Here is the video version, if you prefer it:
In the following posts, we will cover files and their permissions. Then we will cover commands that change those permissions. This is really important, so pay attention. After that, we will cover some additional commands related to users. These are good to know if you ever need them, but not of utmost importance.
Talk soon!
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Let’s look at the output produced by top and explain what does it all mean:
mislav@mislavovo-racunalo:~/Linux_folder$ top
top - 13:15:58 up 101 days, 19:37, 1 user, load average: 0.44, 0.42, 0.44
Tasks: 241 total, 1 running, 239 sleeping, 0 stopped, 1 zombie
%Cpu(s): 0.2 us, 0.1 sy, 0.0 ni, 99.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 7853.8 total, 127.8 free, 4300.2 used, 3425.8 buff/cache
MiB Swap: 8066.0 total, 6694.0 free, 1372.0 used. 2891.4 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
8770 mislav 20 0 3002016 320192 111444 S 1.0 4.0 5:46.85 Web Content
8357 mislav 20 0 3524552 369556 151704 S 0.3 4.6 5:26.81 firefox-esr
14417 mislav 20 0 4222052 321880 168376 S 0.3 4.0 0:31.67 anki
...
load average – how many processes is your computer executing in the last minute, last 5 minutes, last 15 minutesTasks – how many tasks are there on the computer – 1 is running (meaning being actively executed), 239 are sleeping (meaning waiting for something to happen (such as data from a device) to resume their execution), 0 stopped (meaning no processes whose execution was paused manually) and 1 zombie process (zombie process is a process whose parent process doesn’t exist anymore)%Cpu(s) tells us what percentage of the CPU is being used on what kinds of processes: us is for user processes, sy is for system (kernel) processes, ni is for nice (low-priority) processes, id is for the percentage of the CPU that is idle, wa is for percentage of the CPU waiting for some input/output tasks, hi is the time spent processing hardware interrupts, si is the time processing software interrupts and st is relevant to virtual environments – if you don’t know what virtual environments are, that needn’t concern you (“Linux ‘top’ command: What are us, sy, ni, id, wa, hi, si and st (for CPU usage)?,” n.d.)MiB Mem and MiB swap tell you how much RAM and how much swap space is being used (measured in mebibytes (“Mebibyte,” n.d.))Let’s now look at the columns available for each process: (“A Guide to the Linux ‘Top’ Command,” n.d.)
PID – proces IDUSER – user who owns the processPR – process priorityNI – niceness (nice value) of a processVIRT – total amount of memory consumed by the process (permanent storage device + RAM – the basic idea is that if the operating system runs out of RAM, it can use some memory available on the permanent storage device in addition to the RAM; this concept is called virtual memory – look it up on Google if you are interested)RES – memory consumed by the process in RAMSHR – amount of memory shared with other processes (processes can share memory)S – process state (is the process running, is it sleeping, …)%CPU – how much CPU is the process using (in percentages)%MEM – how much memory is the process using (in percentages)TIME+ – total time used by the process since it startedCOMMAND – the name of the processHope you understand the output of top in detail now and hope you refreshed your memory!
References
A Guide to the Linux “Top” Command. (n.d.). Retrieved February 19, 2020, from https://www.booleanworld.com/guide-linux-top-command/#Understanding_top8217s_interface_the_task_area
Linux “top” command: What are us, sy, ni, id, wa, hi, si and st (for CPU usage)? (n.d.). Retrieved February 19, 2020, from https://unix.stackexchange.com/questions/18918/linux-top-command-what-are-us-sy-ni-id-wa-hi-si-and-st-for-cpu-usage
Mebibyte. (n.d.). Retrieved February 19, 2020, from https://en.wikipedia.org/wiki/Mebibyte
Shotts, W. (2019). The Linux Command Line, Fifth Internet Edition. Retrieved from http://linuxcommand.org/tlcl.php. Pages 137-139
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Let’s talk about parent and child processes.
Every process has to be instantiated (created) by some other process. The process which created the process is called the parent process and the created process is called the child process.
Keep this distinction in mind.
Thank you for reading!
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In this short article I would like to mention a few commands that enable you to monitor your computer resource usage. They are: (Ward, 2014)
vmstatiostatiotoppidstatI never used any of the above commands and so I am leaving it here for you to explore if you ever have the need. So far, I haven’t had the need.
A note: To understand the output of some of the commands above, you may need to understand how computer memory works in modern computers. In particular, you need to understand what is virtual memory and what are pages and what is a page fault. A Wikipedia article read on the topics mentioned in the previous sentence and further Googling on the topics that you don’t understand will be enough for you to gain a grasp of what is going on.
Thank you for reading!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 183-188
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Let’s talk about load averages today. Load averages display how many processes (on average) are ready to run at this moment. You can display this information with uptime. (Ward, 2014)
For example:
mislav@mislavovo-racunalo:~$ uptime
21:15:27 up 97 days, 3:36, 1 user, load average: 0.11, 0.14, 0.18
0.11 tells me that in the last minute my CPU (processor) has been dealing with 0.11 processes, in the last 5 minutes it has been dealing with 0.14 processes and in the last 15 minutes it has been dealing with 0.18 processes.
If you have multiple cores (I have 4), then a load average of 1 means that one core has been busy all the time, while other 3 have been “chilling out”. The point is that when considering the load averages, factor in the number of cores in your computer and if you are in fact dealing with a high load average, use top to find out what process is using up most resources (it will be at the top of the top’s list).
Thank you for reading!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 180-181
The question you may have is: “What is the difference between signals and interrupts?”
The difference is as follows: Interrupts are the communication between the CPU (Central Processing Unit – your processor) and the operating system (the kernel), and signals are the communication between processes and the operating system (the kernel). (“Signals and interrupts a comparison,” n.d.)
Let’s go into a bit more depth:
When an interrupt occurs (initiated by either hardware or software) it is actually managed by the CPU itself, which “interrupts” (pauses) the execution of the current process and tells the kernel to invoke the interrupt signal handler (which, to recap, is a program designed to handle interrupts). Signals, on the other hand, are used to communicate between processes. But, when the signal is traveling from the sending process to the receiving process, it is managed by the kernel, which invokes the action appropriate for the signal the process received.
I hope you gained some clarity on the difference between the two. This isn’t so that important and honestly I could have left out the part with the interrupts, but I just wanted for you to know about them since we were already talking about the operating system at such a low level. If you didn’t quite catch it, don’t worry – it won’t be that much of a hinderance.
Hope you learned something useful!
References
Signals and interrupts a comparison. (n.d.). Retrieved February 14, 2020, from https://stackoverflow.com/questions/13341870/signals-and-interrupts-a-comparison
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This is for the curious souls out there. I hope there are some. Even if you are not as curious, you will benefit from reading this as it paints the bigger picture.
You may have pondered something along the lines of: “OK, there exist processes. OK, processes have priorities. But what is the connection between hardware and software? That is, how does the operating system know that we, for example, moved our mouse? Are we dealing with signals?” Not quite, but the concept is very similar.
Here is where the concept of hardware and software interrupts comes in. Basically, hardware and software interrupts tell the operating system (the kernel): “Hey, deal with me!”. For example, pressing a key on your keyboard triggers a hardware interrupt and your operating system processes it. Interrupts also have priorities, because multiple interrupts can occur at the same time and they need to be handled according to their urgency. It is also important to note that there are programs called interrupt handlers that are executed when an interrupt occurs.
An important caveat: This is not exactly how it works, but it paints the picture. In the next post, I will clarify the details, but they are minor and don’t impact your understanding that much.
Hope you learned something new!
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Let’s talk about process priorities today. Why do processes even have priorities?
Let’s say that the world within your computer is like the real world. Let’s further imagine you are going about your day, doing your thing, driving your car, when all of a sudden you hear an ambulance. Uh-oh. You know you have to move yourself out of the ambulance’s way, because it has priority.
By the same token, processes in your operating system have priorities, depending on how important they are. In Linux, processes have priorities which range from -20 to 20, with -20 being the most important. Yes, you read that right: -20 is the highest priority. (Ward, 2014)
There is also something called a nice value, which is added to the process priority. By default, it is 0. This makes sense – as we learned in the previous paragraph, the higher the priority (in terms of its number), the lower it actually is. So if a processes nice factor is 10, then its real priority is whatever priority it has + 10. The higher the nice value, the more nice the process, since it effectively lowers its own priority. You will most likely never have to meddle with the nice level of a process.
Hope you learned something useful!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 179-180
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If you are a software developer, you sometimes want to know how much time does your program need to execute. Or, if you are a “regular” Linux user, maybe you want to know how much time a command takes to execute (albeit most likely not). Here is how to measure it: (Ward, 2014)
time programName
An example:
mislav@mislavovo-racunalo:~$ time ls
...
real 0m0.004s
user 0m0.000s
sys 0m0.004s
There are 3 relevant times: (“What do ‘real’, ‘user’ and ‘sys’ mean in the output of time(1)?,” n.d.)
real time – the amount of CPU time from starting the call to finishing the call of the process; this includes the time your process waited for some resource and the time the processor was executing other processes (your processor can switch to another process and execute a fraction of that process, then get back to the execution of your process)user time – the amount of CPU time spent within the processsys time – the amount of CPU time spent in the kernel within the process; if you wanted to do some stuff that only the kernel can do (remember that a regular user can’t do everything), you call the kernel function to do that and then this gets added up to sys timeThe CPU time your process takes up is user + sys time.
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 178-179
What do “real”, “user” and “sys” mean in the output of time(1)? (n.d.). Retrieved February 14, 2020, from https://stackoverflow.com/questions/556405/what-do-real-user-and-sys-mean-in-the-output-of-time1
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To view threads you are currently running, you can either execute ps m or execute top, then press H to see a list of all the threads. (Ward, 2014) Examples for both:
mislav@mislavovo-racunalo:~$ ps m
PID TTY STAT TIME COMMAND
5008 pts/1 - 0:00 bash
- - Ss 0:00 -
6695 tty2 - 0:00 /usr/lib/gdm3/gdm-wayland-session /usr/bin/gnome-sess
- - Ssl+ 0:00 -
- - Ssl+ 0:00 -
- - Ssl+ 0:00 -
Where there is a dash instead of a PID, that’s a thread.
top - 20:44:14 up 96 days, 3:05, 1 user, load average: 0.58, 0.62, 0.62
Threads: 864 total, 1 running, 862 sleeping, 0 stopped, 1 zombie
%Cpu(s): 3.5 us, 4.1 sy, 0.0 ni, 92.4 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 7853.8 total, 1109.6 free, 4105.2 used, 2639.0 buff/cache
MiB Swap: 8066.0 total, 6836.9 free, 1229.1 used. 3052.6 avail Mem
…
I got to this by pressing H after I ran top.
Side note: I don’t think you will ever have to mangle with threads (I haven’t), but pays to know what they are and how to view them.
Hope you learned something useful!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 177-178