I want to ship a violin from UK to Bangladesh via airplane as a cargo . Is it harmful for violin?
It's always going to be a gamble.
I've had a case marked fragile arrive squashed, with obvious tractor tyre marks over it. For many years after that all my cases were marked
"Fragile - please throw under-arm"
I'd say if the violin is worth more than a seat, buy it a seat, otherwise pack it well, check your insurance, & hope.
There's always the possibility you could get it on as hand-baggage; even if you just have to beg/cajole the check-in desk. I do the same with my camera gear… If they start getting officious, I take out one lens & pointedly announce "3 grand" then reach for another lens. I've never needed to get a second one out ;)
If you're going to risk airline baggage-mangling… ermmm… handling… then at minimum, reduce the string tension; for a violin I'd probably detach the bridge & wrap separately inside the case. Fill as much space in the case as you can with bubble-wrap & then wrap the entire case in more bubble-wrap. No matter how well you pack it, you will probably have to get the post re-seated when you re-assemble it at the destination. The chances of someone, somewhere on the journey not throwing it or dropping it are quite small.
There's a psychological trick you can use.
If you pack the whole thing inside a larger box [again, with space filled with bubble wrap] making it feel light & rather flimsy for its size, that will tend to make people treat it more gently than if it's heavy & looks like it could withstand a tank running over it.
As Tetsujin said, rough handling is always a possibility, and I would worry about that more than about the temperature and pressure difference. So my preferred solution would be really to have it as a carry-on.
I once flew with an ukulele without a problem. This was a low-cost airline of the kind which sells seats within Europe for 19 Euros, and makes its money off people who arrive at the gate with too-large a carry-on and are forced to give it up as cargo at a steep fee. Nevertheless, I flew with a backpack (visibly smaller than their allowed carry-on, I must admit) and the unpacked ukulele held in my hand. I didn't even have a strap to carry it on my shoulder. I had curious glances from fellow passengers, but nobody said anything, and the airline officials didn't request it to be given up as cargo.
You could call the airline ahead, or take your chances at check-in. If you do the second, I would suggest that you take the violin itself "naked" (a case will wake associations with "luggage", which works against you) and have a plan B if they decide to be too firm with you.
As for the physical effects, I can tell you the ukulele was badly out of tune after I arrived. But it has also been badly out of tune when I have taken it along on trips by car, if the climate between start and destination is sufficiently different. So I won't speculate on the effect size of lowered pressure as compared to other factors. But you have to be aware that transportation has an effect on string instruments, and even if you don't do the modifications Tetsujin suggested, you will need a tuning once it has acclimatized. This shouldn't be a reason to never travel with your instrument though - professional musicians fly frequently with instruments that cost orders of magnitude more than your violin. Even if there are long-term effects from the changed climate, they won't be devastating.
I took a look at the case you are considering shipping it in which does not look substantial enough by itself to adequately protect your instrument. However, you might consider building or having built a simple wooden shipping crate that would be large enough to contain the instrument and its case together to protect your precious cargo. Pack with bubble wrap and padding. Another alternative would be to insure it and buy a new one, if it gets damaged.