Posts

Showing posts from 2013

Checking environment variable in shell scripts

Found couple of ways, the common one :

My test scripts :

#!/bin/sh

if [ -n $JJ ] ; then
echo "-n1"
fi

if [ -z $JJ ] ; then
echo "-z1"
fi

JJ=""

if [ -n $JJ ]; then
echo "-n2"
fi

if [ -z $JJ ] ; then
echo "-z2"
fi

JJ="jcrys26"

if [ -n $JJ ]; then
echo "-n3"
fi

if [ -z $JJ ]; then
echo "-z3"
fi
The result is ...

-n1
-z1
-n2
-z2
-n3


-z to test if a string is empty.
-n to test is a string is not empty.

What about if a string is undefined? And the result really confused me. I have checked the environment variable, I don't have one with name of "JJ".

Reference : http://linuxcommand.org/wss0090.php

Have kept this in draft for a long long time due to the -n result. So.... A quick conclusion is, use the -z. Use -n at your own risk. To revisit this next time, if I still remember. :P



"Rename" a user in Linux system

I tried this in Ubuntu 12.04 LTS. Run these commands as Root in a terminal.

To change a user name.

usermod -l<new_user_id> <old_user_id>
To change the home directory

usermod -m -d /home/<new_user_id> <new_user_id>
The above 2 can be combined into a command.

usermod -l <new_user_id> -m -d /home/<new_user_id> <old_user_id>
* I didn't tried this though. :P

Next, change the content of the following files. Edit the content by replacing <old_user_id> to <new_user_id>.

/etc/hosts
/etc/hostname
What is next? Yes, the user name. This can be change via  User Accounts. Click on the upper right people icon and open the User Accounts GUI. The name can be changed via that interface.

After logout, you'll see the new_user_id account is ready to be used. :)

Note, if you are seeing error prompting unable to change the user_id, you'll need to login to the system as Root, not to use sudo su as Root.

Reference : http://www.ubuntututorials.com/c…

Create cdb file using perl

To create cdb file in perl is quite easy when you have the hash ready.

See example below.


#!/usr/bin/perl use strict; use CDB_File; my %t = ( 'a' =>'apple', 'b' => 'banana', ); CDB_File::create %t, "<cdb file name>", "<temporary file name>";

If you have the key-value pairs in a file, you can parse through it and insert into the cdb by getting the hash ready, or by the pairs.

Say your data file contain this.

a,apple b,banana
Perl code for getting this flat file data into cdb :

#!/usr/bin/perl use strict; use CDB_File; my $cdb = new CDB_File("<cdb file name>", "<temporary file name>"); open (DATA, "<data file name>"); while (<DATA>) { chomp(); my @data = split /,/; $cdb->insert($data[0],$data[1]); } $cdb->finish;

Something about object files

Image
This is from a long lost track. I used to deal with object files in my previous job. That was like ages ago. Is VCS still compiling the design into object files to run simulation? Anyway, thanks to Synopsys team that helped me a lot in improving my debugging skills. :)

Some simple steps to create a "test case" here. Let's create a "Hello world!" object file.

#include <stdio.h>

void main() {
    printf("Hello world!\n");
}
Compile into object file.

gcc <source code file name> -o <object file name>

To enable debug mode, compile with -g option. With debug mode, you can set break point when running the program in gdb.



OK, back to the story.

These are a few commands that they taught me to investigate about the object files.

To get the list of the functions in the object file.

strings <object file>
This one is not obvious in my test case. The strings command actually listed all the strings in the object file. Anyway, it was handy during th…

Some sharings on Oracle database and MySQL database

Recently, I am getting involved a lot on database related tasks. My previous jobs mostly deal with gdbm and MySQL. This "new" job makes me deal a lot with Oracle database and cdb. :)

I actually never bother to get to know the special tables in the database. I need to do a tiny query on production server to get to know the number of records of some of the tables today. Imagine a database with thousands of tables. I can't be doing

select count(*) from <table name>
for all of the table names. In addition, I don't know the list of the tables of interest but only the pattern of the table names.

Luckily, found this post : http://stackoverflow.com/questions/6703444/counting-no-of-records-from-multiple-tables-oracle-db

There is a special table host the tables information, namely ALL_TABLES. To get the list of tables in the database and number of rows for each table, run this sql statement in sqlplus.

select TABLE_NAME, NUM_ROWS FROM ALL_TABLES;
For MySQL :

select table_n…

Accessing data in cdb file using Perl

See this post on cdb file related.

Never knew this can be so easy, thanks to the contributors, see here.

To install the library using cpan. Note, must use root to perform this as this will need the root access to create new file/directory.

sudo cpan
install CDB_File
Say you have a cdb file created call abc.cdb with a key summer. Here's how you can get the value of the key summer.

#!/usr/bin/perl
use strict;

use CDB_File;

my $cdb = "abc.cdb";

my %hash;

tie %hash,'CDB_File',$cdb or
die("unable to tie to cdb file");
my $value = $hash{'summer'};

print "$value\n";



Please note, if a key is not found, it will return a null value.


Without the CDB_File library, the tie will not work, and will have this error message :

Can't locate object method "TIEHASH" via package "CDB_File"

So, have fun! :)

Sending picture file in an email

It is easy to attach a picture in an email, from user's perspective, of course. But, what is actually sent over the Internet to the recipient server?

There are 2 types how a picture can be transmitted over an email, as inline picture, or as an attachement. Before they are transfer over, they need to go through some conversion, that is binary-to-text encoding called Base64.

Say, the binaries of the image starts with FF D8 FF, and the translation process can be as below :

Original binariesFF   D8   FFRegroup the binaries111111111101100011111111Associate decimal62613563From the Base64 index table/9j/

Inline image

To send as an inline, the email message shall have the following attributes.

Content-Type: image/gif; name="file name"
Content-Transfer-Encoding: base64
X-Attachment-Id: image-id
Content-ID: <image-id>Followed by the image binaries in Base64. The attributes and the image binaries is placed within the content type boundary.

In the email content, to refer to the imag…

Character set of a html document

To continue with the previous post on character encoding, my actual topic of interest is how the browser detect what is the character set is used before rendering the page when it is not specified in html header.

Below is my "test code", and I saved them into 4 different file formats (the additional one is Unicode).

<html><head> </head> <body> 一 </body> </html>
Note, there's no character set or doctype being specified in the html header. It is rendered in quirks mode.

I tested in 2 different browsers, IE 9 and FireFox 20, surprisingly, I got 2 different results. I am using document.charset and document.characterSet for IE and FF respectively to check for the character encoding of the document.

File formatIE9FF20ANSIbig5windows-1252UnicodeunicodeUTF-16Unicode (big endian)unicodeFEFFUTF-16UTF-8utf-8UTF-8
FF20 is giving Mojibake characters, it only show correctly when the encoding of the browser is changed to Chinese Traditional (Big5). T…

Character Encoding

My study object is the Chinese character "一".

I am using Notepad in Window 7 to save in different formats, namely ANSI, Unicode and UTF-8. My system locale is Chinese (Traditional, Taiwan). I am using Traditional Chinese Google IME as input method.

In simple words, encoding is to represent "something" in some "notation", decoding is to return the "something" from some "notation". Notation that I am refering here, is the binary representation.

I downloaded Hex Edit to observe the differences of the "encoding" or format (used in Notepad).

ANSI

ANSI, which always refer to ASCII character set plus an extended character set. See http://ascii-table.com/ansi-table.php. However, this character set contains only 256 characters, it does not and unable to cover Chinese character. Sometimes, when saving Chinese text in notepad, it will prompt to save in other format, but sometimes, however, I am able to save it in ANSI format. As I search…

rsyslog

Image
Installation

apt-get install rsyslog
The configuration file is at

/etc/rsyslog.conf
Basic syntax

To define log format.

$template <format name>, "<format string>"

To filter and select destination.

if <filter logic> then <destination>;<format name>
For the above, it must be in one line, or with line separator "\".

Example :


For the properties used in the template format and in the filter, please check out the online manual : http://linux.die.net/man/5/rsyslog.conf

My testing :


This is the messages directed to the destination specified in the example :


Some additional notes.

If you are seeing the message trailing by #012, put the below into your configuration file. I do not encounter this in Ubuntu, however, I see this in Debian.

$EscapeControlCharactersOnReceive off
If you see your log happened to have the messages claimed to be repeated as below image :


The default configuration file must have this $RepeatedMsgReduction is turned on. Commen…

syslog-ng on Ubuntu

Image
Installation

apt-get install syslog-ng

The configuration file is at

/etc/syslog-ng/syslog-ng.conf

Some basic syntax

To define the log to be printed out on all terminals. This normally available in the default configuration file.

destination <identifier> { pipe("/dev/xconsole"); };


To define a file where log should be directed to.

destination <identifier> { file("<file name in full path>"); };


To format the log, you can use template in your destination.

destination <identifier> { file("<file name in full path>" template("$ISODATE:$MESSAGE")); };


If you would like to fully format the logline, you can use $MSGONLY. However, please remember to put a newline character at the end of the template.

Filters can be set based on facility, priority, program name, keyword matching and etc. You can refer to this : http://www.balabit.com/sites/default/files/documents/syslog-ng-ose-3.3-guides/syslog-ng-ose-v3.3-guide-admin-en.html/index…