Trenchbroom is available for linux:
https://github.com/kduske/TrenchBroom/releases
As well as EricW's compilers:
https://ericwa.github.io/ericw-tools/

I don't use Linux, but this is what I'm using currently for mapping and am pretty happy with it. 

Does anyone have a pipeline going from Trenchbroom to unreal engine 4? Or failing that, is there a good way to “map” in unreal engine 4 in a vaguely “quakey” manner? 

Don't have any experience with it myself, but you might want to check out HammUEr:

https://nte.itch.io/hammuer 

HammUEr will be your best bet, me thinks. I havent used it personally but it very much seems to be worth the money. 

That’s a terrible name for something that looks really good. Thanks! Will look into it. 

Is there any resource that could help me find a way to change gravity in Quake2 midmap? Either a tutorial on how to create such a custom entity, a way to alter the existing trigger_gravity in order to affect players as well, or some other way?

Thanks 

I'm trying to make a Threewave map. Does anyone know where to find the FGD/DEF files for use with TrenchBroom or another Level Editor? They don't seem to be anywhere. 

How would I create a door that rises upwards when activated by a key? I am using Trenchbroom 2. 

Good question about CTF.

As far as I know, back in the day CTF mappers would just edit their own copy of the fgd to define a few more entities, because there weren't that many new entities to add & they didn't have many interesting keys to set on them.

(Or even manually edit the map file to add the entities.)

You have item_flag_team1 and item_flag_team2 for the red/blue flags. Then info_player_team1 and info_player_team2 for the initial spawns. The spawn entities could have the angle key set to determine their facing. That's about it.

For respawns after the initial team spawns, that just used normal info_player_deathmatch entities. 

Are they just the normal quake runes? Or is there a different info? 

Runes will automatically spawn at randomly selected info_player_deathmatch locations. 

Hello folks,

My goal: Have an explosive box blow a hole in a wall. Simply deleting a brush is all I'm looking for.

The following image shows what I've got:
https://i.imgur.com/fN5jwAk.jpg

I've tried to directly "target" and "killtarget" the func_wall from the misc_explobox, and I've tried throwing a trigger_relay in the middle because some Quake wiki mentioned it. Tried a short delay and I can't really justify why, just throwing things at the func_wall and hoping something sticks. I'm considering using a silent, instantaneous func_plat to "disappear", but there must be a more elegant solution.

Thanks for reading my post 

Create a func_door, choose the appropriate spawnflag for the key color you want and then give the value -1 for the angle key. (If you want the door to go down, give the value -2.) 

I have no idea, logic looks sensible to me. But the first thing I'd do is troubleshoot the explodobox, replace it with a button and see if that works. If it does and there's an issue with explodoboxes targetting stuff, then put an invisible shootable button beneath the explodobox and have that killtarget the func wall? 

Exploboxes won't trigger or killtarget entities when they explode because on death they run the barrel_explode function, which just goes kablooey and deletes itself and is missing the SUB_UseTargets function that allows an entity to trigger and killtarget things. However, with the dark majyyks of map hacks, it's possible to rig up a setup to do what NewFlesh wants, with only one extra entity and thankfully no usual fussing with any info_notnulls.

Normally, you could just manually set the explosive box to killtarget something by sticking SUB_UseTargets someplace in its key/value fields, but just about every field is already taken- th_die is already filled with barrel_explode. th_pain works, but only activates if the barrel isn't blown up in one shot, and then you have the wall already missing and the barrel still clearly not blown up. If your name is Preach and you're a member of Mensa, maybe you could juryrig something with SUB_CalcMoveDone, but it's five in the morning and none of us have time for that crap. The one big field that we want that a boom-box doesn't use by default is use, but how do you use a barrel o' boom? Make a trigger_once around it with a reasonable amount of health (both sizes of explodebox have 20 health, for reference) and make it just big enough to overlap the funbox. Have the trigger_once target both the explodebox and a relay that killtargets the wall- this is necessary because in id1 progs, you can't have one relay both target and killtarget things at once. Then add a use | barrel_explode key/value pair to the explodebox- this will make it go boom like normal when called upon by the trigger_once. In short:

classname | barrel_explode
origin | (wherever the box is placed)
targetname | splodebox <- add this line, change name if desired
use | barrel_explode <- add this line

classname | trigger_once
target | splodebox <- make it match the targetname above
health | 20 <- this is the default explodebox health

classname | trigger_relay
origin | (doesn't matter)
targetname | splodebox <- share a targetname with the actual explosive box so both are hit by the trigger_once
killtarget | splodewall <- use whatever name fits your normal entity naming schemes

classname | func_wall
targetname | splodewall <- this is the wall you want to be 'destroyed,' so link it appropriately

Double check to make sure everything's wired up right and it should work just fine. Note that this really works with anything you want instead of an actual misc_explobox, but by using an actual box (even if the box itself isn't actually shot by the player) you don't have to worry about spoofing a model or including extra textures or anything. Or, you could just add a tiny invisible button like Shambler suggested... if you're a weenie. 

The bit that says
classname | barrel_explode
should actually be
classname | misc_explobox or misc_explobox2
depending on which size box you want to use. I slacked off in proofreading and now I'm the weenie. 

Use the shootable button method.