bash history tricks

I came across an interesting blog entry about maintaining a persistent history of commands across multiple bash sessions using some features of bash I was unaware of.

First there was a variable PROMPT_COMMAND which will run the command stored in the variable before presenting the new prompt.

Then there was the BASH_REMATCH variables, which store substring patterns when used with [[ ... ]] e.g.

test_string="Random string of numbers : 1234"
[[
   $test_string =~ (.*)\:\ +([0-9]+)
]]

echo ${BASH_REMATCH[0]}
echo ${BASH_REMATCH[1]}
echo ${BASH_REMATCH[2]}

This snippet will output the following
Random string of numbers : 1234
Random string of numbers
1234

I have simplified the example from Eli's blog as I am not using the same format for history output , so my code in .bashrc looks like

log_bash_history()
{
  [[
    $(history 1) =~ ^\ *[0-9]+\ +(.*)$
  ]]
  local command="${BASH_REMATCH[1]}"
  if [ "$command" != "$HISTORY_LAST" ]
  then
    echo "$command" >> ~/.persistent_history
    export HISTORY_LAST="$command"
  fi
}
export PROMPT_COMMAND=log_bash_history

Python Command history

I was using the python command interpreter interactively to test out some snippets of python and rather than either typing it out again, copy/paste and removing the prompt markers at the beginning of the line or loosing it entirely, I wondered if there was a way to save the command history like in bash.

Turns out there is an easy way to do this and it is referenced in the python documentation (http://docs.python.org/3/tutorial/interactive.html)

Place the startup script below in a file in your home directory such as .pystartup
and export the environment variable PYTHONSTARTUP pointing to this file

import atexit
import os
import readline
import rlcompleter

historyPath = os.path.expanduser("~/.pyhistory")

def save_history(historyPath=historyPath):
    import readline
    readline.write_history_file(historyPath)

if os.path.exists(historyPath):
    readline.read_history_file(historyPath)

atexit.register(save_history)
del os, atexit, readline, rlcompleter, save_history, historyPath