Linux and Unix How To

[edit] How to roll back to a stable version with aptitude aka How to downgrade a package

If you got it from a PPA that you need to disable, use ubuntu-tweak.

• Open Ubuntu Tweak, and select Package Cleaner in the list on the left.
• Click on the Purge PPAs button near the right.
• Select the PPA in question and click apply. You'll be prompted to downgrade related packages

To do it manually, first find the version you want to use:

sudo apt-cache showpkg <pkg>

Then install the version you need:

sudo aptitude install <pkg>=<version>

[edit] How to enable or disable KDE compositing from the command line

export DISPLAY=:0.0
qdbus org.kde.kwin /KWin org.kde.KWin.toggleCompositing

[edit] How to reinforce directory and file access limits

find . -perm /077 -type d -exec chmod -v 700 {} \;
find . -perm /077 -type f -exec chmod -v o-rwx,g-rwx {} \;

[edit] How to change system wide umask to the most secure one

Edit /etc/profile as root and set

umask 0077

For a personal umask edit ~/.bashrc

[edit] How to cp (copy) in a command line with a progress indicator

rsync -ghop --progress source destination

[edit] How to attach a console/stdout to an already running process

First find out what your console device is

tty

Then connect to the process and modify it's handles

gdb -p `pgrep processname`
p dup2(open("/dev/pts/3",1),0)
p dup2(open("/dev/pts/3",1),1)
p dup2(open("/dev/pts/3",1),2)
quit

Where /dev/pts/3 is the name of your stdout device. To check where a running process is sending it's redirects run

ls -al /proc/`pgrep processname`/fd

[edit] How to make an already running job survive a logout

Ctrl-Z to suspend it, then type

bg

to make it a background job. Then type

disown -a

to make it ignore SIGHUP signals sent on a shell logout (similar to nohup)

[edit] How to check Ubuntu version

lsb_release -a

[edit] How to install OpenCV 2.0 on Ubuntu 9.10 Karmic Koala

OpenCV is a widely used computer vision library for research and development. I've tried using instructions for installing it from the official manual, but it failed miserably. It seems that 2.0 code is incompatible with the latest incarnation of ffmpeg. Couple minutes in googling and I had it working.

UPDATE The following process describes building the latest stable version from the sources. There is an easier way tin install it by using a third-party repository. Configure it in with add-apt-repo and install.

Here is what needs to be done if you want to build OpenCV from sources:

Get all the prerequisites:

sudo apt-get install build-essential libgtk2.0-dev libavcodec-dev libavformat-dev libjpeg62-dev libtiff4-dev
sudo apt-get build-dep libswscale-dev swig

Get OpenCV 2.0, and unpack it somewhere.

Fix ffmpeg links. Make sure you are root (sudo -s) and do the following:

mkdir /usr/include/ffmpeg
ln -s /usr/include/libavcodec/avcodec.h /usr/include/ffmpeg/avcodec.h
ln -s /usr/include/libavformat/avformat.h /usr/include/ffmpeg/avformat.h
ln -s /usr/include/libavformat/avio.h /usr/include/ffmpeg/avio.h
ln -s /usr/include/libavutil/avutil.h /usr/include/ffmpeg/avutil.h

Configure OpenCV. In the folder you've unpacked it to run:

./configure --prefix=/usr/local/opencv --enable-apps --enable-shared --enable-swscale --enable-gpl --with-swig
sudo ln -s /usr/include/libswscale/swscale.h /usr/include/ffmpeg/swscale.h

Make and install it:

make
sudo make installYou could also test it by running make test

Configure your system to look for opencv libraries:

echo /usr/local/opencv/lib > /etc/ld.so.conf.d/opencv.confReload the libraries and make sure opencv shows up now
ldconfig -v | grep opencvYou may get warnings on unaccessible lib paths, that is ok. You are looking for a line that says "/usr/local/opencv/lib" 

Set-up pkg-config:

echo 'export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/opencv/lib/pkgconfig' >> /etc/bash.bashrc
echo 'export PYTHONPATH=/usr/local/opencv/lib/python2.6/site-packages/opencv' >> /etc/bash.bashrcRestart your terminal to get the new paths

Test by building samples:

cd ~/Programs/OpenCV-2.0.0/samples/c
. build_all.sh

You're done.

[edit] How to check why a system reboot/restart is required

The following will tell you which packages require a reboot

cat /var/run/reboot-required.pkgs

[edit] How to prevent a package from being upgraded

Normally it could be done with aptitude

sudo aptitude hold [packagename]

But sometimes aptitude does not work. Use dpkg:

sudo -s
echo linux-image-generic hold | dpkg --set-selections

Check with

dpkgs --get-selection  | grep linux-image-generic

Reset with

echo linux-image-generic install | dpkg --set-selections

[edit] How to change default java used on Ubuntu

Assuming you have more than one java installed. Run:

sudo update-alternatives --config java

Or if you want graphical interface do

sudo apt-get install galternatives
galternatives

[edit] How to enable screen switching functionality on Thinkpads using Fn-F7

Only works for Thinkpads with Nvidia right now. Get disper either from its website or set the repos, apt-get update and then apt-get install. do as super user:

echo "event=ibm/hotkey HKEY 00000080 00001007" > /etc/acpi/events/ibmvideobtn
echo "action=/home/CHANGE ME/bin/thinkpad-screen-switcher.sh fnf7 >/var/log/thinkpad-screen 2>&1" >> /etc/acpi/events/ibmvideobtn
echo "echo enable,0x084e > /proc/acpi/ibm/hotkey" > /etc/acpi/resume.d/91-ibm-hotkey-enable.sh

Add manually "echo enable,0x084e > /proc/acpi/ibm/hotkey before the end /etc/rc.local for the mapping to survive reboot and suspend. thinkpad-screen-switcher.sh is the great Thinkpad Display Script written by Stuart Hopkins

[edit] How to forcefully clear memory used by kernel for caching

sudo sh -c "echo 1 > /proc/sys/vm/drop_caches"

[edit] How to find out biggest memory consuming processes with ps

For the real memory:

sudo ps -eo pmem,vsize,pcpu,rss,vsize,args | sort -rnk 1 | head

For the virtual memory

sudo ps -eo vsize,pmem,pcpu,rss,vsize,args | sort -rn | head

[edit] How to remove Alt-F2 popup after having xfc installed

Run xfc_settings_editor Go to custom and delete the Alt-F2 entry

[edit] How to clean up disk space in Ubuntu

• Clean package infromation and obsolete/unused packages

sudo aptitude clean
sudo aptitude autoclean
sudo apt-get autoremove
sudo apt-get install localepurge
sudo localepurge
sudo apt-get install deborphan
sudo deborphan | xargs sudo apt-get -y remove --purge

Start Synaptic Package Manager. Click the Status button and remove all residiual configurations (if present)

• Clean kernels

Edit /boot/grub/menu.lst and change

# howmany=all

to

# howmany=2

then save it and run

sudo update-grub

this keeps the kernels installed, but limits your grub menu to the latest 2 kernels at all times.

• Other tools that could help: fslint, gtkorphan

[edit] How to add openclipart galleries to open office 3

Install first:

sudo apt-get install openclipart

then run

dpkg-query -L openclipart-openoffice.org

Note the many files in /usr/lib/openoffice/share/gallery/. You should see the same thing. In OpenOffice 3, go to Tools > Options. Click Paths in the left panel, then Gallery in the right. Click Edit. Add a new path '/usr/lib/openoffice/share/gallery' . After a few clicks of OK to accept the changes, restart open office.

[edit] How to edit an apt-get key

sudo gpg --keyring /etc/apt/trusted.gpg --trustdb-name /etc/apt/trustdb.gpg --edit-key 7EBC211F trust

[edit] How to lower ati power consumption

aticonfig --set-powerstate 1

[edit] How to save power under Linux on a ThinkPad

Ubuntu mostly has everything you need. You could get powertop and follow its recommendations. Alternatively you can go to [1], download debian package and install. Then edit

/etc/default/acpi-support

Set

ENABLE_LAPTOP_MODE=true.

[edit] How to set hardware time

echo "About to set date with following parameters :"
echo "YEAR=$2"
echo "MONTH=$3"
echo "DAY=$4"
echo "HOUR=$5"
echo "MINUTE=$6"
echo "SECOND=$7"
date -s "YEAR.MONTH.DAY-HOUR$2.$3.$4-$5:$6:$7" > /dev/null
hwclock -w

[edit] How to get a boot log in Ubuntu

Enable the bootlogd deamon by editing the file in /etc/defaults/bootlogd, and setting

BOOTLOGD_ENABLE=Yes

Reboot. The boot daemon is started at the beginning and stopped at the end of the system uptime. Check out /var/log/boot for all the informations that was pushed out during the boot time.

[edit] How to start OpenVPN on boot

sudo update-rc.d -n openvpn defaults 96 4

then

sudo update-rc.d openvpn defaults 96 4