My Linux wall

How to change the extension to several files at a time

2011-08-11T04:59:00.000-07:00

If you work every day with line commands and pipes, and process several files at a time, perhaps you need to rename them at once, mainly to change file extension, I think.

mmv command is very useful to do this and it is included in all well-known linux distros.

For example, to change file extension from .tmp to .txt to all files inside current directory, you can type:

$ echo "*.tmp #1.txt" | mmv

But mmv has much other capabilities (see man page for details). A simple -c option in command line arguments allows you to preserve original files, that is, to copy (instead of move) serveral files at a time.

How to update Ruby on Rails applications from 2.2 to 2.3 version of Rails

2010-09-02T05:10:00.000-07:00

To update a Ruby on Rails application from 2.2 to 2.3 version of Rails you have to go through the following steps:

Intall the 2.3 version of Rails with gem.
Update file config/environment.rb and specify the new Rails version (RAILS_GEM_VERSION).
Run rails:update task of rake at the project home.
Add/Remove the generated/removed files inside your SCM software.

I have updated an application managed with CVS SCM software from 2.2.3 to 2.3.8, so I did:

(1) gem install rails -v 2.3.8 (as root)

(2) vi config/environment.rb

RAILS_GEM_VERSION = '2.3.8' unless defined? RAILS_GEM_VERSION

(3) rake rails:update

(in /home/user/project)
/home/user/project/app/controllers/application.rb has been renamed to /home/user/project/app/controllers/application_controller.rb, update your SCM as necessary

(4) cvs add app/controllers/application_controller.rb

(5) cvs remove app/controllers/application.rb

(6) commit

Convert .ts files to .avi

2010-08-25T08:02:00.000-07:00

I use this command to convert .ts files recorded from a Blusens T50 device to .avi format in Debian Lenny (with debian-multimedia repositories included):

ffmpeg -i input_file.ts -vcodec libxvid -b 2000k -acodec libmp3lame -ac 2 -ar 44100 -ab 128k output_file.avi

Result .avi files play fine in a wide range of devices.

To increase or decrease video quality you can use a higher or lower value in -b option.

You can select start point and duration of the encoding with -ss and -t options. For example, to encode 30 seconds starting at the minute 10 you can type:

ffmpeg -i input_file.ts -ss 00:10:00 -t 00:00:30 -vcodec libxvid -b 2000k -acodec libmp3lame -ac 2 -ar 44100 -ab 128k output_file.avi

Therefore, to choose the second audio stream instead of the first, you can use the -map option:

ffmpeg -i input_file.ts -vcodec libxvid -b 2000k -map 0:0 -map 0:2 -acodec libmp3lame -ac 2 -ar 44100 -ab 128k output_file.avi

(the first -map choose the video stream and the second one the second audio stream)

Convert .ts (or .avi) files to .3gp

2010-08-17T13:26:00.000-07:00

I use this command to convert .ts (or .avi) files to .3gp format in Debian Lenny (with debian-multimedia repositories included):

ffmpeg -i input_file.ts -s qcif -vcodec h263 -acodec libfaac -ac 1 -ar 8000 -r 25 -ab 32k -y output_file.3gp

Result .3gp files play fine in my Nokia 7610 Supernova.

Sources:
http://goinggnu.wordpress.com/2007/02/13/convert-avi-to-3gp-using-ffmpeg/

Enforce full participation on 1:N relationships in PostgreSQL

2010-07-06T02:34:00.001-07:00

Suppose we have a 1 to N (cardinality) relationship between entities A and B which are represented in our relational model through two tables: the a table with an id attribute, its primary key, and the b one with an id attribute, its primary key, and another one, a_id, the foreign key which links to a:

CREATE TABLE a (
id INTEGER,
CONSTRAINT a_primary_key (id)
);

CREATE TABLE b (
id INTEGER,
a_id INTEGER,
CONTRAINT b_primary_key (id),
CONTRAINT b_foreign_key (a_id) REFERENCES a(id)
);

The question is, how can we enforce full participation of this relationship? That is, how can we guarantee that all tuples of both tables are all linked or, what is the same, how can we avoid to have unlinked tuples?

For b table the solution is easy. We must include an a_id (the foreign key) when we insert a tuple, so we are always relating b tuples to a. If we want to delete all associated tuples of b when removing the corresponding a, we can add ON DELETE CASCADE to the b_foreign_key constraint and that's all. But the a side is more difficult.

First, we must guarantee that a tuple in a is inserted if and only if we are sure a corresponding one in table b is created also. To do this, we must use a transaction which do both insertions or none of them.

And second, how can we enforce that when we remove all tuples of b related with one of a, the latter became removed? One solution to guarantee this in PostgreSQL is to build a PL/pgSQL trigger which checks, each time an update or delete occurs in b, if there are tuples in b related with a which include the same a_id of the b tuple currently being removed, and delete the corresponding a tuple if this occurs:

CREATE OR REPLACE FUNCTION enforce_a_participation() RETURNS trigger AS $$
BEGIN
PERFORM a_id FROM b WHERE a_id=OLD.a_id;
IF NOT FOUND THEN
DELETE FROM a WHERE id=OLD.a_id;
END IF;
RETURN NULL;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER enforce_a_participation_trigger AFTER DELETE OR UPDATE
ON b FOR EACH ROW EXECUTE PROCEDURE enforce_a_participation();

(remember to write CREATE PROCEDURAL LANGUAGE plpgsql; before creating your first PL/pgSQL function)

This way, when the last row of b related with one row of a is removed, the corresponding a is deleted also, and the problem become solved.

Perl script which copies selected files from current to target directory

2010-05-20T03:02:00.000-07:00

This script allows to copy selected files, whose names are included in a names file, from current directory to target directory.

Target directory must exist, each file name of names file must be alone in one line and the lines of this file beginning with -- are ignored.

The script is very useful when you want to pick up a large subset of files from a directory.

Shell scripts to convert file names to lower case and upper case

2010-04-20T00:28:00.000-07:00

To convert to lower case:

#!/bin/sh
for i in *
do
j=`echo $i | tr '[A-Z]' '[a-z]'`
mv $i $j
done

And to upper case:

#!/bin/sh
for i in *
do
j=`echo $i | tr '[a-z]' '[A-Z]'`
mv $i $j
done

Configuring several networks in a laptop

2009-08-28T03:42:00.000-07:00

Nowadays there are several ways to configure network settings for wired and wireless cards. I have proved several applications to do it (mainly Network Manager and ifupdown) but each of them show some problems for my mobility requiriments.

I have to connect my laptop to different wireless and wired networks in my daily work:

DHCP wired nework at home.
DHCP wireless network at home.
Static IP wired usb network at work throw a Conceptronic cable (Prolific PL-2301/PL-2302 device - plusb module).
Static IP wired ethernet network at work.
Several sporadic wireless networks.

I tried to use Network Manager to solve all these requirements, but I had difficulties:

To store several skills for different networks.
To manage usb network connections.

By other hand, it is difficult for ifupdown to manage several wireless connections and it is not easy to change from one wireless network to another one.

So, I decide to use Network Manager to manage wireless connections while ifupdown take charge of the wired ones (usb and ethernet). Network Manager does not manage devices included into /etc/network/interfaces, so it is easy to use together these two applications.

I combine ifupdown application (ifupdown debian package) with resolvconf one (resolvconf debian pagackage) to allow dns entries inside interfaces file. This is my /etc/network/interfaces:

# Loopback

auto lo
iface lo inet loopback

# Ethernet networks

allow-hotplug eth0

iface eth0-home inet dhcp

iface eth0-work inet static
address w.x.y.z
netmask 255.255.255.0
network w.x.y.0
broadcast w.x.y.255
gateway w.x.y.1
dns-nameserver w.x.y.2

iface eth0-dhcp inet dhcp #default

mapping eth0
script /usr/local/sbin/ping-places.sh
map 192.168.1.254/24 192.168.1.1 eth0-home
map w.x.y.z/24 w.x.y.1 eth0-work

# usb network

allow-hotplug usb0

iface usb0 inet static
address 192.168.5.100
netmask 255.255.255.0
network 192.168.5.0
broadcast 192.168.5.255
gateway 192.168.5.1
dns-nameservers w'.x'.y'.z' w''.x''.y''.z''
dns-search domain.com

# wireless

allow-hotplug wlan0

where mapping stanzas and ping-places.sh script are used. I have copied ping-places.sh from /usr/share/doc/ifupdown/examples (ifupdown package) to /usr/local/sbin.

To configure one network, I have to plug the wire and power on the laptop. It will ping several routers to guess the network where it is wired. If, by error, you power on the laptop before connecting it, you can make:

# ifdown <interface>
# ifup <interface>

as root or sudoer, where <inteface> is eth0 or usb0.

For wireless networks you can work with Network Manager as usual.

Perl script to apply a command to a large number of files

2009-07-06T05:22:00.000-07:00

If you need to apply any command or script to a large number of files it is usefull to get a perl script like this one, which applies a command to all files of current directory that verifies a pattern.

The command must obtain its input from stdin and write its output to stdout.

It is very usefull to make massive transformations in text or XML documents. For example, to reformat all xml files of a directory using xmllint command we can do:

$ command.pl "xmllint --format -" xml output

where .xml.output files will include the result of applying xmllint --format to corresponding .xml files.

This script is released under the terms of the GNU General Public License version 3.

D-Link DI-824VUP+ print server

2009-04-25T03:43:00.001-07:00

If you have a D-Link DI-824VUP+ and want to use your USB printer with your Linux computer, you have to do the following:

Create a LPD/LPR printer.
Write the internal IP of your DI-824VUP+ device inside the host field.
Write lpUSB0 inside the queue field.
Select your printer driver (or install it from a ppd file).
Select a name for your printer.

Now you can print into yout printer throw the D-Link device. It means that you can print using a wireless connection. I don't know if it is the same for all printers but, with my Samsung ML-1210, I have to power on the printer before connecting it to the DI-824VUP+ device, otherwise it doesn't work.

D-Link DI-824VUP+ wireless problem

2009-04-25T01:51:00.000-07:00

I have a D-Link DI-824VUP+ which causes me more than one headache for several months.

The problem was that sometimes my two Lenovo laptops, with 4965 and 5100 Intel wireless cards respectively, did not connect to the wireless network of my DI-824VUP+ device. I had to reboot the device and try to connect again and, in some cases, I got the connection to work.

I believed the problem was with Linux Intel wireless driver, which logged an "authenticated timout" problem, but after upgrading to different new kernels the problem remained the same.

So, finally I found out a firmware update in australian D-Link page http://www.dlink.com.au (entering in support section and choosing the model name). I have upgraded from 1.5 firmware to 1.6b, and the problem disappeared completely! The wireless connections of my two laptops have success all times, without rebooting DI-824VUP+ device.