Quick fixes for common Linux problems

Fix Linux wireless networking problems

Networking that doesn't work is a problem that affects all operating systems from time to time. It can be frustrating to deal with, since things often seem to just not work, without giving any clue as to where the chain is broken. The first test is usually that old favourite, ping

ping www.linuxformat.co.uk

This should show packets being sent to an received by the Linux Format server. If it doesn't, try

ping 80.244.178.151

That's the IP address of the Linux Format website, so if that works when the previous command didn't, you know that you're unable to resolve domain names into IP addresses. Check that /etc/resolv.conf contains the addresses of your ISP's DNS servers. If you are using a router with a DHCP server, you may find that it contains the router address, in which case you should check that the router has the correct IP addresses.

If pinging an IP address doesn't work, try pinging one of the ISP's servers, such as the DNS server (ISPs usually give DNS addresses on their websites, and it doesn't hurt to make a note of these). If that works, the problem is probably with your ISP's connection to the rest of the internet. Another possibility is that your system is trying to use IPv6, the newer IP protocol, but your router does not understand it, which causes long delays, long enough to look like it isn't working.

The next step is to check whether you can connect to your router's web interface (if it has one) or ping your modem. If this works, the link between your modem and the ISP is down, which could be a line fault (check that your cat/ significant other hasn't unplugged the ADSL phone line), a problem at the ISP or you haven't paid your bill.

At this point a phone call to your ISP's support desk is in order. If you can't get through, it's most likely a problem at their end and the only solution is patience. Finally, check everything local: are the cables connected? Does ifconfig -a even show your network interface? If not, have you changed anything since your last reboot? A kernel update will stop third-party modules working until they are reinstalled, and some network adaptors, particularly wireless ones, use third-party kernel modules.

Software - what to do when all your processes are trying to run at once

Have you ever noticed how sometimes things just go slower and slower? There's nothing specific but everything seems to take longer than it should. I find caffeine helps here, or sleep in extreme cases, but what about when it happens to the computer? There are three main resources in your computer: CPU cycles, memory and hard disk space, and it is possible that a runaway program, or even general usage, can be using up too much of one of these.

CPU usage is the easiest to check, using the top program (that's its name, not an opinion of its usefulness). Run this from a terminal and you'll see rows and columns of data in the terminal window. The CPU line shows how much of the processor is being used by various types of program: sy is system, us is user and ni refers to programs that are running with a positive nice value. Nice is a way of scheduling programs to use more or less CPU; the higher the niceness, the nicer a program is to other process, letting them have first pick at the available CPU cycles.

It's a little more complicated than that, as nice is only a recommendation to the kernel's process scheduler, but that's too involved to go into here.

Double top

If you have more than one CPU core, press 1 to have top show them all. The figures to look at first are id and wa, for idle and wait. Unless you are compiling software or playing video, the idle figure should be quite high, usually over 90%. If it is down to single figures, or even zero, something is sucking up all your CPU cycles. That's fine if it's something you intend to do, like transcoding a video, but it could also be a runaway process.

The list of processes shows the amount of CPU and memory that each program is using, and by default this is sorted by CPU usage. If something is hogging the processor, you can use top to either renice it, if it is something you want to keep running, or kill it.

The first column shows the PID, which is the program's unique process ID. Press R to renice or K to kill, and type in the PID of the process. Renicing asks for a number to nice the process by, which is added to the existing value (higher number are 'nicer'). Nice values can run from -20 to 19, but only the root user can set a negative value.

Five is a good starting point, and 19 means the process only gets CPU time when nothing else wants it, which is useful for an intensive background task like video transcoding. Killing a task sends signal 15 (the TERM signal), and is the equivalent to pressing Ctrl+C in a terminal. It asks the program to stop, so the program has the opportunity to shut down cleanly. If the program is really out of control, it may not respond to this, so you should send signal 9 (KILL) which will stop the program without giving it that chance to elegantly shut down.

-------------------------------------------------------------------------------------------------------

First published in Linux Format issue 117