How To …
Network Security
by Mostafa on Aug.09, 2007, under How To ..., Linux, Software
A list of very interesting articles:
http://backtrack.offensive-security.com/index.php?title=ExternalHowto
Joining PDF Documents
by Mostafa on Aug.03, 2007, under How To ..., Linux, Software
A quick search on the web reveals that the simplest (and most available) command to do so is:
$ gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=finished.pdf file1.pdf file2.pdf
Source: Putting together PDF files by Scott Nesbitt on NewsForge
[Update: Feb 1, 2011] jpdftweak is probably a better option with many useful features.
Named Pipes (FIFOs)
by Mostafa on May.22, 2007, under How To ..., Linux
A named pipe is a special kind of file on *nix systems that can be used for inter-process communication. They behave like FIFOs and are created using the command “mkfifo“:
$ mkfifo mypipe
$ ls -l mypipe
prw-rw-r-- 1 xxx xxx 0 May 22 10:18 mypipe
The “p” in the attributes list indicates that this is indeed a pipe.
A trivial example of its use may be to redirect the output of a command on a remote server to a pipe and then reading from that pipe from another host via ssh.
Software Keyboard and Mouse (KM) Switcher
by Mostafa on May.15, 2007, under How To ..., Linux, Software
Stumbled across this great tool a few days ago:
Description from the project homepage:
Synergy lets you easily share a single mouse and keyboard between multiple computers with different operating systems, each with its own display, without special hardware. It’s intended for users with multiple computers on their desk since each system uses its own monitor(s).
Redirecting the mouse and keyboard is as simple as moving the mouse off the edge of your screen. Synergy also merges the clipboards of all the systems into one, allowing cut-and-paste between systems. Furthermore, it synchronizes screen savers so they all start and stop together and, if screen locking is enabled, only one screen requires a password to unlock them all.
Mass conversion of images
by Mostafa on May.07, 2007, under How To ..., LaTeX, Linux, Software
The following “one-liner” can be used to mass convert a given image format into another using the convert (part of ImageMagick) and basename tools:
$ for A in $(ls *.$SRC_TYPE); do convert $A $(basename $A .$SRC_TYPE).$DST_TYPE; done
where $SRC_TYPE is the file suffix of the original images (e.g. png) and $DST_TYPE is the file suffix of the type desired (e.g. eps).
Linux and DVD Regions
by Mostafa on Apr.12, 2007, under How To ..., Linux, Software
Typically, Linux DVD playback software are capable of decrypting (libdvdcss must be installed) and playing back DVDs from any region, irrespective of the region code of the drive. Therefore, there should be no need to change the region code of the drive to watch discs from a different region. Regardless, there exists a very handy program that allows the user to change the region code and view other relevant information such as the number of changes remaining. It is called “regionset” and is available for Fedora from the Extras repository. The project website is:
http://linvdr.org/projects/regionset/
There is also a useful article on Linux.com about DVD playback:
Gnome automount options
by Mostafa on Mar.15, 2007, under How To ..., Linux, Software
On a default Fedora installation, Gnome will automatically mount (with some default options) any recognized volumes as soon as an external storage device is plugged in. While the defaults might be fine in general, sometimes it is necessary to supply some more options.
The tool responsible for automating the mount (gnome-volume-manager) gets additional options from the /system/storage/default_options/$fstype$/mount_options key of gconf — the Gnome “system registry”. One can check/modify existing entries (one a per user basis) by running:
$ gconf-editor /system/storage/default_options &
This should launch the graphical configuration editor with the default_options key selected. Under this key are the entries for different filesystems. Default mount options can then be changed by selecting the desired filesystem and editing the mount_options key.
If a required filesystem type is not listed, then it can be added by using the gconftool-2 utility. E.g. if we wanted to add the ext3 filesystem to the configuration database with the options “sync” and “uid=“, we would run the command:
$ gconftool-2 -t list –list-type string \
-s /system/storage/default_options/ext3/mount_options “[sync,uid=]”
Next time an external volume is plugged in, it will be mounted with the additional options specified! It should be noted that regardless of the options supplied via gconf, some mount options are always present: r(o|w),noexec,nosuid,nodev. At the moment I do not know how to change them.
The main motivation for finding out about the defaults is that I wanted to add “sync” as a default option. This causes data to be written immediately to the device, instead of being buffered first — a useful option to have for external devices. It should minimized data loss in case of an accidental removal (without running umount first). However, it should also be noted that for solid state drives (e.g. flash), this may result in a shortening of service life and poorer performance.
Update [Sun Mar 18, 2007]:
I’ve done some rudimentary throughput performance testing with both the sync and async modes and the performance hit with sync appears to be quite severe (at least 20 times slower than async) — even with a high performance HDD as the target. In light of this, I am removing sync from the list of default options. Given the type of data I’m likely to store on the device, speed is certainly more valuable than data integrity.
Test results:
Testing was carried out on a FAT32 volume. For each mode, both the transfer time and the subsequent un-mount time (indicating the time needed to flush the buffer) are shown.
async:
$ time dd if=/dev/zero of=/tmp/usb/dummy bs=8k count=130000 && time sudo umount /tmp/usb/ 130000+0 records in 130000+0 records out 1064960000 bytes (1.1 GB) copied, 45.9035 seconds, 23.2 MB/s real 0m46.026s user 0m0.035s sys 0m2.915s real 0m11.680s user 0m0.003s sys 0m0.263s
sync:
$ time dd if=/dev/zero of=/tmp/usb/dummy bs=8k count=130000 && time sudo umount /tmp/usb/ 130000+0 records in 130000+0 records out 1064960000 bytes (1.1 GB) copied, 969.384 seconds, 1.1 MB/s real 16m13.525s user 0m0.048s sys 0m12.156s real 0m0.842s user 0m0.004s sys 0m0.199s
MATLAB – Interrupting function execution
by Mostafa on Mar.02, 2007, under How To ..., Software
MATLAB itself does not provide any way of arbitrarily interrupting/pausing a function at mid-execution to examine the internal function stack/variables. It is a pity since such a mechanism would be an extremely valuable debugging tool.
Finally, I realized that this is not as impossible as it seems. In fact, it is EXTREMELY simple! All that is required are four lines of code in any script/function:
[statcode, result] = system('ls sometoken'); if statcode == 0 keyboard end
The system command is used to run system console commands. Here, a simple check is made for the existence of the file ‘sometoken’ in the current directory. If the file exists, statcode is set to zero and the if condition is satisfied. The keyboard command is then executed which causes MATLAB to pause execution and return control to the command line! In order to continue execution, ‘sometoken’ must be deleted/removed from the current directory and the command return must be issued.
The best place for this code is perhaps in any loop that might exist in the MATLAB script/function. Then, to cause execution to pause, one simply needs to create a file ‘sometoken’ in the current directory.
Linux NFSv4 Howto
by Mostafa on Feb.27, 2007, under How To ..., Linux, Software
NFS is commonly used to share files on Linux. NFSv4 is the latest protocol and circumvents firewall related complications experienced with NFSv3 by requiring only ONE fixed tcp port open on the server side. It is surprisingly easy to set up:
Server side:
- Edit /etc/exports and add directories to be exported (fsid=0 is a mandatory option for nfs4) and authorized clients (check the exports manpage)
- Open up tcp port 2049 on the firewall
- # /etc/init.d/nfs restart
- # chkconfig –level 345 nfs on
Client side:
- # mount -t nfs4 -o rw,intr,hard server:/ /mount/point
It is not necessary to specify the exact path on the “server:/” with NFSv4.
Useful sites:
Learning NFSv4 with Fedora Core 2
RHEL4 NFS manual
Linux NFS-HOWTO
Linux – Disable Shutdown For Normal Users
by Mostafa on Feb.27, 2007, under How To ..., Linux
Very useful for servers/shared machines:
Disable Shutdown For Normal Users
Addendum:
Setting file mode for /etc/acpi/events/power.conf to “0000” is not sufficient to disable the power button. It’s better to:
- Uninstall gnome-power-manager
- Leave file permissions for power.conf unchanged and simply set the action= line to an empty string
It is also advisable to set the local login screen style to plain in gdmsetup.