The other day I rsync’d one of my folders to back up the documents to an external harddrive. Later, I copied the files back (not using rsync — but cp).

The permissions on the files changed, setting the executable bit on all the files. Now, when I open a file in gedit I am asked if I want to display the contents or run the file. Obviously it is text file, but because the executable bit is set linux treats it differently.

It is easy enough to change a few files:

$ chmod 644 *

To recursively change all of the files, I would use this set of commands:

$ find . -type f | xargs chmod 644

This works great…unless there are filenames which contain spaces in their names. In this case, the output from find will break up the filename into several pieces (separated by the space) and send each one over to xargs.

To prevent we use use the -print0 flag on find and the -0 flag on xargs:

$ find . -type f -print0 | xargs -0 chmod 644

Try it!

So what happens?  Well, there is a library conflict which causes my desktop to revert to a low-graphics, single-monitor mode.  Yuck.

Fortunately, it is easy to correct.

Step 1. After booting up, open up a command prompt

Step 2.  Stop the gdm (x-windows) daemon.

$ sudo service gdm stop

Enter your root password and x-windows will stop and you revert to a terminal login.

Step 3.  Login.

Step 4.  You have to be root to install the driver so go ahead and change to root.

$ sudo su -

Enter your root password

Step 5.  Now, at some point you would have already downloaded the NVIDIA driver.  Hopefully you kept it laying around somewhere.  Change to the directory containing the driver. In my case, it is the Downloads directory.

# cd /home/scott/Downloads

My NVIDIA driver is named NVIDIA-Linux-x86-173.14.25-pkg1.run

Step 6. As root, install the driver

# ./NVIDIA driver is named NVIDIA-Linux-x86-173.14.25-pkg1.run

This starts the installer.  Follow the onscreen instructions.  The installer will detect that the driver is already installed.  Select ‘Yes’ to continue.

It will report an error (which is what caused the problem in the first place!)

ERROR: File ‘/usr/lib/xorg/modules/extensions/libglx.so’ is not a symbolic link.

Accept the error and continue.

You will then get a prompt asking you if you want to run nvidia-xconfig to automatically update your X configuration.  I select No.

Installation of the graphics driver is complete!

Step 7. Exit root

# exit

Step 8. Restart X-Windows

$ sudo service gdm start

Enter your root password

Congratulations!  Your NVIDIA graphics driver is (re) installed and your dual-head display is back the way it was.

Not only do I use SSH, but I often use SCP (secure copy) to copy files from one server to another. Both commands require a password.

There is a way, however, to create a unique key pair so that you can login to a remote server without having to enter a password.

SSH Without a Password

The steps are simple and straightforward.

1. On the client run the following commands:

$ mkdir -p $HOME/.ssh

$ chmod 0700 $HOME/.ssh

$ ssh-keygen -t dsa -f $HOME/.ssh/id_dsa -P ”

NOTE: Those are a pair of single quotes after the -P, which will set an empty password.
This will create two keys, a private key and a public key, the .ssh folder.

2. Copy $HOME/.ssh/id_dsa.pub to the remote server.

3. Log in to the remote server and run the following commands:

$ cat id_dsa.pub >> .ssh/authorized_keys

$ chmod 0600 .ssh/authorized_keys

That’s it! You can log out of the remote server then log in using SSH and will not be prompted for a password.

scott@blackjack:~$ ssh-keygen -t dsa -f $HOME/.ssh/id_dsa -P ”
Generating public/private dsa key pair.
Your identification has been saved in /home/scott/.ssh/id_dsa.
Your public key has been saved in /home/scott/.ssh/id_dsa.pub.
The key fingerprint is:
a5:3c:b6:e9:13:85:62:34:ee:6f:56:47:46:6f:dd:8b scott@blackjack
The key’s randomart image is:
+–[ DSA 1024]—-+
| |
| o . |
| o . … . ..|
| +..o. o o o|
| o .S. o .. .|
| …+. .E . |
| .oo . |
| .= |
| o.. |
+—————–+
scott@blackjack:~$ cd .ssh
scott@blackjack:~/.ssh$ ll
total 24
drwx—— 2 scott scott 4096 2010-08-17 09:01 ./
drwxr-xr-x 55 scott scott 4096 2010-08-17 08:58 ../
-rw——- 1 scott scott 668 2010-08-17 09:01 id_dsa
-rw-r–r– 1 scott scott 605 2010-08-17 09:01 id_dsa.pub
-rw-r–r– 1 scott scott 4714 2010-06-25 14:55 known_hosts
scott@blackjack:~/.ssh$ scp id_dsa.pub sstanger@midnight:/home/sstanger/
sstanger@midnight’s password:
id_dsa.pub 100% 605 0.6KB/s 00:00
scott@blackjack:~/.ssh$ ssh sstanger@midnight
sstanger@midnight’s password:
[sstanger@midnight ~]$ cat id_dsa.pub >> .ssh/authorized_keys
[sstanger@midnight ~]$ chmod 0600 .ssh/authorized_keys
[sstanger@midnight ~]$ exit
Connection to midnight closed.
scott@blackjack:~/.ssh$ cd
scott@blackjack:~$
scott@blackjack:~$ ssh sstanger@midnight
[sstanger@midnight ~]$

NOTE: On some systems the ssh policies may not allow this. If this is the case then you can make this change on the remote host:

[sstanger@midnight ~]$ mv .ssh/authorized_keys .ssh/authorized_keys2
[sstanger@midnight ~]$ chmod 700 .ssh
[sstanger@midnight ~]$ chmod 640 .ssh/authorized_keys2

Manually mounting a USB drive is achieved with a few simple steps in a terminal window:

First we need to know what the name of the file system is. With the device plugged in we use the fdisk command.

scott@blackjack:~$ sudo fdisk -l
[sudo] password for scott:

Disk /dev/sda: 400.1 GB, 400088457216 bytes
255 heads, 63 sectors/track, 48641 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0×6669894e

Device Boot Start End Blocks Id System
/dev/sda1 * 1 47888 384660328+ 83 Linux
/dev/sda2 47889 48641 6048472+ 5 Extended
/dev/sda5 47889 48641 6048441 82 Linux swap / Solaris
Note: sector size is 2048 (not 512)

Disk /dev/sdb: 1016 MB, 1016858624 bytes
16 heads, 32 sectors/track, 969 cylinders
Units = cylinders of 512 * 2048 = 1048576 bytes
Disk identifier: 0×00000000

Device Boot Start End Blocks Id System
/dev/sdb1 * 1 969 992182 6 FAT16
scott@blackjack:~$

The device is called sdb1, and it is this that we will mount. The sequence of commands to mount and then unmount are:

scott@blackjack:~$ cd /media
scott@blackjack:/media$ sudo mkdir thing
scott@blackjack:/media$ sudo mount /dev/sdb1 thing
scott@blackjack:/media$ sudo umount thing
scott@blackjack:/media$ sudo rmdir thing
scott@blackjack:/media$