Help me understand the goal behind USE flags

[DanielC]

Hello all,

I know conceptually what USE flags are, and I know that they are supposed to be one of the big advantages of Gentoo. But in practice I don't know what to do with them. For example, I honestly cannot think of a single program that I use that has optional support for (say) KDE, or ALSA, or what-not.

Can you help me understand USE flags are useful? Can you name a program that you use turned out more to your liking because you had your USE flags set?

Cheers,
Daniel.

[russK]

Usually the use flags will enable or disable chunks of functionality or features. I tend to not mess with the USE flags until I need a feature that is not in the default settings.

A good example might be vlc. The package supports a lot of USE flags. I currently don't have support for bluray, but if I suddenly wanted to use vlc for a bluray movie, I would set the bluray USE flag and rebuild vlc.

Use 'equery' from the gentoolkit package:

[mi_unixbird]

Well, for instance, I turned off policykit, consolekit, udisks, cups, nls, a variety of languages, kde, gnome, ncurses, doc, handbook and a variety of other things that are commonly on.

And lo and behold, I thus have no policykit, consolekit, udisks, cups, nls, a variety of languages, kde, and gnome on my system. I do have ncurses on my system though since it's often nonoptional, I only have it turned off where it is optional. One of the more interesting things is that I lack a top command because I emerged procps without ncurses.

I find it had to believe that not some part of your system has optional support for some of the things listed above.
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[DanielC]

[genterminl]

I'll give another example that might be more common than those already presented. I run a KDE desktop. However, my favorite email program is Balsa, which is a gnome application. If I simply emerged balsa, it would drag in all sorts of gnome libraries and applications, and might (it's been a long time since I set this up) even try to bring in systemd, which I have chosen not to use. So, I globally set -gnome (in make.conf) and balsa only pulls in those gnome pieces which are absolutely necessary for it to compile and run. There are also some specific applications where I have to turn on the gnome use flag (in packages.use) in order to get them to install at all, or to be able to turn on some other use flag I want. It would be similar with kde if you wanted to use some particular kde application, but you use a lighter weight desktop and wanted to avoid the entire kde desktop and associated bloat.

[Abraxas]

I'll give another USE case. I use gnome. Generally I globally disable QT and then re-enable it per application for applications that only have a QT interface that I want a GUI for. I do this for a lot of things. Sometimes there are choices for libraries you can use and if I prefer one I will disable the others unless a particular application can only use that specific library.
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[NeddySeagoon]

DanielC,

Here is an example you can try at home.

After updating your system do

[-Fin-roller]

One package that also demonstrates this is media-video/ffmpeg. You're probably going to decode and encode video with your computer and ffmpeg is used to do a lot of that. Just prepare to get a bit confused and check out