
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Ctrl-a S
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# Close current window
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Ctrl-a k
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# Rename window
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Ctrl-a A
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# Kill all screen sessions
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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:

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

Verify with:

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ip route show

6. Mounting USB Drives Permanently#

One-off Mount#

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

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

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

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

Permanent Mount via UUID#

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

Add to /etc/fstab:

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

Apply:
Terminal window
```
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sudo mount -a
```

Adjust ownership for easy access:

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sudo chown -R pi:pi /mnt/external

7. Refreshing Arch Linux Mirrors#

On Arch Linux, ensure you have the fastest mirrors:

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# Update mirror list with the 5 fastest servers
2
sudo pacman-mirrors -f 5
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# Then update your system
5
sudo pacman -Syu

8. Starting Docker Service#

If Docker isn’t already running, use:

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# Start Docker daemon
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sudo systemctl start docker
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# Enable it at boot
5
sudo systemctl enable docker
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# Check status
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sudo systemctl status docker

9. Working with External Hard Drives#

9.1. Connect, Partition & Format#

Detect drives
Terminal window
```
1
sudo lsblk
```

Partition (if needed)

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

Format partition

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# ext4 for Linux
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sudo mkfs.ext4 /dev/sda1
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# exFAT for cross-platform
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sudo mkfs.exfat /dev/sda1

Create mount point & mount

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sudo mkdir -p /mnt/external
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sudo mount /dev/sda1 /mnt/external
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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
Terminal window
```
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sudo apt-get install hd-idle
```

Configure
Edit /etc/default/hd-idle:

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

Restart service

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sudo systemctl restart hd-idle
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# or for SysV:
3
sudo service hd-idle restart

11. Monitoring Disk Activity & Background Services#

List open files on a mount
Terminal window
```
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sudo lsof | grep /home/pi/USB
```
I/O top
Terminal window
```
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sudo apt-get install iotop
2
sudo iotop
```

Running services

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

Process snapshot
Terminal window
```
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ps aux
2
# or top for live view
3
top
```
System logs
Terminal window
```
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dmesg | less
2
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:

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

Restart or stop Samba:

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