Linux processes.

I want to assume we all have been in a situation whereby we want to run multiple commands in our terminal without opening another terminal window. This seems rather not possible, but in this article, I will be showing you how it is possible, why it is possible and a lot more cool stuff.

First off we need to understand what running a command or a process means in Linux? Google's definition:

A process is simply a running application, command, or any other program

This means that running a simple ls command is a process in Linux. Each process is known to have a parent process id (PPID) and a process id (PID). The process id is used to identify a running process, but there is an exception to this, there is this one process that doesn't have a parent process id this is known as the init process.

if you go to your terminal and type in ps -f 1 you should see this as your result

UID          PID    PPID  C STIME TTY      STAT   TIME CMD
root           1       0  0 Sep18 ?        Ss     0:02 /sbin/init splash

As you can see there is no PPID on this process why? Because it was the Linux kernel that started this process after booting up your system.

Now let's discuss what happens when you run a command for example ls. When you run the ls command the bash terminal gets forked, thereby creating another bash terminal you can't see, now we have two terminals, the initial terminal where the ls command was run is the parent process and the second terminal that got created is the child process. The ls command gets executed on the child process and it exits from there, displaying the output or result on the main terminal you're using. This is known as the fork/exec technique in Linux.

If you run this command exec ls it closes the terminal immediately because the bash terminal isn't forked it runs exec directly, the command ran but nowhere to display the output the bash terminal was exited out from.

Managing processes

To check the current processes in a shell run ps. You should see an output like this

    PID TTY          TIME CMD
  43301 pts/2    00:00:00 bash
  48347 pts/2    00:00:00 ps

If you run ps -f it shows you the hierarchy of the commands.

Now let's see how to run multiple commands in one bash terminal. Let's run sleep 360 for example, when we run it we see we can't really do anything in our terminal again except we kill the process using ctrl + c , but what if we wanted to avoid this.

First we would have to stop the command from running using ctrl + z, then run bg to continue running the process in the background.

Let's test this.

somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ sleep 360
^Z
[1]+  Stopped                 sleep 360
somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ bg 
[1]+ sleep 360 &
somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$

We can see that we started a process by running sleep 360, then we stopped this process by running ctrl + z, finally we continued running the process in the background using the bg command.

Some of you might wonder how do we check if the command is still running, well just type in jobs in your terminal and hit enter. Like this.

somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ jobs 
[1]+  Running                 sleep 360 &
somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$

It shows that the process is still running. What if we changed our mind to bring the process back to where we can see it running, just type in fg which stands for foreground and that's all

somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ fg 
sleep 360

This applies to when we have one process running. Let's say we have three different processes running, it's not really difficult it's basically the same thing we have to do only slightly modified. For example, we have sleep 360, sleep 350 and sleep 340 running. How do we get sleep 350 running in the background?.

All we have to do is run bg {id} where id is the id of the sleep 350 job that has been stopped. Here is an example.

somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ sleep 360
^Z
[1]+  Stopped                 sleep 360
somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ sleep 350
^Z
[2]+  Stopped                 sleep 350
somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ sleep 340
^Z
[3]+  Stopped                 sleep 340
somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ jobs 
[1]   Stopped                 sleep 360
[2]-  Stopped                 sleep 350
[3]+  Stopped                 sleep 340
somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ bg 2
[2]- sleep 350 &
somtochukwu@somtochukwu-HP-Laptop-14-fq0xxx:~$ jobs 
[1]-  Stopped                 sleep 360
[2]   Running                 sleep 350 &
[3]+  Stopped                 sleep 340

So there you have it, this is only the tip of the iceberg compared to what Linux processes are all about. But this should give a little insight into how Linux processes work and to run Linux processes in the background and foreground in your terminal.

Thank you for taking your time to read. No knowledge is a waste. If you have any opinions or comments do leave them down in the comments section. Cheers!