Linux — pophints

Getting started

Check where you are, which user you are, and what is happening on the system.

Where am I?

orientation

pwd                 # current directory
whoami              # current user
hostname            # machine name
date                # date and time
uname -a            # kernel and architecture

Built-in help

documentation

man ssh             # manual page
ssh --help          # program help
help cd             # shell built-in help
apropos filesystem  # search manual page descriptions

In man: /word searches, n jumps to the next match, and q quits.

Files & directories

Check paths carefully. Recursive options such as -r affect everything below a directory.

List files

ls

ls -lah             # detailed list, including hidden files
ls -lt              # newest changes first
du -sh directory/   # total directory size

Change directory

cd

cd /var/log         # absolute path
cd ..               # parent directory
cd ~                # home directory
cd -                # previous directory

Create a directory or file

mkdir · touch

mkdir notes
mkdir -p project/src/components
touch todo.txt

The -p flag creates missing parent directories and does not complain if they already exist.

Copy files

cp

cp report.txt backup.txt
cp -i file.txt /tmp/           # prompt before overwrite
cp -a folder/ folder-copy/     # copy directory and attributes

Move or rename

mv

mv old-name.txt new-name.txt
mv file.txt ~/Documents/
mv -i report.txt archive/

Add -i to prompt before overwriting an existing file.

Delete a file or directory

rm · rmdir

rm -i file.txt
rmdir empty-directory
rm -r directory/   # directory and its contents

Careful: rm has no trash. Check pwd and the target path first.

Create a link

ln

ln -s /path/to/original shortcut
readlink -f shortcut

ln -s creates a symbolic link; without -s, it creates a hard link.

tar.gz archives

tar

tar -czf archive.tar.gz directory/   # create
tar -tzf archive.tar.gz              # list contents
tar -xzf archive.tar.gz -C /tmp      # extract into directory

Text & search

Use less for large files and pipe commands together with |.

Read a file

cat · less

cat config.conf             # print whole file
less /var/log/syslog        # page through a file
head -n 20 file.txt         # first 20 lines
tail -n 50 file.txt         # last 50 lines
tail -f application.log     # follow new lines

Press q to quit less.

Search text with grep

grep

grep -n "error" app.log
grep -Rni "TODO" ./src
grep -E "warn|error" app.log
command | grep "pattern"

-n shows line numbers, -i ignores case, and -R searches recursively.

Find files

find

find . -name "*.log"
find /var/log -type f -mtime -1
find . -type f -size +100M
find . -type f -name "*.tmp" -print

Start from the directory you need rather than / to avoid unnecessary output and permission errors.

Redirect output

shell

command > output.txt        # overwrite file
command >> output.txt       # append to file
command 2> errors.txt       # save errors
command 2>&1 | tee run.log  # display and save output

Compare and count

diff · wc

diff -u old.conf new.conf
wc -l access.log          # count lines
sort names.txt | uniq     # sort and remove duplicates

System, permissions & processes

System changes need administrator privileges. Use sudo only when a command requires it.

File permissions

chmod

chmod u+x script.sh
chmod 644 file.txt
chmod 755 script.sh

Numeric values: 4 read, 2 write, 1 execute. 755 = owner rwx, others r-x.

Owner and group

chown

sudo chown user:group file
sudo chown -R user:group directory/  # recursive

Check the target directory and its contents before a recursive ownership change.

Inspect processes

ps · top

ps aux
ps -ef | grep nginx
top
free -h
df -h

top shows processes and load; free -h shows memory; df -h shows filesystem space.

Stop a process

kill

kill PID
kill -TERM PID       # ask it to exit
kill -KILL PID       # force stop if TERM did not work
pkill -f "process-name"

Find the PID with ps. KILL does not give the process time to save data.

systemd services

systemctl

systemctl status ssh
sudo systemctl enable --now ssh
sudo systemctl restart ssh
journalctl -u ssh -e
journalctl -u ssh -f

The OpenSSH service is usually called sshd on Fedora/Arch and ssh on Debian/Ubuntu.

Network & packages

Package commands need root privileges. Use repositories and signing keys you trust.

Network and listening ports

ip · ss

ip addr
ip route
ss -tulpn
curl -I https://example.com
ping -c 4 1.1.1.1

ss -tulpn lists TCP/UDP sockets; showing process names may require sudo.

Update packages

by distribution

# Debian / Ubuntu
sudo apt update && sudo apt upgrade
sudo apt install package

# Fedora / RHEL
sudo dnf upgrade
sudo dnf install package

# Arch Linux
sudo pacman -Syu
sudo pacman -S package

On Arch, upgrade the whole system with pacman -Syu; do not perform partial upgrades.

SSH & file transfers

Install an SSH client locally and the server daemon on the machine you want to reach.

Connect to a server

ssh

ssh user@server.example.com
ssh -p 2222 user@203.0.113.10
ssh -v user@server.example.com  # verbose diagnostics

Verify the server fingerprint through a trusted channel on your first connection.

Install and enable the SSH server

sshd

# Debian / Ubuntu
sudo apt update && sudo apt install openssh-server
sudo systemctl enable --now ssh

# Fedora / RHEL
sudo dnf install openssh-server
sudo systemctl enable --now sshd

# Arch
sudo pacman -S openssh
sudo systemctl enable --now sshd

For remote access, also allow the SSH port in the host firewall and your provider's network rules.

Create an SSH key

keys

ssh-keygen -t ed25519 -a 64 -C "device-name"
eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_ed25519

Protect the private key with a passphrase. Never share id_ed25519; the server only needs the .pub file.

Install your key on the server

ssh-copy-id

ssh-copy-id user@server.example.com
ssh-copy-id -p 2222 user@server.example.com
cat ~/.ssh/id_ed25519.pub

If ssh-copy-id is unavailable, add the public key contents to ~/.ssh/authorized_keys on the server.

Transfer files with SCP

scp

scp file.txt user@server:/home/user/
scp -P 2222 archive.tar.gz user@server:/tmp/
scp user@server:/var/log/app.log .
scp -r directory/ user@server:/home/user/

SCP uses uppercase -P for the port; SSH uses lowercase -p.

Sync with rsync

rsync

rsync -avh --progress ./site/ user@server:/var/www/site/
rsync -avh --progress user@server:/var/log/app.log .

The trailing slash in ./site/ means “copy the contents.” For a custom SSH port, add -e "ssh -p 2222".

Forward a GUI application

X11 forwarding

ssh -X user@server.example.com
ssh -Y user@server.example.com  # trusted server only
editor

The client needs an X server; the SSH server must allow X11Forwarding yes and set DISPLAY. For a full desktop, use a VPN with RDP (such as xrdp) or a trusted mesh VPN and remote desktop. Never expose unencrypted VNC/RDP directly to the internet.

First-time SSH server setup

Example for a fresh VPS. Replace the username, public IP, hostname, and port. Set up and verify key login before disabling password authentication.

Keep your current SSH session open until a second session successfully connects with the new port and key. A firewall or sshd mistake can lock you out; make sure you have your provider's console access.

01 · Update the system

root or sudo

# Debian / Ubuntu
apt update && apt upgrade -y

# Fedora / RHEL
dnf upgrade --refresh -y

# Arch
pacman -Syu

Run from the provider console or an existing administrator account.

02 · Create an administrator

user · sudo

# Debian / Ubuntu
adduser deploy
usermod -aG sudo deploy

# Fedora / RHEL / Arch
useradd -m -s /bin/bash deploy
passwd deploy
usermod -aG wheel deploy

# Verify group membership
id deploy

On Fedora/RHEL and Arch, confirm the wheel group is allowed in sudoers. Use visudo to validate changes; do not edit sudoers without checking its syntax.

03 · Install and enable SSH

sshd

# Debian / Ubuntu
apt install openssh-server
systemctl enable --now ssh

# Fedora / RHEL
dnf install openssh-server
systemctl enable --now sshd

# Arch
pacman -S openssh
systemctl enable --now sshd

04 · Install your public key

authorized_keys

# On your local computer:
ssh-copy-id deploy@server.example.com

# To add a key manually, on the server as deploy:
mkdir -p ~/.ssh
chmod 700 ~/.ssh
printf '%s\n' 'PASTE_PUBLIC_KEY_CONTENTS_HERE' >> ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys

The public key is the line in id_ed25519.pub. Never put the private key (the file without .pub) in authorized_keys.

05 · Change the port and allow it through the firewall

firewall first

# Choose an unused port, for example 2222.
# Set this in /etc/ssh/sshd_config:
Port 2222

# Allow it before reloading SSH:
# Debian / Ubuntu with UFW
ufw allow 2222/tcp
ufw status

# Fedora / RHEL with firewalld
firewall-cmd --permanent --add-port=2222/tcp
firewall-cmd --reload

# Arch: allow it in your configured firewall (nftables/ufw)

Also open the TCP port in your cloud provider's network firewall. On Fedora/RHEL with SELinux, label a custom SSH port with semanage port -a -t ssh_port_t -p tcp 2222 (policy tools required).

06 · Validate and reload

keep session open

sshd -t
systemctl reload sshd || systemctl reload ssh

If sshd -t reports an error, do not reload the service. Fix the config first and use the service name for your distribution.

07 · Test a second login

required

Confirm that key authentication works and the user has sudo access (sudo -v). Keep the first session open.

08 · Disable password and root login

after key login works

# In /etc/ssh/sshd_config:
PasswordAuthentication no
KbdInteractiveAuthentication no
PermitRootLogin no

# Validate and apply:
sshd -t
systemctl reload sshd || systemctl reload ssh

Check for overriding settings in /etc/ssh/sshd_config.d/*.conf and verify the effective config. Open another fresh key-authenticated session before closing the old one.

Service names and firewall tools vary by distribution and provider image. Before finishing, check the active config with sshd -T, the service status, and firewall rules.