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- [Presenter] There is a lot of information

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available from the Kernel.

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Some of it is messages the Kernel prints,

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some of it is information you ask for

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by looking at proc files,

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or other files and virtual file systems.

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So in this chapter,

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we're going to talk about messages and proc.

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Inside the kernel source code,

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are lots and lots of calls to a function called printk.

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It's a lot like C's printf,

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in case you've seen that.

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And it's a way to print text messages.

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Those messages go into a RAM buffer,

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and you can look at the contents

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of that RAM buffer with a command,

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command called D message.

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Sometimes those messages show up on the system console

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if they're important enough,

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and that's configurable.

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And they're also usually logged for a logging daemon.

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And so you can look at the log file for those messages.

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So D message is the command to show

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the contents of that RAM buffer right now.

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And, if you want to look at the log file,

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these days, it's typically,

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you use the journalctl command, -K,

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and that'll show you kernel logged files.

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If you do journalctl -k, -f, and let it run,

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as the kernel adds more messages, you'll see those.

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Sometimes when you're trying to debug something,

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it's kind of handy to see

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what messages are getting printed.

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Linux has a number of virtual file systems,

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or pseudo file systems.

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They look like file systems with directories and files,

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but they're not stored on disc anywhere.

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They're totally made up when you ask for 'em.

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So you look at a file, underneath proc,

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the kernel right then generates its contents.

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So by definition, it's always up-to-date information.

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Some of these files are writeable.

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So you can tell the kernel something like,

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"Change this variable to this new value."

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And that's often for some sort

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of things we'll call tunables.

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So all the files have some function

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the kernel associated with them

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that gets called when you use that file.

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Proc is the first, and kind of classic

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pseudo file system for Linux.

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The proc file system is normally mounted on/proc,

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and that should happen at boot time,

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so it should be there.

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Proc gets its name from process,

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and the proc file system has lots

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of information about processes and more.

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Every process has a subdirectory underneath proc,

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and the name of the subdirectory is the process ID.

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So if you change directory to /proc,

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you'll see lots of process ID numbers.

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So the information you could see in proc,

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will be information about memory usage of the process,

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what program it's running,

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what files it has, open watch more, lots more.

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There could be hundreds of files

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and directories per process.

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If a process has multiple threads,

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then there's also information about all the threads.

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That's underneath a directory called Task.

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Have a look at proc.

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And then the challenges,

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we will look at it a bit too.