1. SSH connection commands

SSH, or Secure Shell, creates an encrypted connection between your computer and a remote Linux server. Most Linux VPS and dedicated server administration begins with the ssh command.

Connect using the default SSH port

ssh username@server_ip

Example:

ssh root@203.0.113.10

Connect using a custom SSH port

ssh -p 2222 username@server_ip

Connect using a private key

ssh -i ~/.ssh/server_key username@server_ip

Create a modern SSH key pair

ssh-keygen -t ed25519 -a 100

Store the private key securely and protect it with a strong passphrase. Never send a private SSH key through email, chat, or a support ticket.

Copy your public key to the server

ssh-copy-id username@server_ip

For a custom SSH port:

ssh-copy-id -p 2222 username@server_ip

Test the SSH connection with detailed output

ssh -vvv username@server_ip

Verbose output helps diagnose authentication failures, key problems, connection timeouts, and SSH negotiation errors.

Keep an idle SSH connection alive

ssh -o ServerAliveInterval=60 username@server_ip

Run one command remotely

ssh username@server_ip "uptime"

Close the SSH session

exit

2. Check server, operating system, and kernel information

Show the hostname

hostname

Show detailed hostname information

hostnamectl

Change the server hostname

sudo hostnamectl set-hostname server.example.com

Also review /etc/hosts after changing a hostname so local hostname resolution remains correct.

Show the Linux distribution and version

cat /etc/os-release

Show kernel and architecture information

uname -a

uname -r

uname -m

Show CPU information

lscpu

Show memory information

free -h

Show server uptime and load averages

uptime

Show the current date and timezone

date

timedatectl

3. Navigate the Linux filesystem

Display the current directory

pwd

List files and directories

ls

Detailed list with hidden files and readable sizes:

ls -lah

Sort by modification time:

ls -lath

Change directory

cd /path/to/directory

Go to your home directory:

cd ~

Go up one directory:

cd ..

Return to the previous directory:

cd -

Display a directory tree

tree -L 2 /var/www

The tree package may need to be installed first.

4. Create, copy, move, and delete files

Create an empty file

touch filename.txt

Create a directory

mkdir directory_name

Create nested directories:

mkdir -p /var/www/example.com/public

Copy a file

cp source.txt destination.txt

Copy a directory recursively while preserving attributes:

cp -a source_directory/ destination_directory/

Move or rename a file

mv old_name.txt new_name.txt

Move a file into another directory:

mv file.txt /path/to/destination/

Delete a file interactively

rm -i filename.txt

Remove an empty directory

rmdir directory_name

Create a symbolic link

ln -s /actual/path /path/to/link

Inspect a file type

file filename

5. View and edit text files

Display the complete file

cat filename.txt

Read a long file one screen at a time

less filename.txt

Inside less, press / to search, n for the next match, and q to quit.

Show the first lines

head -n 20 filename.txt

Show the last lines

tail -n 50 filename.txt

Follow a file as new lines are added

tail -f /var/log/application.log

Start with the latest 100 lines and continue following:

tail -n 100 -f /var/log/application.log

Edit with Nano

nano filename.txt

Edit with Vim

vim filename.txt

Write output to a file as root

echo "configuration=value" | sudo tee /etc/example.conf

Append instead of overwriting:

echo "new line" | sudo tee -a /etc/example.conf

Compare two files

diff -u old.conf new.conf

6. Search for files, directories, and text

Find a file by name

find /path/to/search -type f -name "filename.conf"

Find files case-insensitively

find /var/www -type f -iname "*.php"

Find large files

find /var -type f -size +500M -printf '%s %p\n' 2>/dev/null | sort -nr | head

Find files modified within one day

find /var/www -type f -mtime -1

Search for text inside a file

grep "search text" filename.txt

Search recursively and include line numbers

grep -Rni "search text" /path/to/directory

Search using an extended regular expression

grep -E "error|failed|critical" application.log

Exclude comments and blank lines

grep -Ev '^[[:space:]]*(#|$)' /etc/ssh/sshd_config

Locate a command

command -v nginx

which php

Search command history

history | grep systemctl

You can also press Ctrl+R in Bash and begin typing part of a previous command.

7. Linux permissions and ownership

View permissions and ownership

ls -l filename

Change file permissions

chmod 644 filename

Typical permissions:

• 644 for normal public files
• 755 for directories
• 600 for sensitive private files
• 700 for private directories or scripts accessible only by the owner

Make a script executable

chmod +x script.sh

Change owner and group

sudo chown user:group filename

Change ownership recursively

sudo chown -R user:group /specific/directory

Inspect permissions on every part of a path

namei -l /var/www/example.com/public/index.php

8. Manage Linux users and groups

Show the current user

whoami

Show user and group IDs

id username

Show logged-in users

w

who

Create a user on Debian or Ubuntu

sudo adduser username

Create a user on AlmaLinux, Rocky Linux, or RHEL

sudo useradd -m username
sudo passwd username

Give sudo access on Debian or Ubuntu

sudo usermod -aG sudo username

Give administrative access on AlmaLinux or Rocky Linux

sudo usermod -aG wheel username

Change a password

sudo passwd username

Lock and unlock a user account

sudo passwd -l username
sudo passwd -u username

Safely edit sudo rules

sudo visudo

9. Install and update Linux packages

Debian and Ubuntu package commands

Refresh package metadata:

sudo apt update

Install available upgrades:

sudo apt upgrade

Install a package:

sudo apt install package_name

Remove a package:

sudo apt remove package_name

Search for a package:

apt search package_name

AlmaLinux, Rocky Linux, and RHEL package commands

Check for updates:

sudo dnf check-update

Install updates:

sudo dnf upgrade

Install a package:

sudo dnf install package_name

Remove a package:

sudo dnf remove package_name

Search for a package:

dnf search package_name

Determine which package installed a file

Debian or Ubuntu:

dpkg -S /path/to/file

AlmaLinux, Rocky Linux, or RHEL:

rpm -qf /path/to/file

10. Manage services with systemd

Check service status

sudo systemctl status nginx

Start, stop, or restart a service

sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx

Reload configuration without a complete restart

sudo systemctl reload nginx

Enable a service at boot

sudo systemctl enable nginx

Enable and start immediately:

sudo systemctl enable --now nginx

Disable a service at boot

sudo systemctl disable nginx

Check whether a service is active or enabled

systemctl is-active nginx
systemctl is-enabled nginx

List failed services

systemctl --failed

Validate configuration before restarting

Nginx:

sudo nginx -t

Apache on Debian or Ubuntu:

sudo apache2ctl configtest

Apache on AlmaLinux or Rocky Linux:

sudo httpd -t

OpenSSH:

sudo sshd -t

Always test configuration syntax before restarting a web server, SSH daemon, database service, or firewall.

11. Monitor processes, CPU, and memory

Interactive process monitoring

top

htop

htop may need to be installed separately.

List all processes

ps aux

Search for a specific process

ps aux | grep nginx

A cleaner alternative:

pgrep -a nginx

Show the highest CPU-consuming processes

ps aux --sort=-%cpu | head

Show the highest memory-consuming processes

ps aux --sort=-%mem | head

Send a normal termination signal

kill PID

Forcefully terminate a process only when necessary

kill -9 PID

Use a normal kill first. Signal 9 prevents the application from completing cleanup or writing pending data.

Inspect memory and system activity

vmstat 1 10

Inspect disk I/O activity

iostat -xz 1 5

The iostat command is commonly provided by the sysstat package.

12. Check disk space, inodes, and storage devices

Show filesystem disk usage

df -hT

Show inode usage

df -ih

A filesystem can report “No space left on device” even with free storage if all available inodes have been consumed by many small files.

Check the size of one directory

du -sh /path/to/directory

Show the largest items in a directory

du -xh --max-depth=1 /var | sort -h

Display block devices and filesystems

lsblk -f

List mounted filesystems

findmnt

Check current mounts

mount | column -t

Check swap usage

swapon --show

13. Linux networking and DNS commands

Show IP addresses and interfaces

ip address show

Short form:

ip a

Show the routing table

ip route

Show the route used to reach an IP

ip route get 1.1.1.1

Test basic connectivity

ping -c 4 1.1.1.1

A failed ping does not always mean the host is offline because ICMP traffic may be filtered.

Trace the network path

traceroute example.com

Run an interactive route quality test

mtr example.com

Generate a report suitable for troubleshooting:

mtr -rwzc 100 example.com

Show listening ports and services

sudo ss -lntup

Find the process using a port

sudo lsof -i :80

sudo ss -lntp 'sport = :443'

Test whether a remote TCP port is reachable

nc -vz example.com 443

Query DNS records

dig example.com A

Return only the resolved value:

dig example.com A +short

Check nameservers:

dig example.com NS +short

Check mail exchangers:

dig example.com MX +short

Check reverse DNS:

dig -x 203.0.113.10 +short

Inspect HTTP response headers

curl -I https://example.com

Follow redirects and show the final response

curl -IL https://example.com

Test a website against a specific server IP

curl -I --resolve example.com:443:203.0.113.10 https://example.com/

This is useful before DNS changes or while troubleshooting a reverse proxy, migration, or virtual host.

Display the server’s public IP

curl -4 https://icanhazip.com

14. Firewall commands

UFW on Ubuntu or Debian

Show firewall status:

sudo ufw status verbose

Allow standard SSH:

sudo ufw allow OpenSSH

Allow a custom SSH port:

sudo ufw allow 2222/tcp

Allow HTTP and HTTPS:

sudo ufw allow 80/tcp
sudo ufw allow 443/tcp

Enable the firewall only after confirming the rules:

sudo ufw enable

firewalld on AlmaLinux or Rocky Linux

Show active rules:

sudo firewall-cmd --list-all

Allow HTTP and HTTPS permanently:

sudo firewall-cmd --permanent --add-service=http
sudo firewall-cmd --permanent --add-service=https

Allow a custom SSH port:

sudo firewall-cmd --permanent --add-port=2222/tcp

Reload after confirming all required rules:

sudo firewall-cmd --reload

View the active nftables ruleset

sudo nft list ruleset

Avoid flushing an active firewall remotely unless you have console or out-of-band access and understand how the firewall is managed.

15. Read logs and troubleshoot failed services

Show recent system errors

sudo journalctl -p err -b

Show logs for one service

sudo journalctl -u nginx

Show recent service logs and follow new entries

sudo journalctl -u nginx -n 100 -f

Show logs from the current boot

sudo journalctl -b

Show logs from a specific period

sudo journalctl --since "2026-07-20 14:00" --until "2026-07-20 15:00"

Inspect kernel messages

sudo dmesg -T | tail -n 100

Authentication logs

Debian or Ubuntu:

sudo tail -n 100 /var/log/auth.log

AlmaLinux or Rocky Linux:

sudo tail -n 100 /var/log/secure

Nginx logs

sudo tail -f /var/log/nginx/access.log
sudo tail -f /var/log/nginx/error.log

Apache logs on Debian or Ubuntu

sudo tail -f /var/log/apache2/access.log
sudo tail -f /var/log/apache2/error.log

Apache logs on AlmaLinux or Rocky Linux

sudo tail -f /var/log/httpd/access_log
sudo tail -f /var/log/httpd/error_log

Search compressed and active logs

zgrep -i "error" /var/log/application.log*

Check recent login history

last -a | head -n 30

Check failed login attempts

sudo lastb | head -n 30

16. Transfer and synchronize files

Copy a local file to a remote server with SCP

scp file.tar.gz username@server_ip:/remote/path/

Download a remote file

scp username@server_ip:/remote/path/file.tar.gz ./

Use SCP with a custom SSH port

scp -P 2222 file.tar.gz username@server_ip:/remote/path/

Notice that SCP uses uppercase -P, while SSH uses lowercase -p.

Synchronize a directory with rsync

rsync -avh --progress source_directory/ username@server_ip:/remote/path/

Use rsync through a custom SSH port

rsync -avh --progress -e "ssh -p 2222" source_directory/ username@server_ip:/remote/path/

Preview an rsync operation

rsync -avhn source_directory/ destination_directory/

The -n option performs a dry run without changing files. Use it before synchronization commands involving deletion or production directories.

Download a file with curl

curl -fLO https://example.com/file.tar.gz

Download a file with wget

wget https://example.com/file.tar.gz

Only download installation scripts or software from trusted sources. Review scripts before running them with elevated privileges.

17. Create and extract compressed archives

Create a gzip-compressed tar archive

tar -czf backup.tar.gz directory_name/

List archive contents before extracting

tar -tzf backup.tar.gz

Extract a tar.gz archive

tar -xzf backup.tar.gz

Extract into a specific directory

tar -xzf backup.tar.gz -C /destination/path/

Create a ZIP archive

zip -r backup.zip directory_name/

Extract a ZIP archive

unzip backup.zip -d /destination/path/

Calculate a SHA-256 checksum

sha256sum backup.tar.gz

Compare checksums before and after a transfer to verify file integrity.

18. Schedule commands with cron

Edit the current user’s cron jobs

crontab -e

List the current user’s cron jobs

crontab -l

Edit root’s cron jobs

sudo crontab -e

Run a script every day at 2:30 AM

30 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

Run a command every five minutes

*/5 * * * * /usr/local/bin/task.sh >> /var/log/task.log 2>&1

Use full executable paths inside cron jobs because cron usually runs with a limited environment. Redirect standard output and errors to a log so failed jobs can be diagnosed.

19. Shell shortcuts and command combinations

Run a second command only if the first succeeds

command_one && command_two

Run a second command only if the first fails

command_one || command_two

Send one command’s output into another

ps aux | grep php

Redirect output into a file

command > output.txt

Append output:

command >> output.txt

Redirect both standard output and errors:

command > output.log 2>&1

Run a command in the background

long_running_command &

List and manage shell jobs

jobs
fg %1
bg %1

Keep a command running after logout

nohup command > command.log 2>&1 &

For long interactive maintenance sessions, consider using tmux or screen instead.

Start a tmux session

tmux new -s maintenance

Reattach later:

tmux attach -t maintenance

20. Common server troubleshooting workflows

Website is not responding

1. Check whether the web service is active.
2. Validate the web server configuration.
3. Review listening ports.
4. Inspect error logs.
5. Test the website locally and externally.

sudo systemctl status nginx
sudo nginx -t
sudo ss -lntp | grep -E ':80|:443'
sudo tail -n 100 /var/log/nginx/error.log
curl -I http://127.0.0.1
curl -IL https://example.com

Server is slow

uptime
free -h
top
ps aux --sort=-%cpu | head
ps aux --sort=-%mem | head
df -hT
df -ih
vmstat 1 10
iostat -xz 1 5

Disk is full

df -hT
df -ih
sudo du -xhd1 / | sort -h
sudo du -xhd1 /var | sort -h
sudo find /var -type f -size +500M -printf '%s %p\n' 2>/dev/null | sort -nr | head -n 20

Investigate before deleting anything. Common causes include oversized logs, old backups, application caches, database files, temporary files, and abandoned website archives.

SSH connection is refused

sudo systemctl status ssh
sudo systemctl status sshd
sudo sshd -t
sudo ss -lntp | grep ssh
sudo journalctl -u ssh -n 100
sudo journalctl -u sshd -n 100

The service is normally named ssh on Debian or Ubuntu and sshd on AlmaLinux, Rocky Linux, and RHEL.

DNS points to the wrong server

dig example.com A +short
dig example.com AAAA +short
dig example.com NS +short
dig @1.1.1.1 example.com A +short
dig @8.8.8.8 example.com A +short

21. Commands that require extra caution

Choose the right server management approach

SSH access provides complete control, but it also makes you responsible for operating system updates, firewall policies, service configuration, backups, security monitoring, and incident response.

An unmanaged VPS is suitable when you are comfortable administering Linux yourself. A managed service is better when you need experienced administrators to handle server configuration, hardening, updates, monitoring, or troubleshooting.

Frequently asked questions