Editing a bash script while it's running

self-extend

#! /bin/bash
#
# Illustrate what happens when editing a script while it's running.
#
# This happens when saving the file in emacs, for example, but not with
# 'sed -i' because the latter deletes the old file before writing a new one.
# See https://unix.stackexchange.com/a/88572/46435 for an explanation.
#

set -eu

me="$0"

# We're about to append this command to the current script.
cmd="date; undo_append"

orig_size=$(cat "$me" | wc -c)

undo_append() {
  echo "Truncating '$me' back to its original state."
  truncate "$me" -s "$orig_size"
}

# Append the 'date' and 'undo_append' commands to this script.
echo "$cmd" >> "$me"