Transferring files

Overview

Teaching: 30 min
Exercises: 10 min

Questions

• How do I upload/download files to the cluster?

Objectives

• Be able to transfer files to and from a computing cluster.

Computing with a remote computer offers very limited use if we cannot get files to or from the cluster. There are several options for transferring data between computing resources, from command line options to GUI programs, which we will cover here.

Download files from the internet using wget

One of the most straightforward ways to download files is to use wget. Any file that can be downloaded in your web browser with an accessible link can be downloaded using wget. This is a quick way to download datasets or source code.

The syntax is: wget https://some/link/to/a/file.tar.gz. For example, download the lesson sample files using the following command:

[yourUsername@cirrus-login0 ~]$ wget https://archer-cse.github.io/2019-12-09-epcc-hpcintro/bash-lesson.tar.gz

Transferring single files and folders with scp

To copy a single file to or from the cluster, we can use scp. The syntax can be a little complex for new users, but we’ll break it down here:

To transfer to another computer:

[user@laptop ~]$ scp /path/to/local/file.txt yourUsername@remote.computer.address:/path/on/remote/computer

To download from another computer:

[user@laptop ~]$ scp yourUsername@remote.computer.address:/path/on/remote/computer/file.txt /path/to/local/

Note that we can simplify doing this by shortening our paths. On the remote computer, everything after the : is relative to our home directory. We can simply just add a : and leave it at that if we don’t care where the file goes.

[user@laptop ~]$ scp local-file.txt yourUsername@remote.computer.address:

To recursively copy a directory, we just add the -r (recursive) flag:

[user@laptop ~]$ scp -r some-local-folder/ yourUsername@remote.computer.address:target-directory/

Transferring files interactively with FileZilla (sftp)

FileZilla is a cross-platform client for downloading and uploading files to and from a remote computer. It is absolutely fool-proof and always works quite well. It uses the sftp protocol. You can read more about using the sftp protocol in the command line here.

Download and install the FileZilla client from https://filezilla-project.org. After installing and opening the program, you should end up with a window with a file browser of your local system on the left hand side of the screen. When you connect to the cluster, your cluster files will appear on the right hand side.

To connect to the cluster, we’ll just need to enter our credentials at the top of the screen:

• Host: sftp://login.cirrus.ac.uk
• User: Your cluster username
• Password: Your cluster password
• Port: (leave blank to use the default port)

Hit “Quickconnect” to connect! You should see your remote files appear on the right hand side of the screen. You can drag-and-drop files between the left (local) and right (remote) sides of the screen to transfer files.

Archiving files

One of the biggest challenges we often face when transferring data between remote HPC systems is that of large numbers of files. There is an overhead to transferring each individual file and when we are transferring large numbers of files these overheads combine to slow down our transfers to a large degree.

The solution to this problem is to archive multiple files into smaller numbers of larger files before we transfer the data to improve our transfer efficiency. Sometimes we will combine archiving with compression to reduce the amount of data we have to transfer and so speed up the transfer.

The most common archiving command you will use on (Linux) HPC cluster is tar. tar can be used to combine files into a single archive file and, optionally, compress. For example, to combine all files within a folder called output_data into an archive file called output_data.tar we would use:

tar -cvf output_data.tar output_data/

We also use the tar command to extract the files from the archive once we have transferred it:

tar -xvf output_data.tar

This will put the data into a directory called output_data. Be careful, it will overwrite data there if this directory already exists!

Sometimes you may also want to compress the archive to save space and speed up the transfer. However, you should be aware that for large amounts of data compressing and un-compressing can take longer than transferring the un-compressed data so you may not want to transfer. To create a compressed archive using tar we add the -z option and add the .gz extension to the file to indicate it is compressed, e.g.:

tar -czvf output_data.tar.gz output_data/

The tar command is used to extract the files from the archive in exactly the same way as for uncompressed data as tar recognizes it is compressed and un-compresses and extracts at the same time:

tar -xvf output_data.tar.gz

Transferring files

Using one of the above methods, try transferring files to and from the cluster. Which method do you like the best?

Working with Windows

When you transfer files to from a Windows system to a Unix system (Mac, Linux, BSD, Solaris, etc.) this can cause problems. Windows encodes its files slightly different than Unix, and adds an extra character to every line.

On a Unix system, every line in a file ends with a \n (newline). On Windows, every line in a file ends with a \r\n (carriage return + newline). This causes problems sometimes.

Though most modern programming languages and software handles this correctly, in some rare instances, you may run into an issue. The solution is to convert a file from Windows to Unix encoding with the dos2unix command.

You can identify if a file has Windows line endings with cat -A filename. A file with Windows line endings will have ^M$ at the end of every line. A file with Unix line endings will have $ at the end of a line.

To convert the file, just run dos2unix filename. (Conversely, to convert back to Windows format, you can run unix2dos filename.)

A note on ports

All file transfers using the above methods use encrypted communication over port 22. This is the same connection method used by SSH. In fact, all file transfers using these methods occur through an SSH connection. If you can connect via SSH over the normal port, you will be able to transfer files.

Key Points

• wget downloads a file from the internet.

• scp transfer files to and from your computer.

• You can use an SFTP client like FileZilla to transfer files through a GUI.