Module: Muxr::ForegroundCommand

Defined in:
lib/muxr/foreground_command.rb

Overview

Looks up the foreground command running inside a PTY by walking from the shell’s pid → its tpgid (foreground process group on the controlling tty) → that process’s command name. Hides the result when the shell itself is foreground so titles aren’t full of “bash” / “zsh” noise.

Two platform paths:

Linux: /proc/<pid>/stat (no fork — runs fast even on the main thread,
       though Application uses a background thread anyway)
macOS: ps -o tpgid=,pgid= -p <pid> + ps -o comm= -p <tpgid>. Two
       fork+execs, ~10–20ms total — exactly the reason callers run
       this off the event-loop thread.

Returns the command name string or nil. nil also covers “couldn’t read” so callers degrade silently rather than risk showing stale data.

Constant Summary collapse

SHELLS =

Command names we never want to surface — these are the empty-prompt case. If a user genuinely runs ‘bash` inside `bash` we’ll under-report rather than mis-report.

%w[bash zsh fish sh dash ksh tcsh csh].freeze

Class Method Summary collapse

Class Method Details

.linux_tpgid(pid) ⇒ Object 



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# File 'lib/muxr/foreground_command.rb', line 74

def linux_tpgid(pid)
  raw = File.read("/proc/#{pid}/stat")
  parse_linux_stat(raw)
end

.lookup(pid) ⇒ Object 



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# File 'lib/muxr/foreground_command.rb', line 24

def lookup(pid)
  return nil unless pid.is_a?(Integer) && pid > 0
  tpgid, pgid =
    if File.exist?("/proc/#{pid}/stat")
      linux_tpgid(pid)
    else
      macos_tpgid(pid)
    end
  return nil unless tpgid && pgid
  return nil if tpgid <= 0
  return nil if tpgid == pgid # shell is its own foreground — empty prompt

  name =
    if File.exist?("/proc/#{tpgid}/comm")
      File.read("/proc/#{tpgid}/comm").strip
    else
      `ps -o comm= -p #{tpgid} 2>/dev/null`.strip
    end
  normalize(name)
rescue StandardError
  nil
end

.macos_tpgid(pid) ⇒ Object 



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# File 'lib/muxr/foreground_command.rb', line 79

def macos_tpgid(pid)
  out = `ps -o tpgid=,pgid= -p #{pid} 2>/dev/null`.strip
  return [nil, nil] if out.empty?
  tpgid, pgid = out.split.map(&:to_i)
  [tpgid, pgid]
end

.normalize(name) ⇒ Object

Public for testing — strips path/dash/whitespace and filters shells.



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# File 'lib/muxr/foreground_command.rb', line 48

def normalize(name)
  return nil if name.nil? || name.empty?
  name = name.strip
  name = name.sub(/\A-/, "") # login shells appear as "-bash"
  name = File.basename(name)
  return nil if name.empty?
  return nil if SHELLS.include?(name)
  name
end

.parse_linux_stat(raw) ⇒ Object

Public for testing — parses Linux /proc/<pid>/stat into [tpgid, pgid]. The comm field can contain spaces and parens, so we slice from the last ‘)’ rather than splitting from the start.



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# File 'lib/muxr/foreground_command.rb', line 61

def parse_linux_stat(raw)
  idx = raw.rindex(")")
  return [nil, nil] unless idx
  tail = raw[(idx + 2)..]
  return [nil, nil] unless tail
  fields = tail.split(" ")
  # After the closing paren the fields are:
  #   state(0) ppid(1) pgrp(2) session(3) tty_nr(4) tpgid(5) ...
  pgid  = fields[2]&.to_i
  tpgid = fields[5]&.to_i
  [tpgid, pgid]
end