Cursed Commands - No SCP? No problem!

Environmental constraints can drive us to do some silly things as humans. In the many years I’ve been a Linux user, I’ve occasionally found myself stringing together some commands that at best stretch the intent of the program authors, and at worst feel downright filthy. I’ve decided to write some blog posts about some of these; partially to demonstrate the flexibility of good old fashioned command line tools, partially in the hopes that some poor fools in a similar situation might find them useful, but mostly for fun if I’m being honest.

For this first example, I wanted to transfer a compressed firmware image from one machine to another, connected via Ethernet. Just use scp, right? Well, not quite.

The Situation Link to heading

My goal was to install OpenWrt on an old APU2 board from the late, great PCEngines. The APU was actively running a live image off of a USB stick, and connected via Ethernet to a workstation for accessing the web UI. But to properly install to the internal storage I needed to transfer a firmware image from the workstation and dd it.

With an active network connection between the two machines, the solution is obvious:

scp firmware.img.gz root@apu2:

Unfortunately, OpenWrt ships with the lightweight dropbear SSH server, and does not play nice with scp out of the box. So what next?

The “Solution” Link to heading

At this point any number of things could have worked, including but not limited to:

• Connect the APU to the internet, use curl to download the image directly.
• Prepare and mount a USB stick with the image on it.
• Reboot into a live image that has scp support (just about anything else).
• Search the OpenWrt documentation for clues.

But, all of those options (with the exception of the latter) would have required physically interacting with the APU in some way like a caveman. Instead, I did this from the workstation:

tar -czf - firmware.img.gz | ssh root@apu2 'tar -xz'

First, tar is called with -f - in order to write the archive to stdout. This then gets piped into ssh, which has been told to have tar read from stdin on the APU.

Did it work? Yes. Did it feel good? Absolutely not. Despite sending a gzipped file, I applied another layer of compression out of habit.¹ Piping anything to ssh feels weird. And worst of all? I actually could have just used scp. Lo and behold, per OpenWrt’s documentation, scp -O.

This exercise was generally just silly. But, in it there is one important takeaway:

If you have two systems that have a network connection, whatever the constraints at play (self-imposed or otherwise), you have the means to transfer files between them.