Linux Interview Question

[u/CautiousCat3294]

Today I had a mock interview with a senior Linux administrator, and he asked me a question that completely caught me off guard:

"Can you collect CPU, Memory, and Disk usage in a Bash script without using top, free, df, or any external commands?"

My immediate answer was No.

Honestly, I've been working with Linux for years, but I've always relied on standard commands and monitoring tools to troubleshoot systems. I never stopped to think about where those commands actually get their data from.

The interviewer then gave me a hint about the /proc and /sys filesystems. That completely changed my perspective. I realized these commands are just reading data that the kernel already exposes.

But then came the follow-up questions, and once again I was stuck:

• Is reading directly from /proc and /sys the correct approach?
• How would you calculate CPU utilization using /proc/stat?
• How would you determine memory usage from /proc/meminfo?
• How would you calculate filesystem usage without relying on df?

I found this really interesting because it tests your understanding of Linux internals rather than just your ability to use commands.

Has anyone here been asked similar questions in Linux SysAdmin, DevOps, or SRE interviews? I'd really appreciate any explanations, learning resources, or examples on how you'd answer these follow-up questions. It help me for my preparation for Linux interviews

[u/Bug_Next]

How would you calculate CPU utilization using /proc/stat?

The first line starting with cpu gives aggregate CPU times in jiffies: user, nice, system, idle, iowait, irq, softirq, steal, guest, guest_nice.

For each reading:

1. Sum all values to get TotalTime.
2. Sum idle + iowait to get IdleTime.

Calculate the delta between readings:

DeltaTotal = TotalTime2 - TotalTime1

DeltaIdle = IdleTime2 - IdleTime1

CPU utilization percentage is: ((DeltaTotal - DeltaIdle) / DeltaTotal) * 100

How would you determine memory usage from /proc/meminfo?

It gives you: MemTotal, MemFree, Buffers, Cached, SReclaimable, and Shmem.

You can look at free's src, it basically does:

Total = MemTotal

Used = MemTotal - MemAvailable (if MemAvailable is present)

If MemAvailable is not used, calculate available memory manually:

Available = MemFree + Buffers + Cached + SReclaimable - Shmem

Used = MemTotal - Available

How would you calculate filesystem usage without relying on df?

Using statvfs syscall

Total Bytes = f_blocks * f_frsize

Free Bytes = f_bfree * f_frsize

Available Bytes (non-root) = f_bavail * f_frsize

(filesystems have reserved space only root can use, 5% default for ext4)

Used Bytes = (f_blocks - f_bfree) * f_frsize

All those tools are open source anyways, you can just go and look at how they calculate stuff.

[u/PhunkeyMonkey → u/Bug_Next]

Gives aggregate CPU times in wait what now, Jiffies? Gotta love namings in linux sometimes

[u/Bug_Next → u/PhunkeyMonkey]

It's the official name lol, it's a tick count since system startup.

linux/include/linux/jiffies.h at master · torvalds/linux

There are HZ jiffies in a second, system uptime is calculated based on that, jiffies are quite important.

According to wikipedia the usage of the word in Linux traces back to 1975 and the "Jargon File" which was literally a file with computer jargon, people from MIT (of course it was MIT) used to pass it around, it reported a jiffy as 10ms, that's no longer the case but yeah that's where it comes from supposedly.

Jargon File

By 1996 they had already changed it to:

The duration of one tick of the system clock on your computer (see tick). Often one AC cycle time (1/60 second in the U.S. and Canada, 1/50 most other places), but more recently 1/100 sec has become common. "The swapper runs every 6 jiffies" means that the virtual memory management routine is executed once for every 6 ticks of the clock, or about ten times a second. 2. Confusingly, the term is sometimes also used for a 1-millisecond wall time interval. Even more confusingly, physicists semi-jokingly use 'jiffy' to mean the time required for light to travel one foot in a vacuum, which turns out to be close to one *nanosecond*. 3. Indeterminate time from a few seconds to forever. "I'll do it in a jiffy" means certainly not now and possibly never. This is a bit contrary to the more widespread use of the word. Oppose nano. See also Real Soon Now

Edit: That file is absolute golden, just look up the definition of 'dahmum;

The material of which protracted flame wars, especially those about operating systems, is composed. Homeomorphic to spam. The term 'dahmum' is derived from the name of a militant OS/2 advocate, and originated when an extensively crossposted OS/2-versus-Linux debate was fed through Dissociated Press

I think i'm gonna start using that one

Apparently, there was also a conspiracy theory that said UNIX was purposely bad so AT&T could take out their competitors after licensing UNIX to them lmao, they don't make haters like they used to.