To collate pages from two scanned documents where one document contains the even pages and one document odd pages, do this:

pdftk A=even.pdf B=odd.pdf shuffle A B output collated.pdf

If the odd pages are in reverse order:

pdftk A=even.pdf B=odd.pdf shuffle A Bend-1 output collated.pdf

Hat tip.

To create a tar archive to a remote host do something like this:

tar zcvf - /wwwdata | ssh root@dumpserver.nixcraft.in "cat > /backup/wwwdata.tar.gz"

or

tar jcvf - /wwwdata | ssh root@dumpserver.nixcraft.in "cat > /backup/wwwdata.tar.bz"

Hat tip.

Use this command:

libreoffice --headless --convert-to doc *odt

The --convert-to switch uses a filter that is internal to libreoffice. There are other filters available e.g. to pdf or docx etc.

Use ImageMagick to convert a jpg to A4 sized PDF for printing:

convert -page A4 input.jpg output.pdf

 

You can prepare an oversized image for the web by doing something like this:

mogrify -strip -quality 70 -resize 75% name-of-image.jpg

-strip deletes meta-data, -quality reduces the quality and -resize reduces the size of the image.

Recently, I had to close down Skype and try to log into it again, but Skype wouldn't let me:

"Another skype instance may exist", it kept telling me.

This is a common problem. Most of the advice on the Internet is to delete the ~/.Skype/shared.lck and ~/.Skype/USERNAME/config.lck files. This didn't work for me.

However, lsof | grep .Skype revealed that files in the Skype folder were being used. ps aux told me that they were being used by Chromium, because I had clicked on a link in a Skype chat window. It's crazy that a Chromium process spawned by Skype should lock the Skype directory but there you have it. I was able to kill the Chromium and log into Skype.

This is why lsof is an essential command.

Hat tip.

Here's how to find all 'DONE' items older than 60 days in org-mode so they can be archived:

C-c a m

This brings up the agenda dialogue. Then at the 'Match:' prompt type:

CLOSED<"<-60d>"

Then, in the agenda window, to archive them all:

• Press '%' to mark regexp.
• Press '.' to select all.
• Press 'B' for 'bulk action'.
• Press '$' for archive.

Hat tip.Another.

To find out what ports are in use do this:

netstat --all --programs --numeric-ports

He's how you change the browser Thunderbird will open links in:

Setting the browser that opens in Thunderbird - Linux

If you are unable to launch Firefox (or another Mozilla browser) from URL links in a Thunderbird mail message, or if you want to change the browser that is launched, add the following lines to the user.js file, located in your Thunderbird profile folder (you may need to create the user.js file). Change the path of the Firefox executable, if yours is not /usr/bin/firefox. [9] [10]

user_pref("network.protocol-handler.app.http", "/usr/bin/firefox");
user_pref("network.protocol-handler.app.https", "/usr/bin/firefox");
user_pref("network.protocol-handler.app.ftp", "/usr/bin/firefox");

Note: You can also use about:config to set these preferences. See Register protocol and network.protocol-handler.app.(protocol) for more information.

If you are still unable to change the browser after modifying these settings, editing the mimeTypes.rdf file, also located in your Profile folder, can fix the problem. Simply use a text editor to search for all references to the browser that links are currently opening in and replace them with references to the browser that you would like links to open in.

If there are no references to browsers to be replaced in the mimeTypes.rdf file, then adding some new sections to this file can fix the problem. Rather than directly editing the file, you can get Thunderbird to automatically add the required sections as follows:

    * Navigate to "Edit --> Preferences --> Advanced" in the Thunderbird menus and click on the "Config Editor" button.
    * Search for the following three entries:
          o network.protocol-handler.warn-external.http
          o network.protocol-handler.warn-external.https
          o network.protocol-handler.warn-external.ftp 
    * Set the value of each of these three entries to true (you can do this by double-clicking on each entry, then close the "about:config" window and click "OK" on the "Thunderbird Preferences" window). 

Having completed these steps, the next time you click on an http, https or ftp link in Thunderbird, you will be presented with the "Launch Application" window. Press the "Choose" button on this window to select your browser of choice. This will add the required entry to the mimeTypes.rdf file. This is a one-time task; having selected the browser once, links of the same type will now always be opened with the selected browser.

Hat tip.

A couple of essential commands for latex headers when you are printing out a draft of your document:

\usepackage{fullpage}
\usepackage{setspace}
\doublespacing

Very simple one-liner timer for counting down things like a Pomodoro:

sleep 25m && paplay /path/to/alarm.wav

Check the sleep manpage. paplay plays wav, raw, flac's and a few other formats, supported by libsndfile.

A few ideas for a timer from here:

Here's a script:

#!/bin/bash
echo -n "How many minutes would you like the timer to run? "
read limit
echo
echo "Timing $limit minutes..."
echo
counter=0
while [ $counter != $limit ]; do
   echo "$counter minutes so far...";
   sleep 60
   let "counter = $counter + 1"
done
if [ $counter = $limit ]; then
   echo
   echo "Time's up - $counter minutes have elapsed!"
   echo -e '\a' >&2
   exit 0
fi

Here is a one-liner:

echo -e '\a' >&2; sleep NUMBER; echo -e '\a' >&2

Replace 'NUMBER' with the time as per the sleep manpage.

Here is a script to clean up a directory, by deleting files older than a certain date:

#!/bin/bash

DIR=~/Download/

# delete files older than 7 days

find $DIR -mtime +7 -exec rm '{}' \;

# delete empty directories

find $DIR -depth -type d -empty -exec rmdir '{}' \;

When installing postgresql be aware that if you have several server instances on the same machine, then postgresql will manage the ports on which each server listens by itself.

From the Debian README:

Please note that the pg_* tools automatically manage the server ports unless you specify them manually. The first cluster which is ever created (by any major version) will run on the default port 5432, and each new cluster will use the next higher free one.

Please use "pg_lsclusters" for displaying the cluster <-> port mapping, and please have a look at the pg_createcluster manpage (the --port option) for details.

The output from pg_lsclusters will look something like this:

Version Cluster   Port Status Owner    Data directory                     Log file
8.2     main      5432 down   postgres /var/lib/postgresql/8.2/main       /var/log/postgresql/postgresql-8.2-main.log
8.4     main      5433 online postgres /var/lib/postgresql/8.4/main       /var/log/postgresql/postgresql-8.4-main.log

Like this:

cdrecord -v dev=/dev/cdwriter isofile.iso

Hat tip.

Do a screen shot from the command line with ImageMagick like this:

import MyScreenshot.png

After hitting return on this command, you draw the portion of the screen you want grabbed with the mouse.

To make a delay while you open windows or something:

sleep 10; import MyScreenshot.png

To take a shot of the entire screen with a delay of 10 seconds:

 sleep 10; import -window root MyScreenshot2.png

Hat tip.

Like this:

convert maphoto.jpg -resize 800x600 -strip -quality 50 -interlace line imageweb.jpg

Hat tip.

Like many people I use calibre as my ebook viewer. Yet, this program is slowly becoming a living nightmare. First is the root exploit. Second is the way that calibre wants to take over every single file. Almost all my files were defaulting to being opened in calibre. Why on Earth would I want an openoffice document opened in calibre? The creators of calibre think I do.

This obviously required fixing.

The file /usr/local/share/applications/defaults.list looks like this:

[Default Applications]
application/x-sony-bbeb=calibre-gui.desktop;calibre-lrfviewer.desktop;calibre-ebook-viewer.desktop
application/x-ruby=calibre-gui.desktop;calibre-ebook-viewer.desktop
text/rtf=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/pdf=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/x-cbz=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/x-mobipocket-ebook=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/x-cbr=calibre-gui.desktop;calibre-ebook-viewer.desktop
text/fb2+xml=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/vnd.oasis.opendocument.text=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/epub+zip=calibre-gui.desktop;calibre-ebook-viewer.desktop
text/plain=calibre-gui.desktop;calibre-ebook-viewer.desktop
text/html=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/xhtml+xml=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/ereader=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/oebps-package+xml=calibre-gui.desktop;calibre-ebook-viewer.desktop

This is a nonsense. It has been setup to open plain text, html and pdfs in calibre. How totally stupid! It needs editing. This is better:

[Default Applications]
application/x-sony-bbeb=calibre-gui.desktop;calibre-lrfviewer.desktop;calibre-ebook-viewer.desktop
application/x-cbz=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/x-mobipocket-ebook=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/x-cbr=calibre-gui.desktop;calibre-ebook-viewer.desktop
text/fb2+xml=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/epub+zip=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/ereader=calibre-gui.desktop;calibre-ebook-viewer.desktop
application/oebps-package+xml=calibre-gui.desktop;calibre-ebook-viewer.desktop

Then run sudo update-desktop-database. That seems to fix it.

An educational institution wrote to me recently asking me to finish my degree. I ordered the prospectus and looked through the courses that passed themselves off as computer science. Very little of it was science and rest was even less about computers. In fact, I can know more and get better information by reading the right books and blogs.

It was interesting to see that others have a similar take on computer science education:

When I started college, it was as a computer science major. I thought I would use college mostly as a convenient way to learn some programming languages and strategies so I could get a degree and a job in the field. But I arrived already burnt out and ended up switching out after a semester, for two very different reasons.

The first is the state of technology today. We don’t deal with the machine; we don’t even deal with abstractions on top of the machine. We deal with layers of abstractions, layers piled so high you can’t even see where they end.

The other reason, of course, is that technology education is bullshit. I can pinpoint exactly the moment of my burnout: it was when, as a sophomore in high school, I used the conditional operator in a program for my AP Computer Science class and got marked down...

The school that asked me to complete my degree wants more students. To get them, it has dumbed its courses so that they appeal to any half-wit on the Interwebs with a pulse. The question for me is whether it's worth my time and money.

Recently my favourite terminal emulator got deleted from my system, because of the continuing problems I am having due to the non-functioning of KDE.

I came across this interesting discussion on the best terminal emulators. Here are the favourites:

• yakuake (depends on kde though)
• tilde (depends on gnome though)
• rxvt or urxvt or mrxvt - nice and lightweight
• xterm (the basic)
• Guake
• LXTerminal
• yakuake (depends on kde though)
• Terminator
• aterm