Bash

Quick commands

Change owner of folder

sudo chown -R <linuxuser>:<linuxgroup> <pathtofolder>

Manage Linux groups

Create group

groupadd <groupname>

Add user to group

usermod -a -G <linuxgroupname> <linuxusername>

List groups

sudo groups -la

Managing tar files

Preview content of tar file

tar -tzf <pathtofile.tar>

Extract content of tar file

tar -xzf <pathtofile.tar>

Copy files and folders

cp -r <sourcefolder1/sourcefile1> <sourcefolder2/sourcefile2> <sourcefolder3/sourcefile3> <destinationfolder>

SCP

scp <username>@<sourcehost>:<sourcefile/sourcefolder> <destinationfolder>

Get folder size

du -s <folderpath>

Get task manager view

SAR
TOP

Move foreground job to background

CTRL + Z

bg -> Move job to background
fg -> Move job to foreground

Generate SSH key

ssh-keygen -t rsa -b 4096 -C "<nameforsshkey>" -f .ssh/<nameforsshkey>

Apt remove insecure repositories

sudo apt autoremove

Replace characters in file

sed -i 's/<oldcharacters>/<newcharacters>/g' <filepath>

Sync clock

sudo hwclock -s

Resolve DNS Server

Set dns server on specific interface

sudo systemd-resolve --set-dns=<yourprefereddnsip> --interface=<yourinterface>

Allow ssh with password

Change parameter in config file

Edit ssh config file:

sudo nano /etc/ssh/sshd_config

image.png

Restart ssh service

sudo systemctl restart sshd

Increase diskstorage for Linux VMs

To increase the storage drive you first have to increase the storagecapacity in the proxmox admin gui. Then you can use the following commands in this order to increase the size of the volume.

fdisk -l
cfdisk

Next choose "Resize" and then "Write".

fdisk -l <pathtomaindevice> #example path: /dev/sda
parted

Next choose "print" -> "Resizepart". Enter number of device. Then choose "quit".

pvresize <pathtonewdevice>
lvextend -l +100%FREE <pathtolocalpartition>
resize2fs <pathtolocalpartition>
df -h

Then reboot the machine.

Make script executable

In order to make sure that a file ending in .sh can be executed, you have to change its permissions. This is necessary if a script is to be executed by an automation like Ansible or Crontab.

chmod +x /path/to/yourscript.sh

Afterwards it can be called by its relative or absolut path. Sometimes you have to specify the interpreter path such as /bin/bash.

/path/to/yourscript.sh
./yourscript.sh
/bin/bash /path/to/yourscript.sh

Backup MongoDB Docker container via Bash script

This short script is to backup a MongoDB database inside a docker container. A command is executed inside the Docker container via "docker exec". The command uses the program "mongodump", which is already installed on most container images. There you can specify the path in the Docker container where the backup should be stored. Here it is important that the folder in the Docker container is mapped to the filesystem of the host.

datetmp=$(date '+%Y%m%d')
dateprod=${datetmp:2}
docker exec <dockercontainername> /bin/sh -c "mongodump --host="localhost:27017" --port=27017 -o '/data/backups/${dateprod} backup'"

This script can be executed regularly by means of a cronjob, so that the backups are available at a regular interval.

Instructions for creating a cronjob can be found here: Quick commands | LNC DOCS (lucanoahcaprez.ch)

All you need to know about SSH for Ubuntu

A practical guide to setting up, securing, and getting the most out of SSH on Ubuntu.

Setup

Install OpenSSH and open the firewall:

sudo apt update && sudo apt install openssh-server -y
sudo systemctl enable --now ssh
sudo ufw allow ssh

Verify it's running with sudo systemctl status ssh, then connect from your client:

ssh user@server-ip

Key-Based Authentication

Always use keys over passwords. Generate one on your client:

ssh-keygen -t ed25519 -C "my-device"

Copy it to the server:

ssh-copy-id user@server-ip

If you set a passphrase (you should), use the SSH agent so you don't have to re-enter it every time:

eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_ed25519

# macOS: persist in Keychain
ssh-add --apple-use-keychain ~/.ssh/id_ed25519

Hardening the Server

Edit /etc/ssh/sshd_config — here are the settings that matter most:

PermitRootLogin no
PasswordAuthentication no
MaxAuthTries 3
LoginGraceTime 30
AllowUsers alice bob

Always test before restarting:

sudo sshd -t && sudo systemctl restart ssh

Keep an existing session open while testing changes. Getting locked out of your own server is not fun.

Tips & Tricks

This is a non-exhaustive collection of things that make working with SSH much nicer. Most of these go into ~/.ssh/config.

Host Aliases

Instead of typing ssh -p 2222 deployer@prod.example.com -i ~/.ssh/id_ed25519_prod every time, define it once:

Host prod
    HostName prod.example.com
    User deployer
    Port 2222
    IdentityFile ~/.ssh/id_ed25519_prod

Now just run ssh prod. This also works with scp, rsync, and git.

You can define as many as you want:

Host staging
    HostName staging.example.com
    User deployer

Host pi
    HostName 192.168.1.50
    User pi

Defaults and Wildcards

Set sane defaults for all connections:

Host *
    ServerAliveInterval 60
    ServerAliveCountMax 3
    AddKeysToAgent yes
    IdentitiesOnly yes

Or scope settings to a domain:

Host *.corp.example.com
    User admin
    IdentityFile ~/.ssh/id_ed25519_work

Port Forwarding (SSH Tunnels)

Local forward — access a remote service through a local port. Great for databases that only listen on localhost:

ssh -L 5432:localhost:5432 admin@db-server
# now connect to localhost:5432 as if you were on the server

In config (runs automatically when you connect):

Host db-tunnel
    HostName db-server.example.com
    User admin
    LocalForward 5432 localhost:5432
    LocalForward 8080 internal-app:8080

Remote forward — expose a local service through the remote server:

ssh -R 8080:localhost:3000 user@vps
# vps:8080 now points to your local port 3000

SOCKS proxy — route all traffic through the server:

ssh -D 1080 user@ssh-server
# configure your browser to use SOCKS5 proxy on localhost:1080

Background tunnels — run a tunnel without an interactive shell:

ssh -f -N -L 5432:localhost:5432 admin@db-server

For tunnels that should survive disconnects, use autossh:

sudo apt install autossh -y
autossh -M 0 -f -N -L 5432:localhost:5432 admin@db-server

Jump Hosts / Bastion

Reach a server that's only accessible through an intermediate host:

Host bastion
    HostName bastion.example.com
    User jump-user

Host internal
    HostName 10.0.0.5
    User admin
    ProxyJump bastion

ssh internal now transparently hops through the bastion. Chain multiple hops with ProxyJump bastion,internal.

Connection Multiplexing

Reuse an existing connection for instant subsequent logins:

Host *
    ControlMaster auto
    ControlPath ~/.ssh/sockets/%r@%h-%p
    ControlPersist 600

mkdir -p ~/.ssh/sockets

The first connection creates a socket. Every connection after that to the same host is near-instant.

Multiple Git Identities

Use different keys for different GitHub/GitLab accounts:

Host github-personal
    HostName github.com
    User git
    IdentityFile ~/.ssh/id_ed25519_personal

Host github-work
    HostName github.com
    User git
    IdentityFile ~/.ssh/id_ed25519_work

Then clone with git clone git@github-work:company/repo.git.

Escape Sequences

These work inside any SSH session (press Enter first):

Keys What it does
~. Force-disconnect a frozen session
~C Open command line to add tunnels on the fly
~# List active forwarded ports
~? Show all escape sequences

Adding a tunnel mid-session via ~C:

ssh> -L 3306:localhost:3306
Forwarding port.

Run Commands Remotely

ssh prod "df -h && free -m"
ssh prod 'bash -s' < local-script.sh

Mount Remote Filesystems (SSHFS)

sudo apt install sshfs -y
sshfs prod:/var/www ~/mounts/prod

# unmount
fusermount -u ~/mounts/prod    # Linux
umount ~/mounts/prod           # macOS

Skip Host Key Checking (Internal Networks Only)

Host 192.168.1.*
    StrictHostKeyChecking no
    UserKnownHostsFile /dev/null

Only use this in trusted networks. It disables MITM protection.

Debugging

When something doesn't work, add -v flags:

ssh -vvv prod

To see exactly which config options apply to a host:

ssh -G prod

Quick Reference

Action Command
Install SSH server sudo apt install openssh-server
Generate key ssh-keygen -t ed25519
Copy key to server ssh-copy-id user@host
Local port forward ssh -L 8080:localhost:80 user@host
Remote port forward ssh -R 9090:localhost:3000 user@host
SOCKS proxy ssh -D 1080 user@host
Background tunnel ssh -f -N -L 8080:localhost:80 user@host
Jump host ssh -J bastion user@target
Debug ssh -vvv user@host