How to find out what process is using a file system

Hi folks,

In this article I will discuss how to fix a problem with unmounting file systems in Ubuntu. Suppose you want to unmount a file system /some/endpoint for example in Ubuntu but run into an error like

umount: /some/endpoint: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)
Mount is denied because the NTFS volume is already exclusively opened.
The volume may be already mounted, or another software may use it which
could be identified for example by the help of the 'fuser' command.

An easy way to find out what exactly is using that endpoint is to run the following command:

lsof | grep /some/endpoint

Once you see what process is using the filesystem, you can then go ahead to kill or end it.

Quick Way to Recover Offscreen Windows in Ubuntu

On my laptop running Ubuntu 14.04, I usually attach a secondary monitor to it while working. I notice that on disconnecting the secondary monitor, some windows tend to go offscreen making it impossible to use them. Pressing the usual Alt + Tab plus clicking the window doesn’t bring it back. So here is a hack for bring back all offscreen windows which I found on Stack Overflow.

Go to Appearance -> Behaviour, then check or uncheck Enable workspaces.

Reference

linux – Window appears off screen on ubuntu – Stack Overflow. http://stackoverflow.com/questions/12477435/window-appears-off-screen-on-ubuntu. 19/12/2016.

How to Reset Network Adapter in Ubuntu

Here’s a quick tip on how to reset your network adapter. This could come in handy if a server has an incorrect ip address and requires a reset to obtain the right ip address.

Simply run the following command:

sudo service network-manager restart

Reference

How do I reset the network adapter using a terminal command?. askubuntu. http://askubuntu.com/questions/54710/how-do-i-reset-the-network-adapter-using-a-terminal-command [04/12/2016].

How to Set Up Login via SSH Keys on Linux Servers

Here’s a tutorial that talks about setting up login via SSH keys on Ubuntu servers as well as how to log in to such servers from Mac and Windows clients. This tutorial builds on DigitalOcean’s excellent tutorial (How To Set Up SSH Keys) [1].

About SSH Keys

SSH keys provide a more secure way of logging into a virtual private server with SSH than using a password alone. While a password can eventually be cracked with a brute force attack, SSH keys are nearly impossible to decipher by brute force alone. Generating a key pair provides you with two long string of characters: a public and a private key. You can place the public key on any server, and then unlock it by connecting to it with a client that already has the private key. When the two match up, the system unlocks without the need for a password. You can increase security even more by protecting the private key with a passphrase.

Step One—Create the RSA Key Pair
The first step is to create the key pair on the client machine (there is a good chance that this will just be your computer):

ssh-keygen -t rsa

Step Two—Store the Keys and Passphrase
Once you have entered the Gen Key command, you will get a few more questions:

Enter file in which to save the key (/home/demo/.ssh/id_rsa):
You can press enter here, saving the file to the user home (in this case, my example user is called demo).

Enter passphrase (empty for no passphrase):
It’s up to you whether you want to use a passphrase. Entering a passphrase does have its benefits: the security of a key, no matter how encrypted, still depends on the fact that it is not visible to anyone else. Should a passphrase-protected private key fall into an unauthorized users possession, they will be unable to log in to its associated accounts until they figure out the passphrase, buying the hacked user some extra time. The only downside, of course, to having a passphrase, is then having to type it in each time you use the Key Pair.

The entire key generation process looks like this:

ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/demo/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/demo/.ssh/id_rsa.
Your public key has been saved in /home/demo/.ssh/id_rsa.pub.
The key fingerprint is:
4a:dd:0a:c6:35:4e:3f:ed:27:38:8c:74:44:4d:93:67 demo@a
The key's randomart image is:
+--[ RSA 2048]----+
|          .oo.   |
|         .  o.E  |
|        + .  o   |
|     . = = .     |
|      = S = .    |
|     o + = +     |
|      . o + o .  |
|           . o   |
|                 |
+-----------------+

The public key is now located in /home/demo/.ssh/id_rsa.pub The private key (identification) is now located in /home/demo/.ssh/id_rsa

Step Three—Copy the Public Key
Once the key pair is generated, it’s time to place the public key on the virtual server that we want to use.

You can copy the public key into the new machine’s authorized_keys file with the ssh-copy-id command. Make sure to replace the example username and IP address below.

ssh-copy-id user@123.45.56.78

Alternatively, you can paste in the keys using SSH:

cat ~/.ssh/id_rsa.pub | ssh user@123.45.56.78 “mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys”
No matter which command you chose, you should see something like:

The authenticity of host '12.34.56.78 (12.34.56.78)' can't be established.
RSA key fingerprint is b1:2d:33:67:ce:35:4d:5f:f3:a8:cd:c0:c4:48:86:12.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '12.34.56.78' (RSA) to the list of known hosts.
user@12.34.56.78's password: 
Now try logging into the machine, with "ssh 'user@12.34.56.78'", and check in:

  ~/.ssh/authorized_keys
to make sure we haven't added extra keys that you weren't expecting.

Note: if you create your private key in a non-standard location, you will want to copy it to the remote server using the ssh-copy-id command with the i flag as follows:

ssh-copy-id -i ~/.ssh/your-id-rsa-file user@123.45.67.222

Now you can go ahead and log into user@12.34.56.78 and you will not be prompted for a password. However, if you set a passphrase, you will be asked to enter the passphrase at that time (and whenever else you log in in the future).

Logging in from Various Clients

Mac

Note: if you are logging in from a Mac client, you may want to store your private key credentials in the keychain so that you don’t get asked for a password henceforth. To do this, run the following command:

ssh-add -K ~/.ssh/you-id-rsa-key

Then entire your passphrase when prompted (if you set one earlier).

Windows

If you are logging from a Windows machine, you may want to use a client like Putty. If you try to use your OpenSSH key directly with Putty, you may run into an error like “Unable to Use key file /Path/to/your/id-rsa-file (OpenSSH SSH-2 private key)”. To fix this, you will need to convert your private key to one that Putty can use. Puttygen is a software you can use for this. Download Puttygen, run the program, go to Conversions > Import key, find your private key and click Open, then click Save private key. This will generate a .ppk file which you can use with Putty. You may find more details in CNX Soft’s excellent tutorial [2].

Optional Step Four—Disable the Password for Root Login
Once you have copied your SSH keys unto your server and ensured that you can log in with the SSH keys alone, you can go ahead and restrict the root login to only be permitted via SSH keys.

In order to do this, open up the SSH config file:

sudo vim /etc/ssh/sshd_config

Within that file, find the line that includes PermitRootLogin and modify it to ensure that users can only connect with their SSH key:

PermitRootLogin without-password

Put the changes into effect:

reload ssh

References

1. How To Set Up SSH Keys. https://www.digitalocean.com/community/tutorials/how-to-set-up-ssh-keys–2. [04/12/2016].

2. How To Use Putty with an SSH Private Key Generated by OpenSSH. http://www.cnx-software.com/2012/07/20/how-use-putty-with-an-ssh-private-key-generated-by-openssh/. [04/12/2016].

How to Scan Images Remotely

Hi folks,

Today I’ll briefly discuss a nice and easy way to scan images remotely. Suppose you have your multifunction printer or scanner hooked up to an Ubuntu server via a USB cable and you need to quickly scan a document while using a remote machine. How can you go about it? It’s easy. Just use the scanimage command over SSH. There’s an API called SANE (Scanner Access Made Easy) that provides a standard way to access raster image scanners [1]. SANE supports Windows, Linux, UNIX and OS/2. scanimage belongs to the SANE package and is available on Ubuntu by default.

First, log in to the server via SSH.

Next, turn on your printer/scanner and insert the document you wish to scan.

Next, run the following command:

scanimage --format=tiff ><your-file>.tiff

This should fire up your printer/scanner, scan the image and save the result in tiff format. From there you can transfer the output file to your machine for use.

Note that there are alternative ways to achieve this, e.g. by setting up a SANE daemon (saned) on your scanner server and then setting up a SANE client for Ubuntu or using TWAIN for Windows and Mac OS, thus allowing you to use compatible scanner software on the client [2].

Sources

  1. Scanner Access Now Easy – Wikipedia, the free encyclopedia. https://en.wikipedia.org/wiki/Scanner_Access_Now_Easy
  2. SaneDaemonTutorial – Community Help Wiki. https://help.ubuntu.com/community/SaneDaemonTutorial

How to Mount External Hard Drives on Boot on Ubuntu

Sometimes you want your computer to load external hard drives automatically on boot, e.g. if it gets rebooted accidentally, recovers from power failure, etc. Here’s how to do that on Ubuntu 14.04.

First, create a desired directory for your mount point using a command like:

 mkdir /your/mount/point

Then, while the external hard drive is connected and the computer is on, find the drive’s UUID by running the command

blkid

This will list all block devices, including your external hard drive. Note the UUID and the type of the drive.

Then, open /etc/fstab and add a line like

UUID=<your uuid> /your/mount/point  <your mount point> users,defaults 0 0

Save and close the file, then test it by rebooting and perhaps logging in from another machine via SSH. Your drive should mount automatically!

How to cleanly remove Exim 4 on Ubuntu

Run the following command:

apt-get remove exim4 exim4-base exim4-config exim4-daemon-light

All your configs stay on the server so that you can easily get mailing running again by installing it:

apt-get install exim4

Source

exim – How to cleanly remove Exim4 mail server on Ubuntu – Stack Overflow. http://stackoverflow.com/questions/12061358/how-to-cleanly-remove-exim4-mail-server-on-ubuntu

How to fix error of missing zip decoder in Django with Ubuntu 14.04

If installing Django on Ubuntu 14.04 and you run into an error like “IOError: decoder zip not available” solve it as follows:

Install libzip and create a symbolic link in /usr/lib using the commands:

sudo apt-get install zlib1g-dev
sudo ln -s /usr/lib/x86_64-linux-gnu/libz.so /usr/lib/libz.so

Next, reinstall PIL using the commands while in bosg virtual environment:

pip uninstall PIL
pip install  --no-cache-dir PIL --allow-external \
PIL --allow-unverified PIL

Finally, refresh project if on producion environment.

How to search for packages in Ubuntu

Sometimes while working in Ubuntu you may want to find a package, but not know its name. There’s a command for that. It’s called apt-cache.

apt-cache search <your package>

Source

apt – How do I search for available packages from the command-line? – Ask Ubuntu. http://askubuntu.com/questions/160897/how-do-i-search-for-available-packages-from-the-command-line