- 413 words - 3 min read

Summary

Learn how to enhance your ZSH prompt with a color-coded indicator showing the number of displays attaching the current GNU Screen sessions, helping you maintain awareness of your terminal multiplexer state.

Introduction

In a setting where more than one person has access to GNU Screen sessions, I often found myself wondering how many terminals were currently attached to the current session. This becomes particularly important when you need to ensure that no conflicting tasks are running in the same session.

The Solution

We'll implement a prompt modification that shows the number of attached displays, color-coded for quick visual feedback:

  • Green for single attachment (ideal state)
  • Red for multiple attachments (potential conflict state)

Implementation

The Screen Counter Script

First, create bin/screen-user-count: 

#!/bin/bash

screen -ls "$STY" | while read scr
do
    if [[ "$scr" =~ ^[0-9]+\. ]]; then
	pid=${scr%%.*}
	cnt=$(ls -l /proc/$pid/fd/ | grep pts | wc -l)
	echo "$scr [$cnt]"
    else
	echo "$scr"
    fi
done

This is a slightly modified version of a solution on StackExchange, which works nicely. I modified it to use the environment variable $STY, so that if called from within a screen session the query is limit to the currently attached session. This ensures that in our scenario we will always get exactly one result.

ZSH Configuration

Add to your ~/.zshrc: 

suc() {
  local count=$(bin/screen-user-count | grep Attached | awk -F'[][]' '{print $2}')
  if [[ $count -eq 1 ]]; then
    echo "%F{green}${count}%f"
  else
    echo "%F{red}${count}%f"
  fi
}

setopt PROMPT_SUBST

precmd() {
  PROMPT="%m %~ $(suc) %# "
}

How It Works

  1. The screen-user-count script checks /proc filesystem for terminal attachments
  2. suc() function extracts the count and applies color formatting
  3. precmd() integrates the count into your prompt

Bonus

As a last line in our ~/.zshrc we have: 

[ -z "$STY" ] && screen -xR && logout

The last line goes beyond the topic of this post. First it guards against running from within a screen session, so the line is only executed upon login. Then it immediately auto-attaches a previous session or creates one, if there is none. Finally after detaching the session it immediately performes a logout. This ensures that a user logged in to the machine will allways be in a possibly shared screen session.

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