The series comes from a recurrence relation that you get by sitting at unix bash terminal session and first typing in
: "!!" '!!'
Then after that just hitting the up arrow key to get the last command expanded by the shell then hitting return; add infinitum.
The series comes from counting the number of '!!' pairs in each successive line.
Now I don't always have unix and bash to hand so I decided to write a Python "shell" program with enough functionality to simulate the recurrence relation.
In the bash shell:
- !! is replaced by the previous command if it is outside of any quotes.
- !! is replaced by the previous command if it is inside a double quoted string, ("...").
- !! is NOT replaced by the previous command if it is inside a single quoted string, ('...').
- Quoted strings do not nest. Single quotes inside double quotes are treated as plain characters and equally double quotes inside single quotes are treated as plain characters; (they don't terminate the current type of quoted string).
- Apart from when !! is replaced as above, characters are copied to form the latest expanded command.
- The up-arrow key should put the previous expanded command on the input line.
The following works on windows for me using Python 2.7 (Anaconda) which has a windows version of the readline module.
# -*- coding: utf-8 -*- """ Created on Tue Aug 20 17:23:37 2013 @author: Paddy McCarthy """ try: # Python 2/3 compatibility raw_input except: raw_input = input import readline prev = ": '!!'" print('$ ' + prev) while True: cmd = raw_input('$ ').rstrip() dquote = squote = skipnext = False result =  for this, nxt in zip(cmd, cmd[1:] + ' '): if skipnext: skipnext = False continue elif this == '"' and not squote: dquote = not dquote result.append(this) elif this == "'" and not dquote: squote = not squote result.append(this) elif this == '!' == nxt and not squote: skipnext = True result.append(prev) else: result.append(this) result = ''.join(result) print('%s # bang-bang count = %i' % (result, result.count('!!'))) readline.add_history(result) prev = result
I have used it to verify the first 6 terms of the series so far.
|cmd.exe running the Python bang-bang shell simulator|