Ubuntu Troubleshooting Cheat Sheet

by Steven J. Owens (unless otherwise attributed)

* Troubleshooting Quick Reference

So I'm helping a guy troubleshoot some problems with his email on a mac osx box (yes, yes, I know this is an ubuntu journal, but) and I'm dredging up various commands, and I thought, I should keep a short summary of general troubleshooting command somewhere. So, I'm going to try to migrate/copy various commands up to this section to create a summary of troubleshooting commands.

Many, if not most, of these commands will work fine on other linux distros.

And here's the summary of the summary:

Also, check out the linuxinfo package, which parses /proc/cpuinfo and displays a human-readable summary.

And now on to the summary:

editing /etc/sudoers with visudo:

Do NOT chmod and manually edit /etc/sudoers anymore.

Use visudo to run the editor.

To run visudo with a custom editor:

$ sudo EDITOR=emacs visudo

To check on a particular process:

$ ps -ef | fgrep -i whatever

(OSX is System V derived, so "ps -aux" instead; in fact, nowadays many versisn of ps support both syntaxes, so you can do "ps aux" even on an ubuntu box).

Note that, since -f lists the full info, including the command, you can grep on the command name, etc.

Related: To see the full command line, i.e. to avoid your terminal truncating each line of output at 80 or however many characters, pipe it through cat:

$ ps -ef | cat

Or:

$ ps -ef | cat | fgrep -i whatever

Check who's listening on what port:

$ netstat -tln 

Also see the -p option to netstat for process ID and name.

List open files:

Another handy command for that (which I keep forgetting about) is lsof for list open files.

$ lsof 

List processes using a file

fuser, on the other hand, does the reverse - starts with the file or socket and shows what processes are using it.

$ fuser filename

Log files

Another trick I'll do is: if I can't find log entries in the logical place, I'll attempt something (send an email through, or a web request, or stop/restart some service) and then check the timestamps in /var/log to see what was touched.

$ sudo ls -l /var/log

Checking disk space:

Also, it's usually a good idea to check and see if you've run out of disk space, as this will often cause odd things to happen:

$ df -h

Spotting problem processes:

Not to mention check top to make sure you don't have some process out of control:

$ top

Hardware

Finally, for hardware troubleshooting, there are a number of handy tips.

Basic hardware info:

First, look in /proc, particularly /proc/cpuinfo and /proc/meminfo, but there's lots of other handy hardware stuff in there.

$ cat /proc/cpuinfo

In general, dmesg shows you the things linux said to itself while it was booting up (most if this stuff also ends up in /var/log/syslog):

$ dmesg

List your PCI devices:

$ lspci

List your USB devices:

$ lsusb

List all installed hardware:

$ lsdev

Comprehensive hardware listing:

lshw compiles a comprehensive listing of your hardware. Some info requires sudo.

$ sudo lshw

Display hard drive ID:

hdparm queries the kernel for info about drives and similar devices connected to your SATA, PATA, SAS, or IDE controllers. To display the identification info for a particular hard drive, find the device ID, let's say it's /dev/sda, and:

$ sudo hdparm -i /dev/sda

Kernel modules:

Use lsmod to list what modules are loaded into the kernel:

$ lsmod

Most of the time that's pretty esoteric stuff, but sometimes it can really help to make sure that, say, your ipw2200 driver module is loaded. (This used to be a problem with ipw2200 under ubuntu when resuming from hibernation, a few years back).

Then, if you actually understand what you're doing with the kernel and modules, you can use "modprobe modulename" and/or "modprobe -r modulename" to load or remove modules. Or vice versa, modprobe -r to remove it and then modprobe to reload it:

$ modprobe -r  ipw2200

$ modprobe ipw2200

Trace system calls:

strace traces system calls, but this is for really deep voodoo. For a command that's already running but appears to have hung, run strace on the process ID:

$ strace -p processid 

Or you can try starting the command with strace:

$ strace sudo aptitude update

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