Howtos, tips and errors

These howtos and tips are normally appropriate for all linux distributions, but when there are packages to install I take the example of gentoo with emerge. But you can probably do the same with apt-get or yum or whatever.

Tips are things that maybe you don't know they exist.

Howtos are things that maybe you don't know how to do it.

Errors are things that maybe you don't understand at all why they occur.


Change quickly your keyboard layout

setxkbmap -layout us
setxkbmap -layout fr

or even :

loadkeys fr

Mouse without X

/etc/init.d/gpm start

Stop properly X

/etc/init.d/xdm stop

To restart X, /etc/init.d/xdm restart or of course the well-known ctrl-alt-backspace.

Suspend a program

ctrl-s suspend (freeze) a running console process
ctrl-q unfreeze a suspended process
ctrl-z interrupt a running console process (sigsusp), it gives you back the prompt
fg uninterrupt and run in foreground the latest interrupted process
bg uninterrupt and run in background the latest interrupted process
jobs display all processes attached to the current session
fg %n bring back to foreground the process with job id n (1,2,3… : given by jobs)
bg %n send to background the process with job id n (1,2,3… : given by jobs)


Identify your usb storage devices by label or id

With Linux, when you plug several mass storage peripherals (external hard drive, usb key, digital camera, mp3 player etc), they are seen as scsi drives and are given the names sda,sdb… according to the order in which they are connected. This is very annoying when you want to always mount your external hard drive partitions in the same folders, and your usb key in another one, particularly if you set it up in your fstab.

But if you have udev installed, it create symlinks based on labels or ids in /dev/disk/…, and you can use it in your fstab.

One other solution with udev could be to write rules in /etc/udev/rules.d, so that you can change the sd* name, but it is much more complicated.

Let running a program after ssh disconnection with screen

When you connect to a machine with ssh and launch a program, if you disconnect before the program has finished running it will be killed. To avoid that you can use the screen program.

  • Connect with ssh to the machine
  • Enter in the screen by typing screen, or screen -S sessionname to give a name to the session
  • Launch your program
  • Press ctrl-a and then d, it will detach the screen
  • Close the ssh connection
  • Reconnect with ssh to the machine
  • Optionally, type screen -list if there are several running screens
  • Type screen -r to recall the last screen, or screen -r sessionname
  • Your program is still running in this window, or if it has stopped you can see its std out

Monitor your resources

tophtop show processes with cpu and mem usage, kill them
netstatnettopjnettop show current tcp and udp connections
iostatdstat show disk usage (io)

Run X applications in your normal user session as root

Normally you can't run X applications from a root shell. This can be annoying when you want to open a document with root credentials.

As root :

export DISPLAY=:0.0

As user :

xhost +localhost

or/and maybe:

xhost local:

Then you can do as root for example :

scite /etc/fstab &

Change separators in your shell

For example to handle filenames with spaces.

# default value : trigger on spaces, tabs and new line
IFS=$' \t\n'
# example to not trigger on spaces :
# now this is working with filenames with spaces :
for i in `cat bestof.m3u`; do cp $i /mnt/mp3; done


Recompile your kernel

Almost all operations must be done in root or with sudo.

First, download your kernel, and uncompress it :

cp linux-2.6.xx.tar.gz /usr/src
cd /usr/src
tar xvzf linux-2.6.xx.tar.gz

Or for Gentoo :

emerge gentoo-sources   # sources patched with gentoo patches
emerge suspend2-sources # OR sources patched with both gentoo patche and suspend2 patches for laptops

Then compile and install it :

rm linux
ln -s linux-2.6.xx linux
cd /usr/src/linux
cp /boot/config .  # get back your old config file (if exists)
make menuconfig    # to change configuration of your kernel
make gconfig       # OR with the GTK GUI
make xconfig       # OR with the X GUI
make && make modules && make install && make modules_install

And that's finished, the make install command did automatically something like :

cd /boot
rm vmlinuz.old
ln vmlinuz vmlinuz.old # save your old kernel which works ;-)
cp linux/arch/x86_64/boot/bzImage /boot/vmlinuz-2.6.xx # or arch/i386/ if it is your case of course
cp linux/ /boot/
cd /boot
ln -s
ln -s vmlinuz-2.6.xx vmlinuz

in order to install the new kernel, and let you access to older kernels in your boot manager (grub or lilo). It is up to you to remove useless versions from time to time.

The associated grub menu.lst with a boot partition is :

title=   Linux
root (hd0,2)
kernel /vmlinuz root=/dev/hda4

title=   Linux Old Kernel
root (hd0,2)
kernel /vmlinuz.old root=/dev/hda4

With this method, you always have the old kernel in case you have a problem with the new one, and you don't have to modify your grub config file when you change kernel.

But if you do another way, you can use :

update-grub -v

to generate it automatically (but it will probably remove your Windows entry)


Command Archive type
tar zxvf <archive-name> .tar.gz ; .tgz
tar jxvf <archive-name> .bz2
tar yxvf <archive-name> .tar.bz2
unzip <archive-name> .zip
tar cxvf <archive-name> <files-names> .tar.gz

Detail of some options :

x extract
v verbose
c compress

Line for Windows in Grub

Grub is a boot manager, to choose at boot which OS you want to launch. Sometime with automatics installations it removes the line which permits to boot Windows.

You have to add in grub/menu.lst :

title         Windows XP
root          (hd0,0)
chainloader   +1

Modify your path variable

echo $PATH
export PATH="$PATH:/sbin"

But if you want the modification to be definitive (still valable after reboot), you have to modify it in the config files of your shell (.bashrc, .zshrc).

Create/Use a patch

Create a patch :

diff -ur <orig-folder> <modified-folder> > name.patch

Use the patch, if you are in the root folder of those you used above :

patch -p1 < name.patch

Dual Display

There are several ways to do dual screen with Linux :

  • Xinerama : equivalent of windows dual screen (?)
  • TwinView : the same as xinerama but with nvidia drivers
  • Dual Desktop : two separate desktops (which can even be different window managers), you can move the cursor between them but not apps link [FR]


Chronic is a task scheduler, which unlike crons use constraints and not only time to schedule tasks. This permits to schedule tasks when you are not using your computer (updatedb, emerge –sync, etc). Constraints can be Inactivity (based on load average, disk use, screensaver), ping etc.

Download the tar.gz archive here :

Then install it :

tar zxvf Chronic-v.vv.tar.gz
cd Chronic-v.vv
perl Makefile.PL
make test
make install

You can then edit the /etc/chrontab file. Format is well described in the man page (

Here is as an example my chrontab file :

command = "updatedb"; \
   constraint = Freq, 70000; \
   constraint = Inactivity, 1200; \
   constraint = Xscreensaver, 600;

command = "eix-sync"; \
   constraint = Freq, 70000; \
   constraint = Inactivity, 1300; \
   constraint = Xscreensaver, 700; \
   constraint = Ping,; \
   constraint = Concurrent, updatedb, 1;

Moving your linux system to another disk

If you don't want to reinstall linux, you can just move it to your new hard drive or partition.

Let's start from a disk with partitions and file systems created.

Copy the system

mkdir /mnt/boot
mount -t reiserfs /dev/sda2 /mnt/boot
rsync -irua /boot /mnt/gentoo
umount /boot
mkdir /mnt/linux
mount -t reiserfs /dev/sda5 /mnt/linux
rsync -irua /* /mnt/gentoo --exclude /proc --exclude /sys --exclude /mnt/gentoo --exclude
/tmp --exclude /mnt/data
mkdir /mnt/gentoo/proc
mkdir /mnt/gentoo/sys

If you do not have a special boot partition, ignore all the parts related to boot.


You should not exclude /dev, see udevd error.

Update some config files

If you changed the partition plan of your new drive, you have to update grub.conf and

/etc/fstab to match the new plan, and only automount necessary partitions at first.

Install Grub in the MBR

grub> root (hd0,0)
grub> setup (hd0)
grub> quit

Other method

Actually it is easier to copy a “dead” partition. So boot first on a liveced, and then mount and copy your system and boot partitions, without any precaution.

But you still have to update your fstab and grub.conf, and to reinstall grub if the disk changed.

Errors and problems

Shell errors with ZSH

Sometimes ZSH refuses some commands that are accepted by Bash. For example if you try :

emerge =package-4.3.2
echo mem > /sys/power/state

It won't work. In this case, and more generally when you have strange errors, just type bash and tries with bash.

For the emerge thing, this is because = is a particular character for zsh. You can either type :

emerge '=package-4.3.2'
# or
emerge \=package-4.3.2

or add an option in your .zshrc file :

unsetopt equals

Command behave differently in script and prompt

Sometimes a command can work when you type it in your shell, and doesn't when you put it in a script. This is probably juste because your shell is different from the one you specified for the script, as most often scripts have the header #!/bin/sh, but you use another shell.

For instance, zsh doesn't deal with . (current folder) and .. (parent folder) in the same manner that sh and bash :

tar -zcf /home/user/.*

will pack all your home with sh/bash, because . is part of .*, but with zsh it will only pack your config files, because . and .. are not in ls .*.


During boot :

udevd[509]: get_ctrl_msg: unable to receive user udevd message: Socket operation on non-socket
udevd[509]: get_netlink_msg: unable to receive kernel netlink message: Socket operation on non-socket

Some critical files are missing in /dev. I don't know exactly which ones, it happened to me when I tried to move my linux system to another disk.

Emerging baselayout install some of them, but it doesn't seem to be enough, I had to copy all my original /dev to the new partition. You can use a live CD :

cp -rp /dev/* /mnt/gentoo/dev

Touchpad doesn't work

Check that PS/2 mouse is enabled in the kernel in Device drivers / Input device support / Mouse / PS/2 mouse.

No sound in headphones

If sound works with the speakers, but not with the headphones, check with alsamixer that the headphones output is not muted (M key), and that the volume not at the minimum (up/down keys) (you can change output with left/right keys).

Speakers are not disabled when plugging headphones

If your speakers keep playing sound even if headphones are plugged, check with alsamixer that “Headphone Jack Sense” is not muted…

Full screen doesn't stretch image in MPlayer

Check you compiled mplayer with the xv use flag.

updatedb freezes with rlocate

rlocate is an implementation of locate that updates in-line the database by using a kernel module that detects all file creations. Then updatedb just merges the main database with the in-line database.

But sometimes updatedb freezes and doesn't do anything. It seems that this occurs when you restarted X, and then several instances of rlocated (the rlocate daemon) are running.

To fix this, kill all rlocated instances and restart the rlocate service (/etc/init.d/rlocate restart).


Do not wake up on lid open

Add to /etc/local.d/misc.start:

echo " LID" > /proc/acpi/wakeup

Ensure that the file misc.start is executable.

Fix laptop wakes up immediately after suspend

May be due to some USB devices, even if not plugged. Look if some USB* or EHC* or XHC* are enabled in /proc/acpi/wakeup, and disable them:

echo EHC1 > /proc/acpi/wakeup

You can run it in a script at boot time right before suspending:

for usb in "EHC1" "EHC2" "XHC"; do
        state=`cat /proc/acpi/wakeup | grep $usb | cut -f3 | cut -d' ' -f1 | tr -d '*'`
        echo "device = $usb, state = $state"
        if [ "$state" == "enabled" ]; then echo $usb > /proc/acpi/wakeup; fi

Keep screen off when lid shut

If your laptop don't physically switch off your screen when the lid is closed, it sends an ACPI event. Your system will probably switch off the screen by software (if it does not uncomment the “xset” line in the script), but even if it does when you will move your mouse it will switch back on, even if the lid is still closed. One solution is to use “vbetool”, but when I use it a few seconds later I cannot interact with the computer anymore. So instead I disable the mouse, with “xinput” you can selectively disable input devices, such as a mouse, the touchpad, the keyboard, the webcam… (use xinput –list to find ids).

Create /etc/acpi/actions/ with execution permissions:

case `cat /proc/acpi/button/lid/LID/state | awk '{print $2}'` in
        #vbetool dpms on
        DISPLAY=:0.0 XAUTHORITY="/home/cyril/.Xauthority" xinput set-int-prop 12 "Device Enabled" 8 1
        #xset dpms force off
        #vbetool dpms off
        DISPLAY=:0.0 XAUTHORITY="/home/cyril/.Xauthority" xinput set-int-prop 12 "Device Enabled" 8 0

Edit /etc/acpi/ to add:

 case "$group" in
         case "$action" in
+            lid)
+                /etc/acpi/actions/
+                ;;

Lock on wake up

With hibernate-scripts

(hibernate-scripts are probably deprecated)

Add to /etc/hibernate/ram.conf:

LockGnomeScreenSaver true

See “man hibernate.conf” for more options.

With pm-utils

When using the button or when it is automatic apparently gnome-power-manager and gnome-screensaver deals correctly with it if you configure it to lock when lid closed. But it doesn't work when you manually call “pm-suspend” for instance.

So you can add this script /etc/pm/sleep.d/lock:



    DISPLAY=:0.0 XAUTHORITY="/home/cyril/.Xauthority" su cyril -c "(gnome-screensaver-command --lock)"

case "$1" in
    *) exit $NA

A more complete one to automatically detect the user name is available here:

With systemd

Create service /usr/lib/systemd/system/slimlock.service:

Description=Lock X session using slimlock



Ensure right permissions and enable it:

chmod a+r /usr/lib/systemd/system/slimlock.service
systemctl enable slimlock

Now when you suspend the system with systemctl suspend the screen will be locked.

Hard Drive power management

Copy and edit /usr/lib/pm-utils/power.d/harddrive files to /etc/pm/power.d

Root mail alias

Add to /etc/mail/aliases:

root: <your-real-email-address>

And execute



Edit the file /usr/share/hddtemp/hddtemp.db, find the line with the hard drive model that is the closest to yours, duplicate it and modify the model into yours.

Gnome Screensaver

If you don't have gnome-session, it will lock correctly the session but won't monitor idle time to start automatically. You can start the script available here instead of gnome-session. I had to make the following change to correctly detect the configured idle delay:

-gvalue = gclient.get('/apps/gnome-screensaver/idle_delay')
+gvalue = gclient.get('/desktop/gnome/session/idle_delay')

Automatically enable/disable second screen

linux/howtos-tips.txt · Last modified: 2014/03/28 16:18 by cyril
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