OS commands helper

Table of contents

1. Ubuntu installation
2. User management
3. File and disk commands
4. View file content
5. Copy, Move, Delete
6. Disk navigation
7. Make file executable
8. Process management
9. SSH connection
10. Copy file via SSH
11. Crontab and at
12. Record your terminal
13. Network troubleshoot
14. Docker minimal setup
15. Bash scripts

Ubuntu installation

Set /boot approximately 0.5 - 1Gb

Set [SWAP] equal to your RAM

Set /home approximately 60% of total disk

Set / all of the rest space

No empty space left on .

Check disk partitions:

lsblk

should see something like:

User management

1. Create one more user and make it root

sudo useradd -m -s /bin/bash vasya

-m - makes home folder

-s - sets the default shell

This will add user vasya to the system.

2. Set password for the user

sudo passwd vasya

Then enter the password

3. OPTIONALLY. you may set password expiration date for the user

passwd -e vasya 

OPTIONALLY : you may install fish as your default shell for the user

sudo apt-get install -y fish

To use it:

add shell in the /etc/passwd for this user or edit with usermod:

sudo usermod -s /usr/bin/fish vasya

4. Make user vasya root

usermod -aG sudo vasya

Options means -a -- append to -G group sudo for user vasya

Options -a and -G splits together to -aG

check user is root :

id vasya

Should see, that vasya has (sudo) group

switch between users

switch to user vasya

su vasya

Switch to root user

sudo -i

File and disk commands

Create a file:

touch file.txt

Creates empty file if not exist, otherwise updates the timestamp of file access time (atime)

Create a file in text redactor:

nano file.txt

Will open file.txt in nano redactor, if file not exist, it will be created.

Starting nano without filename will create empty file, and after editing on exit will ask which filename to save.

Saving file as a result of some command:

date > date.txt

Will execute date command and save the output to date.txt file.

View file content

Show the content of file.txt in the terminal

cat file.txt

Show the first 25 lines of file.txt

head file.txt -n 25

Show the last 25 lines of file.txt

tail file.txt -n 25

Tail has a beautiful option -f which will show the file content in real time, and will update the content if file is updated.

tail -f log_file.txt

Especially useful for logs, when you want to see the updates in real time.

Copy, Move, Delete

Copy:

cp <source> <destination>

Move:

mv <source> <destination>

Delete:

rm path-to-file/file.txt

Delete recursively:

rm -r folder/

Create Folder

mkdir foldername

Delete Folder

rmdir foldername

Disk navigation

Change directory (go to some folder)

cd foldername

foldername is path , when it starts with / sign it will be the absolute path.

If no / on the start it will look for the folder (path) in the current directory.

Make file executable

To make file executable script, it should be modified with chmod command

chmod +x script.sh

.sh extension is not necessary, but it is a good practice to use it for shell scripts

Process management

Process is a running instance of a program. It operates in the system memory, and has its own PID (process identifier). It occupies some system resources, like CPU, memory, files etc. Some processes started by the user (like commands you perform in terminal), some of them are system processes.

To view all processes dynamically use top command or htop command.

To stop (kill) the process use kill command with the process ID (PID) of the process.

Important to know about kill signals:

• Option -9 means sending SIGKILL signal to the process, which is the most powerful signal, and it will kill the process immediately - becauses it is uncatchable and unignorable. Other signals like SIGTERM or SIGINT can be caught by the process, and it can handle them, but SIGKILL is not.
• Not every process can be killed, some of them are system processes, and they are necessary for the system to work properly. Killing them is restricted from user space, because they are processing in the kernel space
• Zombie processes cannot be killed by kill -9

To view static list of processes use ps command with options:

ps -eo pid,ppid,user,comm,stat,etime --sort=-etime | head -n 20

-eo means watch Every process with selected format output.

--sort means what metric to sort, may be used with - sign, meaning descending order.

Very useful columns to view (not the all possible but mostly used ):

• pid -- PID of the process, (process ideintifier)
• ppid -- Parent PID
• user -- user who initiated the process
• etime -- elapsed time since the process was started (or etimes -- in seconds)
• comm -- command (only the executable name), or exe - path to the executable.
• pmem -- percent of memory

See the man ps manual for exhaustive reference

What is zombie process?

A zombie process in Linux is a process that has completed execution but still has an entry in the process table. This happens when the process's parent process has not read its exit status via a system call like wait().

Key points:

• Lifecycle: When a process finishes execution, it enters a "zombie" state to allow its parent to retrieve its exit status.

• Process Table Entry: The zombie process remains in the process table to store its exit information until the parent acknowledges it.

• Resource Usage: A zombie process doesn't use system resources (CPU or memory), but it occupies a slot in the process table.

• Resolution: If the parent process does not handle the zombie, it can lead to resource leaks. The init process (PID 1) will eventually adopt and clean it up if the parent terminates.

• To list zombie processes, use:

ps aux | grep Z

You can not just kill the zombie process, because it is already dead.

How to Handle Zombies Instead

1. Signal the Parent Process: Use kill -s SIGCHLD <PPID> to inform the parent process to clean up the zombie by reading its exit status.

2. Terminate or Restart the Parent Process: If the parent is not responding or malfunctioning, terminate it with kill <PPID>. This causes the zombie to be adopted by init (PID 1), which will automatically clean it up.

3. System Reboot: As a last resort, rebooting the system will clear all zombie processes since the process table is reset.

SSH connection

Install SSH server

sudo apt-get install openssh-server

Change SSH port

Edit ssh configuration file:

sudo nano /etc/ssh/sshd_config

In the file change # Port 22 to Port 3005 for example , set <30NN> where NN - is your number in Journal

restart service

sudo service ssh restart

Add rule in the firewall to allow traffic on 3005 port:

sudo ufw allow 3005

Check firewall settings with ufw

sudo ufw status

If you try to connect, error will be shown, because of the ssh is running on another port

ssh [email protected]
ssh: connect to host 172.20.10.6 port 22: Connection refused

Connect to the server with the new port

ssh -p 3005 [email protected]

Copy file via SSH

Use scp command to copy files via SSH.

from remote to local

scp -P 3005 [email protected]:/home/ssh-admin/host-copy-file.txt ~/programm/tutorials/OS_tutorial 

Common syntax is: scp <options> <source> <destination>

OPTIONS:

-P - (capital P) -- specifies port number

-r - copy recursively, e.g. entire folder

Look closer what <source> consist of:

<username>@<ip_address>:<path>

<destination> -- is path

~ in the start of the path means home directory of the user

/ in the start of the path means root directory, absolute path

./ in the start means current directory

../ in the start means parent directory

from local to remote

The same syntax, but source is your local path, and destination is remote.

Notice, source and destination are swapped and splitted with space.

scp -P 2002 ~/programm/tutorials/OS_tutorial/OS-helper/README.md [email protected]:/home/ssh-admin/

Crontab and at

Watch and check syntax here https://crontab.guru/

First of all check your host machine time:

date

Should see the current time of the host machine, notice if it will have different time zone or different time.

To set the time zone of the VM, use the following command:

sudo timedatectl set-timezone Europe/Kiev

how to edit crontab

crontab -e

how to list crontab

crontab -l

Example of the script to run every day at 09:00

0 9 * * * /bin/bash /home/everyday_video/silence_minute_play.sh

How to run a script at a specific time

To run at in terminal and provide the command to run at a specific time:

at 09:00

Then provide the command to run at this time:

/bin/bash /home/everyday_video/silence_minute_play.sh

To finish editing, press Ctrl+D

View the scheduled tasks

To view the task that were scheduled with at command, use the following command:

atq

Remove the scheduled task

To remove the task, use the following command:

atrm <task_number>

The <task_number> you can get from the atq command output.

Record your terminal

To start recording with script, provide two destination files: a timing file and a typescript file:

script --timing=time.tm myscript

Script will record all the commands, you perform in terminal, their outputs etc. All you have done will be saved to the myscript file, and timing to the time.tm file.

To replay the script, use the following command:

scriptreplay --timing=time.tm myscript

You will see the same commands, you have performed in the terminal, and their outputs.

Magic!

Network troubleshoot

Example how to set IP address to the interface manually:

sudo ip addr add 192.168.1.111/24 dev enp0s8

IP address should be in the same network as the router (host PC), and the same network mask.

Docker minimal setup

To install follow instructions from https://docs.docker.com/engine/install/ubuntu/#install-using-the-repository

After the installation, it is very helpful to add your username to docker group, so you can run docker commands without sudo:

sudo usermod -aG docker $USER

Now you can run docker commands without sudo

docker commands

To download some image, for example nginx:

docker pull nginx

To run some container:

docker run --name nginx_container -d -p 8080:80 nginx

This command will start container from image nginx, and will redirect port 8080 from host machine to 80 port inside docker container.

--name - sets the name of the container

-d - runs container in detached mode

-p - redirects ports from host to container

To check running containers:

docker ps

You should see the running container nginx_container:

 docker ps
CONTAINER ID   IMAGE     COMMAND                  CREATED       STATUS       PORTS                                     NAMES
d58d48de7838   nginx     "/docker-entrypoint.…"   7 seconds ago   Up 7 seconds   0.0.0.0:80->80/tcp, :::8080->80/tcp         nginx_container

To stop the container you may use docker stop command with name of the container or container ID:

docker stop nginx_container

or

docker stop d58d48de7838