File Transfer


To interactively copy a few files between machines the following commands are often sufficient.

ftp and sftp

Useful Contents

  • ftp – ARPANET file transfer program
  • sftp – Secure file transfer program

scp and wget

Useful Contents

  • scp – secure copy
  • wget– GNU Wget Manual

However, if you need to transfer a substantial amount of data between machines, the weakness of these commands (sftp, scp, and wget) become apparent.

SSH Tar pipe

For one-time bulk transfer of data, the best solution is to use a ssh tar pipe. Basically the idea is to pipe together two tar commands with a ssh pipe. One of the tar commands streams the files to be tranferred to stdout and the other tar command unpacks the files from stdin.

Here are the two main ways I use this technique to transfer files. You can either transfer files from the machine you are on to a remote machine or you can transfer files from a remote machine to the one you are on.

  1. When I want to transfer data from the machine I am on to a remote host. More specifically, imagine that I am logged in as ‘user1’ on a machine named ‘local’. In my home directory, I have a big data directory or file called ‘something’ that I want to copy onto a remote machine named ‘remote’ into some path ‘/somewhere’. I could do this via the following command:

    [user1@local ~]$ tar cf - something | ssh remote " ( cd /somewhere ; tar xf - ) "

    This command doesn’t give you any output, which can be very frustrating when transferring a lot of data. So I recommend adding the verbose flag ‘-v’ to the second tar command:

    [user1@local ~]$ tar cf - something | ssh remote " ( cd /somewhere ; tar xvf - ) "

    Also, if this is going to take a long time you may want to use a screen session.

  2. When I want to transfer data from a remote host to the machine I am on.

    ssh remote "( cd /somewhere ; tar cf - something ) " | tar xf -

Using rsync to mirror remote directories

If you want to mirror some data, then rsync is a better solution than a ssh tar pipe. However, you may want to use a ssh tar pipe to initially transfer the the data in bulk and then use rsync to keep the two data sets in sync.

What is rsync?

rsync is a fast, powerful, and configurable file transfer tool. In particular, rsync is widely used due to its ability to only transfer the differences between files at the source and existing files at the destination. It has a large number of options, which allow you to control nearly every aspect of its operation. Given its extreme versitality there are numerous online tutorials describing how to use it in a large variety of ways. In particular, please take a look at the following:

For this tutorial, we will just be discussing mirroring files between two UNIX systems. We will also only discuss using rsync with a remote shell. So, if you want to:

  • mirror between two systems with different file permission models
  • are interested in setting up or using the rsync daemon

you may need to refer to the man page or other online documentation.

Basic mirroring between two UNIX systems using ssh

Pulling a directory from a remote server:

rsync -avH -e ssh user@server:test .

Here is a brief description of the above options (see the rsync man page for more details):

Option Description
-a archive mode; equals -rlptgoD (no -H,-A,-X)
-v increase verbosity
-H preserve hard links
-e specify the remote shell to use

If you want to delete local files, which have been deleted on the remote machine, simply add the --delete option:

rsync -avH --delete -e ssh user@server:test .

Better remote shell options with ssh

If you are going to be using the rsync command regularly to transfer larger files, there are a number of options to ssh that you should use:

rsync -avH \
-e 'ssh -ax -c blowfish -o ClearAllForwardings=yes' \
user@server:test .

Here is a brief description of the above options (see the ssh man page for more details):

Option Description
-a Disables forwarding of the authentication agent connection.
-x Disables X11 forwarding.
-c cipher_spec Selects the cipher specification for encrypting the session.
-o option Can be used to give options in the format used in the configuration file.

The blowfish cipher is used to make the encryption faster. The option ClearAllForwardings is used to prevent possible automatic port forwards.


On a remote machine,, new imaging data is being added to a directory named /home/jarrod/Maryland/MRI by the jarrod account. Another user, ttest wants to keep a local directory, /home/ttest/B-SNIP/Other-sites/Maryland/MRI on his machine in sync with the remote directory. ttest has an old copy of the remote directory all ready and doesn’t want to download everything again, but only the new or changed files.

This is how test can use rsync to copy new or changed files to his local directory from the remote server, while keeping local copies of files that have been deleted from the remote server.

First ttest creates a couple of temporary variables to simplify his commands and then changes into the directory where his local copy of the data is:


ttest can now start the transfer using the following command:

rsync -avH \
-e 'ssh -ax -c blowfish -o ClearAllForwardings=yes' \


It is always good practice to test commands before executing them. Many UNIX commands have a dry-run option, which perform a trial run with no changes made. For example, the rsync command can be executed in a dry-run mode the -n option.

Here is a short script:



for SITE in Maryland Hartford Detroit
  echo "Synching ${REMOTEDIR}..."
  rsync -avH \
    -e 'ssh -ax -c blowfish -o ClearAllForwardings=yes' \

echo "Synching ${REMOTEDIR}..."
rsync -avH \
  -e 'ssh -ax -c blowfish -o ClearAllForwardings=yes' \