Linux¶

User Accounts¶

Root user has unlimited access to the system
Root user needs to grant a normal user root previlege by making an entry in the /etc/sudoers file. The normal user can then escalate their previlege by prefixing their command with sudo

Package managers download the necessary dependencies when installing packages in Linux
CentOS uses Yum, an RPM (RedHat Package Manager) based package manager
Commands
- yum install ansible - install a package
- yum repolist - list the repositories
- yum list - list installed packages
- yum list <package-name> - search for a package in the list of installed packages
- yum remove <package-name> - remove a package
- yum --show-duplicates list <package-name> - list all the installed versions of a package
- yum install ansible-2.4.2.0 - install a specific version of a package

Services in Linux help us configure software to run in the background and ensure that they run automatically even if the servers are rebooted. Eg. Docker
If there are multiple interdependent services, they should follow the order of startup.
systemctl command is used to manage systemd services
Commands - might need to use sudo
- systemctl start httpd - start an service (HTTPD)
- systemctl stop httpd - stop a running service
- systemctl status httpd - check the status of a service
- systemctl enable httpd - configure the service to start at startup
- systemctl disable httpd - configure the service to not start at startup
- systemctl daemon-reload - reload the systemd

Let's assume a Python server app that runs at port 5000 and prints "Hello World"
We want to configure this app as a service
Create a new file my_app.service in the /etc/systemd/system directory and paste the following in it
```
[Service]
ExecStart=<command to start your app>
```
systemctl daemon-reload - reload the systemd to let it know that a new service has been configured
systemctl start my_app - start your app as a service
To configure the service to run when the system boots up, add the following to the service file
```
[Install]
WantedBy=multi-user.target
```
To add description
```
[Unit]
Description=My best Python app
```
To restart the application if it crashes
```
[Service]
Restart=always
```

To run scripts before and after running the app

[Service]
ExecStartPre=/opt/code/configure.sh
ExecStartPost=/opt/code/result.sh

Once Docker is installed, the Docker daemon runs as a background process that listens for docker commands
Docker installation creates a service file with the following contents

Last updated: 2022-09-22