The problem#

When writing a blog post like last time's, I often will be at least partway through the process before I realize it's interesting enough to write a post on. Then I need to somehow go back and reconstruct a narrative of the troubleshooting steps I performed.

The solution#

As long as I still have the terminals or screen sessions open, I can at least capture a snapshot of the scrollback history that's still available.

In Xfce Terminal (and likely similar in other terminals), I can save the history by going into the "Edit" menu and selecting "Select All" and then "Copy" (plain text) or "Copy as HTML" (to also capture styles) and pasting into a file or writing the clipboard to a file using xclip:

xclip -o -selection clipboard > some_file.html

In screen, the hardcopy command can be used to dump the full history to a file. Type Ctrl+a, : to get into command mode and run the command hardcopy -h to include the entire scrollback buffer. Unfortunately, this is plain text only.

If you use tmux instead of screen, the equivalent is the capture-pane command which does support saving styles if you provide the -e option. Note this encodes the styles so cating the file to a terminal will look right, not in HTML as Xfce Terminal does; you could convert it to HTML using aha.

The details#

The problem#

While in my last post, I said serverless WebRTC was too cumbersome, I wanted to try to see how streamlined I could make the process. While researching, I did find a variant of serverless-webrtc called serverless-webrtc-qrcode as well as a similar demo, webrtc-qr that both use QR codes as an easy way to transmit the offer strings. But both require that both sides have a camera to scan QR codes, while my use case is a WebRTC connection between my desktop without a camera and my smartphone.

The solution#

minimal-webrtc now has a checkbox to enable serverless mode. In that mode, the QR code shown by the host is a much longer URL that includes the initial WebRTC offer. Opening that URL on another device (or in another browser window) will show another QR code along with a "Copy" button. With the first device, either press the "Scan QR code" button and point it at the QR code or use some other mechanism to copy the text and paste it into the text area labeled "Paste offer here:".

To run it locally, download the source code and run a web server to serve the wwwroot/ directory. If both devices can run a web server, then you can just access it via localhost on each, but, as before, because WebRTC requires HTTPS, to run it on your local network, you may need to set up a self-signed certificate.

The details#

X clipboard#

The X Window System, the basis for the GUI on most desktop Linux systems, defines how the clipboard works for copying and pasting between applications in Linux. One notable quark of X clipboard is that there's actually two clipboards in common use: the one you expect explicitly accessed via Copy and Paste menu items or key shortcuts called the CLIPBOARD and another one where you copy by selecting text and paste by pressing the middle mouse button called the PRIMARY selection.

X clipboard utilities#

Occasionally it is useful to be able to read or write the clipboard at the command-line. For most uses, your terminal emulator's copy and paste options are probably enough. The primary use case I have for using a command-line program to interact with the clipboard is when I am uploading a file as a Gist:

<file xclip

The xclip utility will copy the contents of the file onto the clipboard (PRIMARY, not CLIPBOARD, by default) and then I can paste it on the Gist website.

My system also has xsel which is very similar to xclip. Wikipedia actually lists several such programs, including the unfortunately named xcopy, not to be confused with XCOPY.

GNU Screen copy mode#

GNU Screen provides its own clipboard for copying information between the different windows of a screen session. ctrl+a, [ enters copy mode. In copy mode you can move the cursor using the arrow keys and page up/page down keys. Screen keeps a history (of configurable size), so you can scroll back pretty far. In fact, I use Screen's copy mode far more often for viewing the history in a terminal than for actually copying anything. You can exit copy mode either by using esc to cancel or enter once to mark the start of the selection and again to mark the end of it. Once you have copied something, ctrl+a, ] pastes the contents of the clipboard.