How to automatically run program on Linux startup

Modern Linux will first boot into systemd while older versions of Linux uses System V init. Both method will run cron and rc.local before the desktop environment such as GNOME or KDE is loaded. Server-based Linux distributions will run the default shell such as Bash after the user logged in at the console instead of loading the desktop environment.

You can choose to automatically run your programs or scripts at any of the above stages.

systemd is the standard for system and service manager in modern Linux systems. It is responsible for executing and managing programs during system boot-up, among many other things. Compatible programs will provide service unit files which is used by systemd to manage the execution of the program.

You can configure systemd to automatically run these programs during Linux startup via these steps:

1. Check if service unit for your program exists (optional).

   $ sudo systemctl list-unit-files --type=service
   [sudo] password for user:
   UNIT FILE                              STATE
   accounts-daemon.service                enabled
   apparmor.service                       enabled
   apport-autoreport.service              static
   [email protected]                static
   apport.service                         generated
   apt-daily-upgrade.service              static
   apt-daily.service                      static
   atd.service                            enabled
   [email protected]                        enabled
   blk-availability.service               enabled
   bootlogd.service                       masked
   bootlogs.service                       masked
   bootmisc.service                       masked
   checkfs.service                        masked
   checkroot-bootclean.service            masked
   checkroot.service                      masked
   cloud-config.service                   enabled
   cloud-final.service                    enabled
   cloud-init-local.service               enabled
   cloud-init.service                     enabled
   console-getty.service                  disabled
   ##### snipped #####

   You'll have to create your own service unit if it's a custom program or if your program doesn't come with one during installation
   Creating and modifying systemd unit files

2. Check if service unit enabled (optional).

   $ sudo systemctl is-enabled mysql
   disabled

   enabled service unit is executed during boot

3. Enable service unit that you want to execute during startup.

   $ sudo systemctl enable mysql
   Synchronizing state of mysql.service with SysV service script with /lib/systemd/systemd-sysv-install.
   Executing: /lib/systemd/systemd-sysv-install enable mysql
   Created symlink /etc/systemd/system/multi-user.target.wants/mysql.service → /lib/systemd/system/mysql.service.

4. Check if service unit is enabled to confirm (optional).

   $ sudo systemctl is-enabled mysql
   enabled

cron is a daemon to execute scheduled commands. The commands are stored in the cron job table or crontab and is unique for each user in the system. It's started during system boot either by systemd or System V init and you can schedule your job or program to be executed right during the system boot itself by following these steps:

1. Open the default crontab editor.

   $ crontab -e

   You're required to select an editor for the crontab if this is the first time the user uses the command.

   $ crontab -e
   no crontab for user - using an empty one
   
   Select an editor.  To change later, run 'select-editor'.
     1. /bin/nano        <---- easiest
     2. /usr/bin/vim.basic
     3. /bin/ed
   
   Choose 1-3 [1]:

   A crontab will be created for the user running the command and will be executed using the privileges of the user. If you need your program to run as the root user, run crontab -e as the root user itself.

2. Add a line starting with @reboot.

   # m h  dom mon dow   command
   @reboot

   @reboot defines the job to be executed during system boot.

3. Insert the command to start your program after the @reboot.

   @reboot /sbin/ip addr | grep inet\ | tail -n1 | awk '{ print $2 }' > /etc/issue
   echo "" >> /etc/issue

   Use full path for your programs when possible and write your commands in a single line.

4. Save the file to install it to the crontab.

   $ crontab -e
   crontab: installing new crontab
   $ 

   The file is saved in /var/spool/crontab/<username>

5. Check if crontab is properly configured (optional).

   $ crontab -l
   # m h  dom mon dow   command
   @reboot /sbin/ip addr | grep inet\ | tail -n1 | awk '{ print $2 }' > /etc/issue
   echo "" >> /etc/issue

rc.local is a legacy from the System V init system where it is the last script to be executed before proceeding to a login screen for the desktop environment or a login prompt at terminal. It's normally a Bash shell script and you can basically run anything from the script.

You can configure your rc.local script following these steps:

1. Open or create /etc/rc.local file if it doesn't exist using your favourite editor as the root user.

   $ sudo vi /etc/rc.local

2. Add placeholder code into the file.

   #!/bin/bash
   
   exit 0

   It must start with interpreter (/bin/bash) and ends with an exit code (0 is for success)

3. Add command and logics to the file as necessary.

   #!/bin/bash
   
   /sbin/ip addr | grep inet\ | tail -n1 | awk '{ print $2 }' > /etc/issue
   echo "" >> /etc/issue
   
   exit 0

4. Set the file to executable.

   $ sudo chmod a+x /etc/rc.local

   The file will be executed as the root user during system boot

GNOME is the default desktop environment for Linux distributions such as Ubuntu and Red Hat. GNOME can be configured to run programs when a user logs in and can be configured by following the below article:

KDE is another popular desktop environment for Linux and is the default in Kubuntu and openSUSE. It can easily be configured to run programs when a user logs in as detailed in the following article:

A new shell program will be spawned when you start your terminal session. Bash is the default shell for most Linux distribution, and when started it will look for the following files in the particular order and executes them.

1. /etc/profile
2. ~/.bash_profile
3. ~/.bash_login
4. ~/.profile

These files contains commands and logics to set up proper environment variables and run required programs in Bash language. It's also configured to normally execute other files such as /etc/bashrc, /etc/bash.bashrc and ~/.bashrc.

You can edit any of these files to run your program when a Bash session is started. Below is a part of a typical ~/.bashrc file:

PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\[email protected]\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\] \$ '
 
PATH=/home/user/bin:$PATH
 
export EDITOR=/usr/bin/vim
 
alias ll="ls -l"