Never touch anything in /run directory.

I'd say a good rule of thumb for a beginner is not to touch anything outside of their own home directory. Modifying or deleting files in other locations is an easy way to break your system.

https://serverfault.com/questions/24523/meaning-of-directories-on-unix-and-unix-like-systems

• /bin - Binaries.
• /boot - Files required for booting.
• /dev - Device files.
• /etc - Et cetera. The name is inherited from the earliest Unixes, which is when it became the spot to put config-files.
• /home - Where home directories are kept.
• /lib - Where code libraries are kept.
• /media - A more modern directory, but where removable media gets mounted.
• /mnt - Where temporary file-systems are mounted.
• /opt - Where optional add-on software is installed. This is discrete from /usr/local/ for reasons I'll get to later.
• /run - Where runtime variable data is kept.
• /sbin - Where super-binaries are stored. These usually only work with root.
• /srv - Stands for "serve". This directory is intended for static files that are served out. /srv/http would be for static websites, /srv/ftp for an FTP server.
• /tmp - Where temporary files may be stored.
• /usr - Another directory inherited from the Unixes of old, it stands for "UNIX System Resources". It does not stand for "user" (see the Debian Wiki). This directory should be sharable between hosts, and can be NFS mounted to multiple hosts safely. It can be mounted read-only safely.
• /var - Another directory inherited from the Unixes of old, it stands for "variable". This is where system data that varies may be stored. Such things as spool and cache directories may be located here. If a program needs to write to the local file-system and isn't serving that data to someone directly, it'll go here.

/run contains all sorts of virtual stuff, it doesn't persist over a reboot,

I would advise against deleting anything in it as those files are used by programs running as whether user has the ID of 1000 (most likely you)

it contains things such as sockets and lock files so that programs can interact with each other

Something to realise when starting with Linux is that everything is a ‘file’. Sockets, processes, input, output etc. That’s very different from Windows and part of why scripting on Linux is so powerful. You can interact with anything.
So some directories are filled with things that aren’t necessarily files but look like it. Someone else posted a whole list, just realise that under those directories/paths shouldn’t be messed with unless you know what it’s for.
Generally when you’re getting used to Linux, /home/$user (aka ~) is where you put personal things. The rest is managed by OS and applications, don’t worry about it.
Edit: spelling

Only thing i saw here are general answers. /run/user/1000 resp. $XDG_RUNTIME_DIR is a tmpfs (somt. like a ramdisk) created by PAM (pluggable authentication module) on login, 1000 being your user id. It is more restricted in security and size than cache and should be used for small files where quick access or security matters. A common example are sockets.

You can delete it, it will just be regenerated next log in, but your very session will run into trouble, you should log out after (if you don't just get booted out, that is).

And yeah, like others said; this is not Windows, you don't have to, and shouldn't, clean up anything outside your home dir.

Then I got a lot of duplicate files. For example in subfolders relating to my mp3 player I even discovered my whole NextCloud 'drive' is there again: /run/user/1000/doc/by-app/org.strawberrymusicplayer.strawberry/51b78f5c/N

In Linux a file can show up several times in the filesystem without being duplicated. Symbolic links and hard links will cause this to happen, and they're a normal part of organizing the filesystem. Just because you see a file in several places, that doesn't mean disk space is wasted with duplicates. There may be only one physical copy of the file, appearing in multiple places. With hard links you need to be especially careful about deleting, since you'll think you're deleting one of several duplicates but you'll in fact delete the only copy of the file.

Don't delete it. It's an area of the filesystem where the current user session data is kept. This includes things like sockets to communicate with other session components and lock files. It's usually hosted on a ram disk so takes up no space in the system and goes away when you shutdown your machine.

You're going to want to look up things like symlinks, hard links, fuse filesystems, and bind mounts among other concepts. Your "whole directory" and other duplicates are artifacts of how the filesystem and process management works, and simply running fsearch or find over them is going to be confusing if you don't know what you're looking at.

One Unix concept that carries over to Linux is that everything is a file. Your shared memory space, process data, device driver interfaces, etc, all of it is accessible somewhere in the same virtual filesystem tree as the actual files.

Because of this, there's very little reason to have the whole filesystem indexed from root. If you're worried about space usage, you want to work with packages through the package manager. If you're worried about system integrity, you'll want package validators.

Other people already answered the question, I just want to say that this question was incredibly well asked.

Do not touch that as you will break things. It varies per setup on how it is used but everything /run is virtual

The most important thing to realize about the "file system" in Linux is it does a lot more than just persist your documents and app data. You shouldn't index your root directory because almost everything other than your home directory is some kind of Linux distro/application-specific directory that is often not a normal directory stored on a storage device. If you run the mount command with no arguments, every line of output is a separate file system, mounted at some specific directory of the current "mount namespace". Kinda confusing, but every process in Linux has a mount namespace that has a list of mounted file systems, often that namespace is shared between many/most processes, such as your terminal shell. Most of the file systems will be virtual i.e. not representing anything in storage. For example sysfs (always mounted at /sys), proc (always mounted at /proc), devtmpfs (mounted at /dev), etc. are all completely virtual and are ways for system services and applications to access state and devices exposed by the Linux kernel. They should never be indexed, treated as normal files, or modified by the user.

That's probably even more confusing, sorry. But the gist of it is, the only directory on your system you can really count on actually being stored on disk and always available to you is your home directory. Basically everything else exists as an implementation detail of the operating system and software applications.

If I were you, I'd stick to only indexing your user home directory. Indexing /usr or /tmp or /etc or whatever is like indexing C:\Windows and C:\Program Files, except even weirder since at least on Windows those are actually files stored on disk whereas in Linux they may not even be actual files (although most of them in /usr and /etc are actual files on disk).

Thanks everybody, I learned a ton these 2 days. Like a ' jump' in understanding. Not only the specific answer to my concrete question but also on a conceptual level as well.

The thing that makes Linux next level for me now is the extra 'abstraction layer'.

Thing is, for me, digital files always were as tangible as the analog object they represent. A digital document is as 'real' as a paper document. An email as real as a letter. But untill now files where 'real' digital artefacts. And thats ... a bit different with 'virtual' files, sort of.

Anyway, new concepts to explore which is great!

I think others have answered what the folder should do.

FSearch is great, but I wouldn't index the entire file system. There isn't much point in indexing things you won't be using such as all the system files and the representations of hardware processes. It's a bit like on Windows indexing c:\windows - you just don't need all that clogging up your search results. But the Linux filesystem encompasses much more so you'd get even more stuff.

On my system I index my home folder (where all your own files will be kept) and my mount points (for me a series of drives I mount under /mnt/). You could also index /media (or variants) as that is where USB drives, and CDs etc would mount to - but I don't tend to index USB sticks etc.

I can see circumstances where you might want to index other locations depending on how you use fsearch and Linux, but I think for most users it'd just be unnecessary indexing and results.

Edit: I saw someone else mention /etc too. That can be useful if you want to find system config files. They also mentioned /usr/share/docs which contains a lot of the Linux manual/distro docs amongst others. If you want to access that then it's not a bad idea to index it, although most people are online all the time now on multiple devices so it may be a bit redundant for most users day to day; I tend to just search online documentation.

You're using flatpak, right? Flatpak uses "portals" to provide access to other parts of your system. When you open files in flatpak apps, you'll see this folder used for those.

These shouldn't actually take up any meaningful space, and I wouldn't delete anything unless you're experiencing an issue.

Nice that you are using FSearch :) I would put more excludes in it when you really want to index / In fact, apart from /home I would not index anything else than /etc /usr/share/doc and maybe /var/run/media or /media (depending on which Linux distribution you are using, for example Arch Linux will use /var/run/media and Ubuntu will use /media for removable devices).

Flaptak apps installed?