README

bpx fakes a zsh-like hook function system for interactive bash shells. It works with the bind builtin command to modify the readline buffer. See Examples and GIFS.

bpx works best in bash >=4.4 (PS0) and has been tested with the emacs line editing mode in an interactive instance, that was not running in an Emacs shell buffer. Please let me know, how to do it with the vi mode (which is, btw, also the POSIX line editing mode). bash must run without its --noediting option, of course.

BUGS & REQUESTS

Feel free to open an issue or put in a pull request on https://github.com/D630/bpx. See also the comments in the script.

bpx suppresses bash from using the secondary prompt, when more input is needed to complete the command line. Instead, bpx runs its own emulation of Readline's edit-and-execute-command. In order to use your preferred editor, modify the function __bpx_edit, that is:

function __bpx_edit {
    command vim -f \
     '+set ft=sh' \
     "+call cursor(1,$READLINE_POINT+1)" \
     "${1?}" < /dev/tty > /dev/tty;
};

GIT

To download the very latest source code:

git clone https://github.com/D630/bpx

In order to use the latest tagged version, do also something like this:

cd -- ./bpx
git checkout $(git describe --abbrev=0 --tags)

INSTALL

Just put the shell procedure/script bpx.bash elsewhere on your PATH and then execute it with . or source in your configuraton file for interactive bash sessions (usually .bashrc).

HOOK FUNCTIONS

Bash is able to handle signals and other conditions (trap), to invoke the function command_not_found_handle, and to expand special variables at special times (prompting). But it has no "real" hook mechanism like you can see working in tcsh's special aliases, in zsh's hook functions or fish's event handling.

Because of that, the usual way to emulate zsh-like preexec and precmd hook functions is to set a trap on DEBUG and a special value in PROMPT_COMMAND respectively. But the DEBUG trap has many pitfalls, and it aims badly the real bahviour of zsh: preexec functions in zsh are executed after the command line (aka. command list or parse tree) has been read and is about to be executed; they are not beeing executed before each command/pipeline execution of the list. Actually, the second and third parameters of its functions don't even obtain the whole expansion; alias expansion in command and process substitutions, for example, is not performed. That is: one command line, one zsh like preexec hook!

The variable PS0, that was introcuced in bash 4.4, can be a replacement for the DEBUG trap, if the user is satisfied with running commands in command subtitution (a corresponding PRE_PROMPT_COMMAND is missing). The alternative (but still hacky) way used in bpx is to involve Readline by using the bind builtin command to work with the readline buffer. The preexec hook is then plugged into a wrapper around the Readline command accept-line. As a by-product, we are able to define some additional hooks:

Order	Hook	Method	Description
1	preread	bind	Executed in a "keyseq:shell-command" binding before the readline-function accept-line is invoked. READLINE_{LINE,POINT} may be modified
2	preexec	bind	Executed in a "keyseq:shell-command" binding before the readline-function accept-line is invoked. READLINE_{LINE,POINT} may not be read and modified anymore
3	debug	trap	Works with DEBUG and is executed for every command in the command list; BASH_COMMAND available (not recommended)
4	prompt	variable	Works with the PROMPT_COMMAND variable and is executed before each prompting of the primary prompt (PS1)
5	postread	bind	Executed in a "keyseq:shell-command" binding after the readline-function accept-line has been invoked. READLINE_{LINE,POINT} of the next command line may be modified

Similar to zsh, those hook functions execute function names in an array, which has the same name as the hook + "_functions" appended: preread_functions=(foo bar). bpx takes care to redirect to stderr and to pass the correct status codes to them ($?). Your settings to history expansion will also be respected.

internal variables

bpx sets some global variables for internal purposes:

Variable	Description
bpx_var	Integer attributed indexed array variable
"[0]	Is one when command line isn't valid
"[1]	Is one when complete command line has been read
"[2]	Number of BASH_COMMAND in a command line (debug)
"[3]	Last status code

preread, preexec, and postread

As part of the wrapper, bpx binds a number of key sequences (__bpx_set_binds), which will be running after entering the key sequence \C-x\C-x1. Simply use this key or bind it to a key or key sequence of your choice:

bind 'C-j: "\C-x\C-x1"'

Then make sure bpx has sane internal variables, when the next hook takes place:

# set bpx_var to 0
PS1='${_[ bpx_var=0, 1 ]}\u@\h \w \$ '

Before preread, preexec, and postread hook functions are about to be executed, the wrapper provides some functions and variables for them:

Name	Object	Description
__bpx_command_line	Normal function	The body contains the full command line with aliases expanded outside of command and process substitutions
__bpx_set_rl1	Normal function	Assigns to rl1
__bpx_set_rl2	Normal function	Assigns to rl2; implies __bpx_set_rl1
__bpx_read_again	Normal function	Forces rereading and editing of the command line from start
__bpx_read_abort	Normal function	Stops the editing and execution of the current command line
__bpx_read_accept	Normal function	Undoes _bpx_read{again,abort}
rl0	Normal scalar variable	Holds the the full command line without alias expanded
rl1	Normal indexed array	Contains the body of __bpx_command_line