Readline Proxy

This enables custom interactive REPLs, while still providing (as far as possible) normal shell/readline/history behaviour and without leaving the current process.

Example: Python w/o Shell Expansion

This is an example where we decided to run expressions within a python interpreter if their first line starts with a space.

asciinema rec --title="readline_proxy" -w 0.5

This is for sure not super useful as is, but think of python -c being replaced by e.g. interactions with a (stateful) (co)process via bash functions.

Usage

source the script and supply the functions on top with your own versions. You are getting called for

• the welcome message
• the possibility to specify keywords for special mode (rl_special_keywords), triggering special mode if first line begins with one of those
• the decision if an entered line is 'special' and also if it is the first of a multiline
• each line in a multiline expression where you can decide if it is complete
• before such a 'special' expression is pushed to the history and run, so that you can decide what to really run (and put into the history).

WTFs

Here are known differences to a normal shell session after the script is sourced.

Those only apply when you enter statements interactively

Behaviour when sourcing scripts or running them should be of no difference.

Multiline Expressions

Short Version

• (De-)indent your multline code you enter correctly
• multiline assignments like myvar="foo\nbar"

and you'll normally won't have problems.

Details

If a line is not 'special', i.e. entered bash expressions we must support multiline entry mode. We are by far not as smart as bash regarding that - but hopefully tolerable.

Currently we look at (whitespace cleared) endings of those lines to derive that we shall switch to multiline mode automatically and evaluate it when it is complete.

How?

We set multiline mode if the right-stripped first line ends with ';' '{' '(' '[' ' do' We stop multiline mode if the right-stripped last line ends with \n> plus 'done' ']' '}' 'fi' 'esac' ')'

You can force start and end of multiline mode by putting '$' at start and/or '.' at the end of a line. Both characters are removed before evaluation.

Note:Sorry, not yet understood how exactly bash knows when to start / stop multiline mode. Trying to catch syntax error: unexpected end of file on stderr was a dead end road in deed (tee-ing stderr when user command is e.g. bash is a blocker).

• Here document first lines and terminators are detected without any begin/end indicators.

• We store multiline expressions "as is" - i.e. NOT merging them into a single line, using semicolons as bash does.

Prompt

• Normally you see your command on stdout before the result is shown. Now you get it on stderr and the prompt is just a '$' infront of it

Tech

bash allows to provide custom key-bindings via the bind function.
This passes any enter-key hit into a custom function:

bind -x '"\C-M": "rl_handle_enter"'

while normal behaviour is this:

1 ~ $ bind -P |grep accept
accept-line can be found on "\C-j", "\C-m".

Since it seems not possible to invoke readline's standard accept-line behaviour in a custom enter handler, this was created.

What it does is basically:

• checks for customizable criteria if the current line should be treated "special"
• checks if it looks like a bash multiline expression and starts asking for more input until that multiline expression is deemed complete
  • when an expression is deemed complete it is pushed to the history - and run via fc -s.
• fc repeats the command on stderr, we just add a "$" prompt before it.