In order to interact with the external world, Kakoune uses the shell,
mainly through the %sh{ … } string type, and its control socket.

Basic interaction

For synchronous operations, %sh{ … } blocks are easy to use, they
behave similarly to $( … ) shell construct.

For example, one can echo the current time in Kakoune’s status line

:echo %sh{ date }

For asynchronous operations, the Kakoune Unix stream socket can be used.
This is the same socket that Kakoune clients connect to. It is available
through the kak_session environment variable: the socket is

For example, we can echo a message in Kakoune in 10 seconds with:

:nop %sh{ {
    sleep 10
    echo "eval -client '$kak_client' 'echo sleep ended'" |
        kak -p ${kak_session}
} > /dev/null 2>&1 < /dev/null & }
  • The nop command is used so that any eventual output from the
    %sh{ … } is not interpreted by Kakoune

  • When writing to the socket, Kakoune has no way to guess in which
    client’s context the command should be evaluated. A temporary
    context is used, which does not have any user interface, so if we
    want to interact with the user, we need to use the eval command,
    with its -client option to send commands to a specific client.

  • For the command to run asynchronously, we wrap it in a sub shell
    with braces, redirect its std{in,err,out} to /dev/null, and run
    it in background with &. Using this pattern, the shell does not
    wait for this sub shell to finish before quitting.

Interactive output

It is a frequent interaction mode to run a program and display its
output in a Kakoune buffer.

The common pattern to do that is to use a fifo buffer:

evaluate-commands %sh{
     # Create a temporary fifo for communication
     output=$(mktemp -d -t kak-temp-XXXXXXXX)/fifo
     mkfifo ${output}
     # run command detached from the shell
     { run command here > ${output} } > /dev/null 2>&1 < /dev/null &
     # Open the file in Kakoune and add a hook to remove the fifo
     echo "edit! -fifo ${output} *buffer-name*
           hook buffer BufClose .* %{ nop %sh{ rm -r $(dirname ${output})} }"

This is a very simple example, most of the time, the echo command will
as well contain

set buffer filetype <...>

and some hooks for this filetype will have been written

Completion candidates

Filetype specific completion should be provided by external programs.

External completions are provided using an option to store completion,
which have the following format.

line.column[+len]@timestamp candidate1|select1|menu1 candidate2|select2|menu2 ...

the first element of this string list specify where and when this
completion applies, the others are a triplet
<completion text>|<select cmd>|<menu text>

The select command is executed whenever this menu item gets selected,
and is usually used to display an item specific documentation with
info -placement menu '<menu item description>'

The menu text is a markup string (see :doc faces markup-strings), so it can contain {face}

To effectively use that completion option, it should get added to the
completers option.

— set -add buffer completers option=my_option_name —

As a completion program may take some time to compute the candidates, it
should run asynchronously. In order to do that, the following pattern
may be used:

# Declare the option which will store the temporary filename
decl str plugin_filename
# Declare the completion option
decl completions plugin_completions
# Add plugin_completions to completers for files of good filetype
hook global BufSetOption filetype=my_filetype %{
    set -add buffer completers option=plugin_completions
evaluate-commands %sh{
    # ask Kakoune to write current buffer to temporary file
    filename=$(mktemp -t kak-temp.XXXXXXXX)
    echo "set buffer plugin_filename '$filename'
          write '$filename'"
# End the %sh{} so that its output gets executed by Kakoune.
# Use a nop so that any eventual output of this %sh does not get interpreted.
nop %sh{ { # launch a detached shell
    # run completer program and format the output in a list of completions
    candidates=$(completer $buffer $line $column | completer_filter)
    # remove temporary file
    rm $buffer
    # generate completion option value
    completions="$line.$column@$kak_timestamp $candidates"
    # write to Kakoune socket for the buffer that triggered the completion
    echo "set buffer=${kak_bufname} plugin_completions $completions" |
        kak -p ${kak_session}
} > /dev/null 2>&1 < /dev/null & }