Linux

Basic Linux Terminal Commands

To find out more about any linux command, type:

	> man <command>

• cat - Catenate; display text file in terminal
• cp - Copy
• ls - List the contents of the current directory
• less - Display text file in terminal, with scrolling
• mkdir - Make directory
• mv - Move file
• rm - Remove (delete) file

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Remote Login (SSH)

At the bash prompt type:

	> ssh <username>@<hostname>

and then you will be prompted for a password. For example, if I have the username "bob" on a machine called "computer", which is connected to the network (domain) "site.co.uk", I would type:

	> ssh bob@computer.site.co.uk
	Password:

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Remote Login (NX)

Assuming the machine has already been set up with an nxserver, you can simply connect remotely with nxclient (available from NoMachine).

If you wish to set up your own server, NoMachine offer a free edition of their nxserver, however it has a maximum user limit of 2 (in total, not just concurrent connections). Therefore you may wish to use the FreeNX server, which once installed, should be setup using:

	> nxsetup --install --setup-nomachine-key --clean 

If you have difficulty connecting to an NX session ("Info: Aborting the procedure due to signal '15'") and you are using NoMachine's nxclient, please try the following:

• Click "Configure..."
• Select the "Advanced tab"
• In the "Network" frame, UNCHECK "Disable encryption of all traffic"

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Remote File Copying (SCP)

Use SCP to copy files to and from remote computers. At the bash prompt type:

	> scp [-r] [<username_1>@<hostname_1>:]<directory/[filename]> \\
		[<username_2>@<hostname_2>:]<directory/>

and then you will be prompted for a password. For example, if I want to copy pic.jpg from my current directory, to a directory called "pictures" in my home on a remote machine, "computer.site.co.uk", I would type:

	> scp pic.jpg bob@computer.site.co.uk:/home/bob/pictures/
	Password:

Alternatively, if I want to copy the entire pictures directory from my home on the current machine to the same place on a remote server, I would type:

	> scp -r /home/bob/pictures bob@computer.site.co.uk:~/
	Password:

Note the tilde (~) can be used to replace the path to your home directory.

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Mounting an ISO

First create the mount point, then mount the iso as root:

	> su
	Password:
	> mkdir /mnt/iso
	> mount my.iso /mnt/iso -t iso9660 -o loop

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Mounting a floppy disk

	> su
	Password:
	> mount /dev/fd0 /media/floppy

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Read raw mouse data

Student projects often seem to use standard input devices to perform weird and wonderful tasks. However, if you are new to linux, trawling through the kernel sources to find out how to interpret data from device files can be quite daunting. Therefore, I have provided some sample code that you may use as a starting point.

Here is a tarball of the (simple) source code and Makefile: mouse_read.tgz. If you don't know how to use tar, I suggest you look it up, because it's very useful. However, all you need to do to decompress this archive is to type the following in a terminal:

	> tar -zxvf mouse_read.tgz

Then, to run:

	> make
	> ./mouse_read

If you use additional mice, but only want one of them to actually move the mouse pointer on the screen, you will have to edit /etc/X11/xorg.conf. In 'Section "Files"', change "/dev/input/mice" to "/dev/input/mouse0". In 'Section "InputDevice"', again change "/dev/input/mice" to "/dev/input/mouse0".

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Change default device permissions

The proper way to sort out device permissions is to edit the appropriate file in /etc/udev/rules.d/ . For example, if you want any user to have access to the firewire ports (useful for the firewire cameras we use in the lab), you need to edit the file 50-udev-default.rules and include the MODE directive.

	# IEEE1394 devices
	KERNEL=="raw1394*",	GROUP="video", MODE="0666"
	KERNEL=="dv1394*",	SYMLINK+="dv1394/%n", GROUP="video"
	KERNEL=="video1394*",	SYMLINK+="video1394/%n", GROUP="video", MODE="0666"
		

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Allow users to run scripts requiring root permissions (sudoers)

If you add the script to the sudoers file, then any user can run the script without needing to know the root password. As root, use visudo to edit the sudoers file. Insert the following lines:

	# Cmnd alias specification
	Cmnd_Alias SETDEVPERMS = /usr/local/bin/setDevPermissions

	# ...

	# User privilege specification
	%users ALL = NOPASSWD: SETDEVPERMS

It is important to store the script that performs the operation somewhere where only root has write access (e.g. /usr/local/bin/setDevPermissions ), to maintain security. (If you don't do this, anyone could edit the script to perform any operation as root!). The script can then be run by ANY user, without requiring the root password, as follows:

	> sudo setDevPermissions

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Disable autorefresh for all Zypper repositories

Rather annoyingly, Zypper version 0.8.23 (SuSE 10.3) does not allow you to disable the auto-refresh of ALL the existing repositories in one go. You can disable them one at a time using:

	> sudo zypper mr --disable-autorefresh <repository>

...but this gets tedious, so I wrote a simple bash script zypper-no-refresh.sh to save a lot of typing/clicking. Make sure you rename it, set the correct permissions and run as root.

	> mv zypper-no-refresh.sh.txt zypper-no-refresh.sh
	> chmod +x zypper-no-refresh.sh
	> sudo ./zypper-no-refresh.sh

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Encoding video

Mencoder is really useful for encoding/transcoding videos, but I'm often in a hurry, when I use it, so here are a few common configurations.

Re-encoding an AVI captured from DV into MS-MPEG4. You might wonder why I use the MS version, rather than the standard (especially being an avid Linux user); this is simply because it will play on the widest range of basic systems.

	> mencoder input.avi -ovc lavc -lavcopts vcodec=msmpeg4v2 -nosound -o output.mpg

Here's a simple bash script to create an AVI from a sequence of PPM images makePPMvid.sh. Make sure you rename it and set the correct permissions. Run it in the same folder as the sequence of PPMs and it will create a video called output.avi.

	> mv makePPMvid.sh.txt makePPMvid.sh
	> chmod +x makePPMvid.sh
	> ./makePPMvid.sh

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Configure Postfix/Sendmail for use with PHP

In order to use a specific relay host, make sure there is only one relayhost entry in /etc/postfix/main.cf of the form:

	relayhost = [mxhost.domain]

Additionally your relay host may check that mail is sent from a specific user on your machine (e.g. root). To enable other users, (e.g. wwwrun) to "send as root", you can include the -f argument in the sendmail_path for /etc/php5/apache2/php.ini:

	sendmail_path = /usr/sbin/sendmail -froot -t -i

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Merge PDF files using PDFTK

PDFTK is a great linux-based utility for editing PDF files from the console. I find it useful when writing scripts for processing a batch of files. A particularly useful feature is the abillity to merge two or more PDF files:

	> pdftk <first>.pdf <second>.pdf cat output <out>.pdf

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