pymultiplexer

#!/bin/bash
# -*- mode: bash -*-
#
# Wrapper for multiplexing a python or pip to use the same version as the most
# recent caller in its process hierarchy.
#
# This lets us have a single command (pip/python) work for both python2 and
# python3 notebooks, eg
#   !pip install ...
# will install for the version of python currently in use.

# This is the default: assume python2.
SUFFIX=2

# We know *this* process isn't python, so we start with the parent.
NEXT_PID=${PPID}

while true; do
  # Determine where to look up the parent of this process (if it exists).
  PID_STAT="/proc/${NEXT_PID}/stat"
  if [[ ! -f ${PID_STAT} ]]; then
    break
  fi
  
  # Figure out the parent PID: proc(5) says this
  # is the fourth field in /proc/${NEXT_PID}/stat.
  PARENT_PID=$(cut -d\  -f4 <${PID_STAT})
  PARENT_EXE="/proc/${PARENT_PID}/exe"
  if [[ ! -f ${PARENT_EXE} ]]; then
    break
  fi
  
  # We want to know the real filename, so resolve any symlinks.
  CALLER=$(readlink -fnq ${PARENT_EXE})
  if [[ ${CALLER} == *"/bin/python3"* ]]; then
    SUFFIX=3
    break
  elif [[ ${CALLER} == *"/bin/python2"* ]]; then
    SUFFIX=2
    break
  fi

  # Repeat with the parent process.
  NEXT_PID=${PARENT_PID}
done

COMMAND="$(basename $0)${SUFFIX}"
exec "${COMMAND}" "$@"