If you are somewhat familiar with using the Linux or UNIX command line interface it is quite likely that you have used the history command. For those not familiar with it the history command allows you to search through the commands you have already run on your computer’s command line.

An example of its usage is:

# history | grep “kill”

This will search for a command containing the term kill through the history of commands you have run on your computer. This is a basic example of the history command. There are certain limitations in this command. Let’s take a look at how you can tweak it a bit to work more effectively for you. One of the limitations I find bugging me quite often is that if you are running two terminal session at the same time the commands from the first session are erased from the history of commands. Not the smartest way the command should work, I think. Well let’s fix it then.

Log in to your account and launch a shell terminal. In your user’s home directory you should find a file called .bashrc. If this file does not exist, create it. To open the file use a command like the following, replacing user with your username:

# vim /home/user/.bashrc

Add the following lines to the .bashrc file without any changes whatsoever:

shopt -s histappend
PROMPT_COMMAND=’history -a’

And voila, problem solved. Henceforth all your commands from single sessions as well as simultaneous sessions will be written to the command history file, and no overwriting will happen.

Another useful change I like to make to the history command’s configuration is removing duplicates from the history of commands. This way I get access to a lot more data and get it quicker. Open the .bashrc file once again and add the following lines in a new line:

export HISTCONTROL=”ignoredups”
export HISTIGNORE=”&:ls:[bf]g:exit”

Save the file and exit the text editor. Now the history command will check to see if a command already exists in the log file or not before adding any new information. A last modification I make to the history command’s configuration is adding spell check. Add the line shown below to the file .bashrc to help the history command check for spelling errors in command and suggest a valid command:

shopt -s cdspell

Save and exit. Now your Linux machine will be able to figure out when you type gerp instead of grep.

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A really neat tool I was introduced to a few days back is wkhtmltopdf, a freely distributed tool that allows you to convert HTML pages into PDF documents. This is not something I’ve found a lot of use for too often, but when I’ve needed it it’s been a life saver. Let’s take a look at how to set it up and use it.

The installation of wkhtmltopdf is a lot less difficult that trying to pronounce the name of the project. As the project is not yet distributed as installable binaries you will need to compile it.

Requirements:

- QT 4.4 or above
- CMake 1.6 or above
- Subvertion

On Ubuntu do the following to install the tools required to compile:

# sudo apt-get update
# sudo apt-get install libqt4-dev qt4-dev-tools build-essential cmake

Now check out the latest code for the application from the project’s SVN repositories.

# svn checkout http://wkhtmltopdf.googlecode.com/svn/trunk/ wkhtmltopdf

Time to build and compile it:

# cd wkhtmltopdf
# cmake -D CMAKE_INSTALL_PREFIX=/usr .
# make
# sudo make install

Once you are done with the previous steps using wkhtmltopdf is a piece of cake. To convert a web page from HTML to PDF all you need to do is to run the following command:

# wkhtmltopdf http://www.simplehelp.net simplehelp.pdf

The previous command will access the website http://www.simplehelp.net and create a PDF document with its contents. You obviously need a working Internet connection for this to work. You can also modify the above command slightly to convert an HTML file stored on your computer into a PDF document:

# wkhtmltopdf /home/calvin/sevenacross.html sevenacross.pdf

Pretty neat stuff, eh! wkhtmltopdf has several options allowing you to make the PDF document in Portrait mode or Landscape, and so on. Visit the project’s website for more info on it.

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Oftentimes my Ubuntu desktop loses track of time. Really. What happens is that the time it maintains goes a few minutes here and there every month. So I like to use a very useful Linux service called NTP to help keep the clock on my Ubuntu desktop in sync with well maintained clocks over the Internet. Let’s look at how to set this up.

You need an Ubuntu computer with Internet access. The access need not be on all the time, it’s fine even if the computer gets access on and off so that it can check with the clocks online and get in sync. Log into your Ubuntu computer and click on the System button at the top of you screen. Navigate to Administration -> Time and Date. You’ll see a window pop up with the time and date setup in it. Click on the Unlock button and enter your password to proceed.

In this window click on the drop down menu next to item titled Configuration You’ll see two options – Manual and Keep synchronized with Internet servers. Choose the second option. If you don’t have the NTP tools installed Ubuntu will inform you of it and ask if you want it to install them for you.

Click on the button titled Install NTP support. It will ask you for your password then begin installing the required tools. Now we can proceed with setting up time synchronization. the next step is to tell Ubuntu which server you want it to sync the time with.

Click on the Select Servers button to pick which server you want to Ubuntu to use.

The window shown in the screenshot below will pop up, giving you several options. Note that you can pick more than one server. This way if a server goes down for some reason Ubuntu will try getting the time from another server.

Click on the Close button once you are done picking servers. One last thing you need to check is that the NTP is set to run at boot. To do this go to System -> Administration -> Services and make sure that the item titled Clock synchronization service (ntp) is checked.

Voila. Now the clock on your Ubuntu computer will sync with the clocks online periodically, making sure you are always on time.

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If you have a blog or a website with a lot of content sometimes the images can take a toll on the load time of the pages. There are a number of steps you can take to speed things up. If you analyze the loading of your website you might notice that the images take up a huge part of the load time. Let’s take a look at how you can compress your .png images to improve on this figure.

We’ll use the tool pngcrush for this purpose. pngcrush is a free and open source command line tool that, as the name suggests, compresses PNG images. Take a look at the projects website for detailed information on how to use it. We will cover the basics in this article.

Installation

Most modern distributions of Linux such as Fedora and Ubuntu have pngcrush available directly in their default repositories. So, to install it on you Ubuntu computer execute the following command:

# sudo apt-get install pngcrush

On Fedora do the following:

# yum install pngcrush

Usage

pngcrush optimizes a PNG graphics file by reducing the size of the files IDAT chunk. The process is not simple. The great thing about pngcrush is that the authors of the tool have simplified things for you. They ship it with a great option – brute. When you use this option you effectively tell pngcrush to try and use the most suitable method to compress the image at hand. There are about 114 different filters that can be used, so this option is mighty useful. Here’s how you would use it to compress an image:

# pngcrush -brute -e “.compressed.png” image01.png

The two options used here are -brute which I just explained, and -e. The second option tells pngcrush what extension to add the the filenames. So after the process of crushing is complete the compressed version of the file image01.png will be called image01.compressed.png. you can change the extension if you like.

You can also slightly modify the above command to compress several PNG files in a directory. Execute the following command:

# pngcrush -brute -d “/var/www/html/website/images/” *.png

The above command uses a new option, -d. This option tells pngcrush where to place the compressed image files after the compression is complete. The above command will compress all the PNG file in the current directory and place them in /var/www/html/website/images/.

I strongly suggest that you read up more about this tool and its options on the projects website. There are a lot of options that might help you speed up the process of compression or do a better job of it. Put the new images into your website and watch your pages load faster.

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For anyone familiar with the Linux or UNIX command line interface it’s quite likely that grep has crossed your path sometime. grep is a wonderful command line tool that helps you search for a word or words through files. It ships with almost all flavors of Linux by default. For those who aren’t familiar with grep here’s a quick primer.

# grep www /etc/httpd/conf/httpd.conf

#ServerName www.example.com:80
DocumentRoot “/var/www/html”
# e.g., www.apache.org (on) or 204.62.129.132 (off).
Alias /icons/ “/var/www/icons/”

In the example above we are using grep to search for the term www in the file /etc/httpd/conf/httpd.conf. 4 results were returned. It’s a pretty straightforward command. grep ships with a lot more functionality. Feel free to browse through the manpage for grep, which you can get to by running man grep.

Now we’ll look at a feature of grep that allows you to search through a document for two terms, using command only. Let’s modify the command shown above to search for www, and also for the term server.

# grep ‘www\|server’ /etc/httpd/conf/httpd.conf

# SetHandler server-status
# Allow remote server configuration reports, with the URL of
# http://servername/server-info (requires that mod_info.c be loaded).
#ServerName www.example.com:80
DocumentRoot “/var/www/html”
# e.g., www.apache.org (on) or 204.62.129.132 (off).
Alias /icons/ “/var/www/icons/”
# SetHandler server-info
# enable the proxy server:
# (”Full” adds the server version; “Block” removes all outgoing Via: headers)
# use only name-based virtual hosts so the server doesn’t need to worry about
# server name.

So as you can see the output was much larger in the second command (12 lines were returned). Note how we modified the first command and changed the search term to ‘www\|server’. Remember to use the forward slash ( \ ) before the pipe, or else you will not get the same result. Alternately you can also use the tool egrep to run a similar search without using the forward slash. Try this:

# egrep ‘www|server’ /etc/httpd/conf/httpd.conf

# SetHandler server-status
# Allow remote server configuration reports, with the URL of
# http://servername/server-info (requires that mod_info.c be loaded).
#ServerName www.example.com:80
DocumentRoot “/var/www/html”
# e.g., www.apache.org (on) or 204.62.129.132 (off).
Alias /icons/ “/var/www/icons/”
# SetHandler server-info
# enable the proxy server:
# (”Full” adds the server version; “Block” removes all outgoing Via: headers)
# use only name-based virtual hosts so the server doesn’t need to worry about
# server name.

You can also add another pipe and search for another term also. You command would look like # egrep ‘www|server|apache’ /etc/httpd/conf/httpd.conf.

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The other day I was doing some cleaning on my Linux server and I noticed that over time people had created and left empty files. So I decided to create a shell script that would run through the system and let me know how many files like this existed, along with their path.

Here’s a simple command that will allow you to run a similar search:

# find -L /home/stewiegriffin -maxdepth 1 -type f -size 0
/home/stewiegriffin/temp.log
/home/stewiegriffin/brainstorm_notes.txt
/home/stewiegriffin/mail.log
/home/stewiegriffin/niptuck.html

What the above shown command does is that it searches for files that have a size of 0. By default, the find command excludes symbolic files, so we use the -L option to include them. The option maxdepth tells the command to search only in the main directory and not go into sub directories. type -f tells the command to only look at regular files.

If you run the same command without the maxdepth 1 option it will search for empty files in all the directories inside of /home/stewiegriffin. You can also play with the options maxdepth and mindepth if you want to regulate the depth of the search. This is really very helpful if you have a lot of levels of directories and a lot of files to go through.

Of course, if you want you can also reverse this search and ask the command to look for all non-empty files. All you need to do is use the above command and add an exclamation mark in the size parameter:

# find -L /home/stewiegriffin -maxdepth 1 -type f ! -size 0
/home/stewiegriffin/apache.log
/home/stewiegriffin/how_to_log_your_actions.txt
/home/stewiegriffin/maillog.log
/home/stewiegriffin/niptuck01.html
/home/stewiegriffin/index.html
/home/stewiegriffin/test.php

Just be careful while running this. It may return many, many results.

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If you run or administer a production MySQL server it’s quite likely that it sometimes runs a little slow, and can be a very time consuming effort to figure out why. To be able to pinpoint the query that slows the DB down can be hard to track down. MySQL has an option to help you out. It comes with an option to log slow queries. This way when you find your database server running slower than usual you can open this log file in a text editor and see what queries are running slow. Let’s look at how to set this up.

First open your MySQL server’s configuration file, my.cnf. On Linux this should be in the /etc directory. Look to see if the parameter log-slow-queries has already been set. If not, then set it to something like this:

log-slow-queries = /var/log/mysql/mysql-slow.log

You also need to set the length of time before a query is fit to be considered a slow query. On my server I have set it to 30 seconds, like this:

long_query_time = 30

Restart your MySQL server, and then watch the mysql-slow.log file. Whenever a query takes longer to complete than 30 seconds, or whatever you set in my.cnf, the query will be logged in mysql-slow.log. Now debugging MySQL queries should be a lot simpler and quicker for you. Do remember to check this file every once in a while. What might help is to setup a cron job to mail you this file once every few days.

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If you use or administer a Linux machine it can be quite useful to be able to take a look at the history of commands that have been executed on a machine. This can be pretty useful when debugging and issue. I usually use the command history to help me with this.

# history
…
946 rpm -R mutt-1.4.1-11.rhel4.i386.rpm
947 rpm -qR mutt-1.4.1-11.rhel4.i386.rpm
948 sudp rpm -qa | mail
949 sudo rpm -qa | mail
950 sudo rpm -qa | grep mail
951 rpm -qR sendmail-8.13.1-3.2.el4
952 cd scripts/log_parse/
…

This command gives me a straightforward list of the commands executed on this machine by the user I’m currently logged in as. However, sometimes I need a lot more information than just this. I need to know which command was executed at what time. For this I make a small modification to the commands settings, adding the date and time to the information output by the command:

# export HISTTIMEFORMAT=”%F %T “

Now run the history and see the difference:

# history
…
946 2009-07-20 08:12:33 rpm -R mutt-1.4.1-11.rhel4.i386.rpm
947 2009-07-20 08:09:33 rpm -qR mutt-1.4.1-11.rhel4.i386.rpm
948 2009-07-20 09:49:44 sudp rpm -qa | mail
949 2009-07-20 10:37:33 sudo rpm -qa | mail
950 2009-07-20 10:17:13 sudo rpm -qa | grep mail
951 2009-07-20 10:12:23 rpm -qR sendmail-8.13.1-3.2.el4
952 2009-07-20 10:22:43 cd scripts/log_parse/
…

Add this command to your users .bashrc file to make the change permanent.

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The other day I was migrating my hosting from one server to another. My previous hosting only gave me FTP access. The new host came enabled with SSH and a complete Linux environment. I wanted to migrate the data from my old host to the new one. Here’s how I moved all my data.

I used the following command for the migration:

# ncftpget -R -v -u “remoteuser” -p “password” ftp://remotserver.com /home/localdir /remotedir

NC FTP is a really cool FTP client for the Linux command line. It ships by default with most Linux distributions. It comes installed with a few command such as ncftpget and ncftpput. In this example we’re using the first of the two commands. Here’s a breakup of the command.

-R -v These two tell ncftpget to get the files recursively, and make the output verbose.
-u -p These two parameters are followed by the username and password for the FTP session.
ftp://remotserver.com This is the IP address or the domain of the remote FTP server.
/home/localdir This is the path of the directory in your local machine where you want to copy the remote files
/remotedir This is path of the directory you want to copy in the remote server

Just be careful not to mix up the local and remote directories when you run the command. Otherwise, it should be pretty straightforward.

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Sometimes when you are trying to uninstall or upgrade some software on a Linux machine you run into dependency related issues. Just about everyone I know who works with RPM based distributions has experienced what is known as “rpm hell” at one time or another. Here’s something to help you a bit if you are stuck in one of those situations.

Let’s look at how to check what files an installed RPM depends on.

Say you are trying to upgrade or uninstall the RPM of sendmail from your Linux computer. You might run into dependency issues when you run the uninstall command:

# rpm -e sendmail-8.13.1-3.2

In such a case it might be useful to take a look at what files this package depends on. Use the following command:

# rpm -qR sendmail-8.13.1-3.2
/bin/bash
/bin/mktemp
/bin/sh
/bin/sh
/bin/sh
/bin/sh
/bin/sh
/usr/sbin/alternatives
/usr/sbin/useradd
bash >= 2.0
chkconfig >= 1.3
config(sendmail) = 8.13.1-3.2.el4
cyrus-sasl
fileutils
gawk
libc.so.6
libc.so.6(GLIBC_2.0)
libc.so.6(GLIBC_2.1)
…

Now you can see a list of files that the sendmail package depends on and you’ll be better equipped to debug the failed uninstallation process.

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If you administer a Linux server or are a developer it is quite likely that you might need to watch a file to check for live updates to it. It could be a log file which you want to keep an eye on to see if any errors or messages appear, or it could just be a file with some data written into it. Here’s a way you can have that file update itself rather than your having to hit the refresh button constantly.

Say you want to monitor the Apache web server log file, /var/log/http/access.log. Use the following command to get live updates:

# tail -f /var/log/http/access.log

When you hit the Return key you should see the last few entries in the file (if there are any) and then the command will wait for further updates to the file and update automatically. One drawback of this command is that you can not scroll up or down using tail. Another drawback of this command is that if you are monitoring a file that gets rolled (the file gets moved to another file and a new blank file is used instead of the first one), then this command will stop the updates. We have a solution for the second problem. Use the following syntax:

# tail -F /var/log/http/access.log

The upper case -F ensures that even if the file rolls it will continue reading from the new file, which is quite useful. I usually just use this option regardless of whether I think the file might roll or not.

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If you store important information on your Linux server and want to make sure it is for your eyes only you need to password protect these files. Let’s see how you can do this with Linux.

First, pick a good password. There are a few websites on the Internet that help you generate strong passwords. Try and pick an alphanumeric password. Something such as Rv7fkcxASW8h would be a good choice.

Now check to see if you have the package gpg installed on your computer. Run the following command to check:

# whereis gpg
gpg: /usr/bin/gpg /usr/share/man/man1/gpg.1.gz

If you get an output like the one shown above it means you have the required package installed. If not, check your Linux distribution’s documentation to see how you can install the GPG package.

Say you want to password protect a file called dbbackup.zip with the password Rv7fkcxASW8h here’s how you would go about it:

# gpg -c dbbackup.zip

When you enter this you will be requested to enter a passphrase, which is the password you want to protect the file with. Enter the password twice. Now you should see a file called dbbackup.zip.gpg in the same directory as the original file. This is the encrypted and password protected copy of the original file. You can store this file on your hard drive or ship it to someone on a DVD knowing that it will be very hard and nearly impossible for most people to crack it.

When you want to read the file you will need to remove the password protected encryption. For that execute the following command:

gpg dbbackup.zip.gpg

You will be asked to enter the password using which the file was protected. Enter that and you should be in business. Note that this only works for files and not for folders. You should create a zip or tar archive of the files you want to secure and then add the password protected encryption to it.

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From the Qimo home page:

Qimo is a desktop operating system designed for kids. Based on the open source Ubuntu Linux desktop, Qimo comes pre-installed with educational games for children aged 3 and up.

Qimo’s interface has been designed to be intuitive and easy to use, providing large icons for all installed games, so that even the youngest users have no trouble selecting the activity they want.

Qimo needs a minimum of 256MB of memory to run from the CD, or 192MB to install. At least 6 GB of hard drive space is recommended, and a 400MHz or faster CPU. Because of its very minimal system requirements, it’s a fantastic OS to install on that old PC sitting in your closet and put in your childs room.

This tutorial will guide you through installing Qimo, and give a brief overview of the apps that are included, as well as instructions on removing some of the ones you may not want your child to access.

1. To get started, download the Qimo .iso file. You’ll need to burn this .iso to a CD or DVD (it does fit on a CD). Once completed, insert the CD/DVD into your CD/DVD-ROM and boot up your computer. Make sure that your PC is set to boot from the CD/DVD drive first (before the hard disk or other drive).

   You’ll be presented with the initial Qimo boot screen. Because Qimo is a LiveCD, you can try it before you install it, if you’d like. Select Try Qimo without any change to your computer if you’d like to give it a run before you actually install it. This tutorial will actually walk you through the complete installation process – to do so click the down arrow on your keyboard to highlight Install Qimo and then hit Enter.


click to enlarge

2. The Qimo installer will begin to load.


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3. The first step of the installation is to select your language. Do so from the list on the left side of the screen, and then click the Forward button.


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4. Now select the city closest to you (in your time zone) from the drop-down menu, and click the Forward button.


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5. Select your keyboard layout from the list in the left column, and again, click the Forward button.


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6. Now you’ll need to decide how much disk space to allocate to Qimo. Because I’m going to dedicate the entire hard drive to Qimo, I selected Guided – use entire disk. If you don’t want to use your entire hard drive for Qimo, select Manual and use the ’slider’ to determine how much space Qimo gets vs. any other operating system(s) you may already have installed. When you’re ready, click the Forward button.


click to enlarge

7. Now you’ll need to create a profile. Enter your name, a user name, password and name for the computer. NOTE: do NOT place a check in the box labeled Log in automatically. Qimo will automatically create a username named “qimo” which is the user that your child will log in with. This username automatically signs in each time Qimo starts – your child doesn’t need to enter a user or pass. The account created on this screen is for you to use to administrate the PC. Click the Forward button to continue.


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8. That’s it – you’re ready to install Qimo. Click the Install button after you’ve reviewed all the info on the summary page.


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9. Qimo will now install. This may be a good time to go grab a cup of coffee.



10. Once the installation has completed, click the Restart now button.



11. Qimo will now start up.


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12. Because Qimo automatically logs in with the ‘qimo’ account (the account your child should/will use) you’ll be taken directly to the desktop.


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13. The “Dock” at the bottom of the screen includes links to the apps your child will use most.



14. The first app in the Toolbar is Text Editor which is exactly as you’d guess – a text editor. Your kids can create brief notes or use it to write out homework.
15. The second app in the Toolbar is GCompris, a collection of general educational games that are appropriate for children ages 2 and up.


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16. Childsplay is a collection of educational games.


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17. TuxPaint is a very simple Paint program that allows your children to create colorful pictures, posters, cards and letters.


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18. Tux Math teaches mathematics through a series of fun games.


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19. Certain programs are off-limits to the ‘qimo’ user (your childs account). When/if they try to run those programs, they’ll get an error message similar to the one displayed in the screenshot below.



20. However, some programs are not off limits. For example, Firefox can be used by the qimo user. The easiest solution to avoid having your child use programs like Firefox, Transmission (bittorrent), IRC or Instant Messaging, is to keep the PC offline entirely.



21. To quickly update the system, open a Terminal and enter the command su your-user-name (where your-user-name was the one you selected back in step #7). Enter your password when prompted, and then issue the command sudo apt-get update


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22. Now enter the command sudo apt-get -f install


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23. If you’d like to remove programs from qimo entirely, enter the command sudo /usr/sbin/synaptic to launch the Synaptic Package Manager.


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24. From here you can easily search for and remove any programs that you simply don’t want on the PC.


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25. You can also administer Qimo by logging out of your current session, and logging back in with the username and password you created in step #7.


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Ubuntu Linux ships with the tool sudo all setup for you to use to perform any administrative tasks instead of giving you root user privileges. If you have used other flavors of Linux and performed any administrative tasks on them you will quite likely feel a bit constrained by sudo, even though you can do just about anything you can as root with sudo. In such cases you can enable the root user account which has been disabled by Ubuntu by default. Note: this could pose a security risk and is not recommended for beginners.

There are two levels at which you can enable root access – the command line and in the Gnome desktop. To enable the root user login on the command line run the following command:

# sudo passwd root

The command line will prompt you to enter your password, and then the new root user password twice. Enter them correctly and your root command line login should be good to go.

To enable the root user to now log into Gnome, open the file gdm.conf which will most likely be at /etc/X11/gdm/gdm.conf and change the AllowRoot condition to:

AllowRoot=true

Save the file. Log out of Gnome and log back in to activate your new settings.

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If you use Ubuntu Linux it is quite likely that at some point or another you may have been frustrated at its asking for your password when trying to perform an administrative tasks, such as installing programs or changing system settings. Ubuntu uses the sudo tool to manage administrative rights rather than letting users easily log in as the root user. This is actually a good security measure. However, if you need to perform administrative tasks frequently it can get rather annoying. Here’s how you can set Ubuntu to not prompt for your password when using sudo.

Note: this is something that is not recommended if you are using your computer in a public place or at your work, as this will compromise the security of your data. You’re better off setting this at your home desktop where you are likely to trust more people.

Launch a terminal window and enter the following command:

# sudo visudo

Visudo is a tool used to edit the configuration file of sudo . The command will prompt your for your password. Enter it one last time. The configuration file will open in a text editor, most likely Nano. Scroll down to the end of the document and add the following line:

username ALL=NOPASSWD: ALL

Replace username with the username of the user you want to allow a passwordless sudo. So if the user is calvin, you would enter:

calvin ALL=NOPASSWD: ALL

Save and exit the file. Log out, log in as the user calvin and test out your new passwordless sudo.

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