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title	date	tags
SSH to Idle Screen Window	2022-07-19T15:42:31-07:00	[homelab]

I've [written before]({{< ref "/posts/auto-screen" >}}) about setting up my ssh config so that I'll automatically join an existing screen session when ssh-ing to certain hosts, by setting RemoteCommand screen -D -RR -p +

However, this has a couple of issues:

It will always create a new window within the session, even if an idle window exists. More often than not, I find myself immediately killing the new window and switching to an existing one.
It doesn't restrict the rejoin to a named session - in my current usage, I typically only have a single screen session open at once, but that could change!

This didn't seem like too hard of a problem to solve - in pseudo-code, the algorithm should look like:

if !(named session exists):
  create and join named session
else
  for window in named session:
    if window is idle:
      join window
  else:
    create and join new window in named session
end if

Translated into shell code, that looks like:

Host host_nickname
  HostName hostname.avril
  RequestTTY force
  RemoteCommand sessionName=main; sessionSocket=$(ls /run/screen/S-pi | grep -E "^[[:digit:]]+\.$sessionName$"); if [ -z "$sessionSocket" ]; then screen -S "$sessionName"; else idleWindows=$(pgrep -P $(echo "$sessionSocket" | cut -d "." -f1) | xargs -I {} sh -c "echo -n '{}:'; pgrep -P {} | tr '\n' ':'; echo" | grep -E -v ':[[:digit:]]' | sed -n 's/:$//p'); if [ -z "$idleWindows" ]; then screen -D -RR -S "$sessionName" -p +; else screen -D -RR -S "$sessionName" -p $(tr '\0' '\n' </proc/$(echo $idleWindows | tr '\n' ' ' | cut -d ' ' -f1)/environ | grep ^WINDOW= | cut -d '=' -f2);fi;fi

OK, that's....that's pretty gross. Let's break it down.

Breakdown

Variables

sessionName=main - by declaring this right upfront, we make it easy to change the session name if desired.
sessionSocket=$(ls /run/screen/S-pi | grep -E "^[[:digit:]]+\.$sessionName$") - the directory /run/screen/S-<username> contains sockets for each session started by <username>, with names of the form <PID>.<sessionName>. If we wanted, we could declare sessionUser=pi to make this configurable, but I think it's very unlikely I'll be interested in sessions started by other users.

Create the session if it doesn't exist

if [ -z "$sessionSocket" ]; then screen -S "$sessionName" - that is, "if the variable $sessionSocket is empty (i.e. there was nothing in /run/screen/S-pi/ matching the sessionName - no such named session is running), then create a session with that name"

That's the simple case. Moving into the else branch, where the session already exists and we need to join it, gets a bit more complicated.

Find idle windows

idleWindows=$(pgrep -P $(echo "$sessionSocket" | cut -d "." -f1) | xargs -I {} sh -c "echo -n '{}:'; pgrep -P {} | tr '\n' ':'; echo" | grep -E -v ':[[:digit:]]' | sed -n 's/:$//p')¹ ², broken down, is:

echo "$sessionSocket" | cut -d "." -f1 gets the processId of the session
pgrep -P <sessionPID> finds all processes that are children of that session - that is, the processes of the windows.
- This works, but I have the feeling that I'm misunderstanding or glossing over some complexity of screen's operation (or, indeed, Unix process design) by relying on this fact. Note that this SO answer recommends finding windows-in-a-screen by examining the output of sudo w, but it wasn't clear how to map from that output to the window numbers, nor how to tell whether an entry in sudo w corresponds with a screen window or not. In particular, pts/<number> entries seem to appear for regular ssh connections, for screen session parents, and for each window, so that's pretty unclear. If you understand how to parse sudo w to identify screens-and-windows, please do let me know!
pgrep -P <sessionPID> | xargs -I {} sh -c "echo -n '{}:'; pgrep -P {} | tr '\n' ':'; echo" - for each window, list the child processIds of that window, colon-separated, on the same line. For instance, if there are windows with PID 001 and 002, and window 001 has child process 003, the output would look like:

001:003:
002:

pgrep -P <sessionPID> | <list children of window PIDs> | grep -E -v ':[[:digit:]]' | sed -n 's/:$//p filters the preceding output to any lines that don't (-v) contain "a colon followed by a digit" (that is - it filters to only the lines referring to a window with no children; an idle window), then sed removes the trailing colons from each line so we recover the processIds of the idle windows.
- The grep here could have equivalently been grep -E -v ':[^$], to search for "any lines that don't contain a colon not at the end of the line" => "all lines whose only colon is at the end of the line".

After all that, we're left with a variable named $idleWindows which is a newline-delimited list of the processIds of idle windows of the screen session.

We're nearly done!

Open screen to appropriate window

if [ -z "$idleWindows" ]; then screen -D -RR -p +; else screen -D -RR -p $(tr '\0' '\n' </proc/$(echo $idleWindows | tr '\n' ' ' | cut -d ' ' -f1)/environ | grep ^WINDOW= | cut -d '=' -f2);fi³:

if [ -z "$idleWindows" ]; then ... - if the $idleWindows variable is empty, then:
- screen -D -RR -S "$sessionName" -p + - open a new window (-p +) in session $sessionName and attach to it. Else (if the $idleWindows variable isn't empty)...
- ...screen -D -RR -S "$sessionName" -p $(tr '\0' '\n' </proc/$(echo $idleWindows | tr '\n' ' ' | cut -d ' ' -f1)/environ | grep ^WINDOW= | cut -d '=' -f2) - the file /proc/<processId>/environ contains the environment variables of that process. From that, we can grep out the WINDOW variable (which is just a 0-indexed number), then cut it, to provide the value to pass to -p in the screen command (thanks again to the example in this SO question!).
  - EDIT: When I first implemented this, $idleWindows showed up as a space-delimited variable, so I could just do echo $idleWindows | cut -d ' ' -f1. The next day (after, AFAIK, no changes on my Raspberry Pi), it showed up as (as-expected) newline-delimited. Rather than figure out the inconsistency, I added in a tr '\n' ' ' to coerce both cases into a standard form. ☆☆☆ SOFTWARE ENGINEERING ☆☆☆

This is a travesty, why would you do this?

A good question, to which my Signal autocomplete tried to provide an answer when I told a friend about this:

Arguably, a better way to do this would be to create a script containing the appropriate logic (the filtering logic could probably be better expressed in a higher-level language!), deploy it to the appropriate hosts, and then call that script as the RemoteCommand (perhaps falling back to screen -D -RR -p + if it's absent). This would allow the logic to be split out nicely into appropriately named and commented functions. That script deployment is the rub, though - I'd rather "store" this logic in one place (my ~/.ssh/config file) than have to handle deploying a script to every host I want to use this logic on. If I change my mind about that and want a more legible and debuggable source, I could add this script to the pi-tools repo I use for setting up common tools on my homelab, or host the script on my NAS.

No, really, why did you do this?

Most of the tasks I take on for this homelab project are motivated by a desire to learn. It's not so much that they're useful in-and-of-themself (as I said above - I could have stuck with the simple screen -D -RR -p + command which opens a new window, and closed the window as needed), but rather that they're a prompt to learn something new. And, indeed, in this case I've learned a good few things:

The $'...' quoting syntax for bash that allows for double and single-quotes within a single string
The [[:digit:]] character class for grep. I could have sworn it was \d, but that didn't work - maybe that's only for an older version of grep? Regardless, this spelling is more explicit.
How to get the environment variables of a running process from /proc/<processId>/environ. I imagine that has a few nefarious purposes for black-/grey-hat applications...
pgrep - here I've been, using ps aux | grep like some kind of chump!
Hopefully, this has now cemented in my brain the if [ -z "$variable" ] syntax for checking emptiness - but I doubt it...
echo $$ to find the process of the current process

And a few things I have yet to learn:

I'm still a bit puzzled about why perl appears to behave differently when called in an ssh command than when called on the machine itself. When called on the machine, referencing capture groups uses the $1 syntax that I'm used to; but, when perl is used in a command in ssh, I needed to use \1 instead (apparently, there is no standard across engines to insert capture groups into replacements - and note that that article lists Perl as using both). I even checked that the perl --version in both contexts was the same. This is a real puzzler! Because of this, you'll notice I haven't used perl here even though it could have replaced and consolidated a few commands.
As I called out in [an earlier section]({{< ref "#find-idle-sections" >}}), I don't yet know how I could have parsed the output of sudo w or information from netstat to extract the information I was looking for here. I'm also a little stumped about why this questioner found screen -Q commands to be so slow, but they appeared to take negligible time for me.

I had to use xargs sh -c "<command>" here, rather than the more normal xargs <command>, because the command contained semi-colons - without the sh -c wrapper, only the first part of <command> would have been passed to xargs, and the following parts would have been executed independently, outside the xargs context. ↩︎
While testing this, I ran into a problem - I was testing by running ssh <host> <command> from the command line, and I needed to enclose the command in quotes to prevent the ssh command only reading up until the first semi-colon and then stopping. However, I'd already used both double-quotes (to wrap the sh -c command) and single-quotes (within the sh -c command) - how could I combine both of those and wrap them in a string identifier? Luckily, this SO answer provided a solution - by wrapping a string in $'...', you can use both double-quotes and escaped-single-quotes within it. This also required that the slashes in \0 and \n be escaped, too - so it ended up looking like \'\\0\'. Not recommended - but good to know when necessary! ↩︎
There's another fi at the end of the full command to close the first if which created the session if it didn't exist. ↩︎

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