Preventing Accidental Overwriting Of Files In Bash Shell

How many times has this happened to you? It used to happen once in a while with me. A Linux user learns to use the redirection operators such as '>' and '>>' but accidental overwriting starts to become common in commands you use and shell scripts you write.

The accidental overwriting of files that happens unintentionally is known as clobbering and it commonly happens while using the '>' redirection operator.

samar@Techgaun:~$ mycmd > myfile

In the above example, the mycmd clobbers any existing data in the myfile file if that file exists already. Worse things may happen sometime. Imagine accidentally typing

samar@Techgaun:~$ mycmd > /etc/passwd

instead of possibly using other redirection operators (like >> or <). Thankfully, you could recover /etc/passwd from either /etc/passwd- or /var/backups/passwd.bak if you hadn't rm'd these files.

To prevent such accidental overwriting, we can set the noclobber environment variable. Below is a session of enabling this variable:

samar@Techgaun:~/Desktop/test$ echo "www.techgaun.com" > myfile
samar@Techgaun:~/Desktop/test$ echo "Overwriting techgaun.com" > myfile
samar@Techgaun:~/Desktop/test$ set -o noclobber
samar@Techgaun:~/Desktop/test$ echo "Retrying to overwrite" > myfile
-bash: myfile: cannot overwrite existing file

As seen above, you have to turn on the noclobber variable using the set -o noclobber command in your shell. However, you might want to intentionally overwrite contents of certain files even when the noclobber is turned on.

samar@Techgaun:~$ mycmd >| myfile

Notice the >| in place of your normal > redirection operator. Using this operator, you can however overwrite the existing files even if the noclobber is turned on.

If you want to turn off the noclobber variable, type the following:

samar@Techgaun:~$ set +o noclobber

You can also permanently turn on the noclobber by the following command:

samar@Techgaun:~$ echo "set -o noclobber" >> ~/.bashrc

Moreover, such accidental overwriting can be prevented by enabling the interactive mode which is available in most of the linux commands. For example, you can write the alias for many commands that are likely to cause accidental overwriting. See some examples of aliases below:

samar@Techgaun:~$ alias rm=rm -i
samar@Techgaun:~$ alias mv=mv -i

You could even keep these aliases in your ~/.bashrc file permanently. Enabling such interactive modes by default in the commands that are more likely to cause accidental overwriting can prevent clobbering in many cases.

I hope this proves useful to you :)


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Install XAMPP 1.8 From PPA In Ubuntu

Since apache friends has released the v. 1.8 of XAMPP for linux and windows, its important you guys upgrade your XAMPP. In this post, you will find the instructions to install XAMPP 1.8 from PPA.

The most important updates of v. 1.8.0 of XAMPP are: Apache 2.4.2, MySQL 5.5.25a, PHP 5.4.4, and phpMyAdmin 3.5.1. Since the software components are updated, I strongly recommend to upgrade your XAMPP.
All you have to do is follow the following steps in order:

sudo add-apt-repository ppa:upubuntu-com/xampp
sudo apt-get update
sudo apt-get install xampp

Alternatively, you can download the tar file for XAMPP from Apache Friends and follow their instructions to install XAMPP 1.8.0. In case you're looking for upgrading your previous XAMPP installation, be sure to follow this How To.

I hope this helps :)


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How To Manually Install Flash Player 11 In Linux

This post will provide a step by step instructions for installing flash player 11 plugin in ubuntu 11.04 and other different versions and distros. This will be helpful for everybody who are having trouble with the software center like I had.


Make sure no firefox process is running and then fire up the terminal and type the following commands in order:

mkdir -p ~/flash && cd ~/flash

wget http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_11.2.202.238.orig.tar.gz

tar -zxvf adobe-flashplugin_11.2.202.238.orig.tar.gz

sudo cp -r libflashplayer.so /usr/lib/firefox/plugins

sudo cp -r usr/* /usr

Once you have finished copying the shared object and other necessary files in their respective target directories, you can open the firefox and you're good to go. :)


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Screen Recording Software Solutions For Linux

Windows users have several options to choose from when it comes to the desktop recording (and only paid ones are good generally) but Linux users have fewer options but robust, simple, and best of all, free and open source desktop screen recording tools that we can trust on.

Below are some of the screen recording tools you might want to try:

recordMyDesktop

recordMyDesktop is a desktop session recorder for GNU/Linux written in C. recordMyDesktop itself is a command-line tool and few GUI frontends are also available for this tool. There are two frontends, written in python with pyGtk (gtk-recordMyDesktop) and pyQt4 (qt-recordMyDesktop). recordMyDesktop offers also the ability to record audio through ALSA, OSS or the JACK audio server. Also, recordMyDesktop produces files using only open formats. These are theora for video and vorbis for audio, using the ogg container.

Installation under debian and ubuntu:

sudo apt-get install gtk-recordmydesktop

XVidCap

XVidCap is a small tool to capture things going on on an X-Windows display to either individual frames or an MPEG video. It enables you to capture videos off your X-Window desktop for illustration or documentation purposes.It is intended to be a standards-based alternative to tools like Lotus ScreenCam.

sudo apt-get install xvidcap

Istanbul

Istanbul is a desktop session recorder for the Free Desktop. It records your session into an Ogg Theora video file. To start the recording, you click on its icon in the notification area. To stop you click its icon again. It works on GNOME, KDE, XFCE and others. It was named so as a tribute to Liverpool's 5th European Cup triumph in Istanbul on May 25th 2005.

sudo apt-get install istanbul

Vnc2Flv

Vnc2flv is a cross-platform screen recording tool for UNIX, Windows or Mac. It captures a VNC desktop session (either your own screen or a remote computer) and saves as a Flash Video (FLV) file.

Wink

Wink is a Tutorial and Presentation creation software, primarily aimed at creating tutorials on how to use software (like a tutor for MS-Word/Excel etc). Using Wink you can capture screenshots, add explanations boxes, buttons, titles etc and generate a highly effective tutorial for your users. It requires GTK 2.4 or higher and unfortunately is just a freeware(could not find any source code for it).

Screenkast

Screenkast is a screen capturing program that records your screen-activities, supports commentboxes and exports to all video formats.

If you got any more suggestions, please drop the comment. :)


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Why Alias Command With Itself

Aliasing the command to itself to suppress the original functionality of the command and provide it new added sets of functionality can come quite handy for linux users and administrators.

If you have been using linux shell for a while, I'm pretty sure you are now familiar with the `ls` command, if not I think you have just learnt to use man pages. Probably you've been using `ls -l` command to list files with the files size as well. Too bad, you won't just be able to instantly make the sense of the file size displayed using this command so why not alias `ls` command to always provide human readable file sizes. So here is my alias:

alias ls='ls -lh'

This is what I always want to see as the output with `ls` command. The same kind of alias can be used with `du` and `df` commands. There are number of other cases where aliasing a command with itself is good choice.

Another example is the less command. By default, you need to press q to exit less which can be quite annoying if the entire content can fit in a single screen. However, adding -F flag will gracefully quit after displaying the content if the content fits in a single screen. So I have my alias for less as below:

alias lesss='less -F'

If something shoots in your mind, feel free to share here as a comment :)


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Stack-based Directory Switching For Easy Reversal

So how many times have you used the `cd` command repeatedly to go back and forth of two or more directories. Probably you are already familiar to the `cd -` command which lets you switch between the current and the previous directory. But, many times this current and previous directory switching restriction will not suffice and hence a better option in such case is to use the `pushd` command instead of `cd`.

For example, just use the `pushd somedirA`, `pushd somedirB`, ... and like that. Now if you need to switch back, you can just use `popd` command and you'll be switching back easily. The `pushd` command saves the current directory path and then cds to the supplied path.

If you dig more, you'll come to know about the -n and -N switches you can combine with these commands so I will let you explore on this. Also, you can use the `dirs` command to view the stack of directories. If you are some computer student or enthusiast, you have already gotten an idea from a famous data structure called stack. Anyway, I hope this comes handy sometimes like it does to me :)


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Fix "trying to overwrite '*', which is also in package *"

Today I was updating few stuffs in edubuntu and dpkg was continually throwing me the problem while trying to install kdelibs-data. The error read as "trying to overwrite 'A', which is also in package X" and the fix was pretty straightforward but still I thought it would help someone out there.

Below is the exact error I was getting while trying to install kdelibs5-data from the deb file.

dpkg: error processing /var/cache/apt/archives/
kdelibs5-data_4%3a4.4.5-0ubuntu1.2_all.deb (--unpack):
trying to overwrite '/usr/share/polkit-1/actions
/org.kde.kcontrol.kcmremotewidgets.policy', which is also in package kdebase-runtime-data 4:4.6.5-0ubuntu1

The fix was pretty simple. Add the --force-overwrite switch in the dpkg command as below:

dpkg -i --force-overwrite kdelibs5-data_4.4.5-0ubuntu1.2_all.deb

I hope this comes useful sometimes.


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Recover Deleted Files From An NTFS Volume Using Ntfsundelete

Ntfsundelete is a part of ntfsprogs, a suite of NTFS utilities based around a shared library. It lets us recover the deleted files from any NTFS volumes without making any changes in the NTFS volume itself.

Generally when a file is deleted from disks, it is some kind of pointer to the physical file that gets deleted and the actual content still remains in the disk unless it is overwritten by new files so it is possible to recover those files.

ntfsundelete has three modes of operation: scan, undelete and copy. By default, it will run in the scan mode which simply reads an NTFS volume and looks for the files that have been deleted.



To use ntfsundelete, you'll have to install the ntfsprogs suite with following command in ubuntu and debian-based distros:

samar@Techgaun:~$ sudo apt-get install ntfsprogs

You'll have to first figure out which drive you want to recover. A handy command for this is:

samar@Techgaun:~$ sudo fdisk -l

Once you know the NTFS volume you want to recover, you can first run the scan mode to list the filenames that can be recovered.

samar@Techgaun:~$ sudo ntfsundelete /dev/sda4

The optional -f switch can be specified for the forceful scanning. There is a nice percentage field which gives the information on how much of the file can be recovered. You can apply the time and percentage filters to scan specific files. For example, you can use the following command to search for the files which can be recovered 100%

samar@Techgaun:~$ sudo ntfsundelete -p 100 /dev/sda4

And, you can apply the time filter to list the files altered/deleted after the specified time. For example, following command will scan and list the files deleted in the last 14 days.

samar@Techgaun:~$ sudo ntfsundelete -p 100 -t 2d /dev/sda4

Other suffices you can use are d, w, m, y for days, weeks, months or years ago respectively.

Once you get the files to be recovered, you can use the -u switch to undelete or recover the files. An example of recovering files by pattern matching is as below:

samar@Techgaun:~$ sudo ntfsundelete -u -m *.jpg /dev/sda4

Similarly you can recover by providing inode or inodes range using the -i switch. You can get the inode values from the first column in the scan mode.

samar@Techgaun:~$ sudo ntfsundelete -u -i 161922 /dev/sda4

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