readline - a simple interface to the readline and history libraries


 local RL = require 'readline'
 RL.set_options{ keeplines=1000, histfile='~/.synopsis_history' }

-- the Standard Interface
 local str = RL.readline('Please enter some filename: ')
 local save_options = RL.set_options{ completion=false }
 str = RL.readline('Please type a line which can include Tabs: ')
 str = RL.readline('Now tab-filename-completion is working again: ')

 -- the Alternate Interface
 local poll = require 'posix.poll'.poll
 local line = nil
 local linehandler = function (str)
    line = str
 RL.handler_install("prompt> ", linehandler)
 local fds = {[0] = {events={IN={true}}}}
 while true do
    poll(fds, -1)
    if fds[0].revents.IN then
       RL.read_char()  -- only if there's something to be read
       -- do some useful background task
    if line then break end
 print("got line: " .. line)

 -- Custom Completion
 local reserved_words = {
   'and', 'assert', 'break', 'do', 'else', 'elseif', 'end', 'false',
   'for', 'function', 'if', 'ipairs', 'local', 'nil', 'not', 'pairs',
   'print', 'require', 'return', 'then', 'tonumber', 'tostring',
   'true', 'type', 'while',
 line = RL.readline('now it expands lua reserved words: ')

 RL.save_history() ; os.exit()


This Lua module offers a simple calling interface to the GNU Readline/History Library.

The function readline() is a wrapper, which invokes the GNU readline, adds the line to the end of the History List, and then returns the line. Usually you call save_history() before the program exits, so that the History List is saved to the histfile.

Various options can be changed using the set_options{} function.

The user can configure the GNU Readline (e.g. vi or emacs keystrokes ?) with their individual ~/.inputrc file, see the INITIALIZATION FILE section of man readline.

By default, the GNU readline library dialogues with the user by reading from stdin and writing to stdout; this fits very badly with applications that want to use stdin and stdout to input and output data. Therefore, this Lua module dialogues with the user on the controlling-terminal of the process (typically /dev/tty) as returned by ctermid().

Most of readline's Alternate Interface is now included, namely   handler_install,   read_char   and handler_remove.
Some applications need to interleave keyboard I/O with file, device, or window system I/O, typically by using a main loop to select() on various file descriptors.   To accommodate this need, readline can also be invoked as a 'callback' function from an event loop, and the Alternate Interface offers functions to do this.
The Alternate Interface does offer tab-completion; but it does not add to the history file, so you will probably want to call RL.add_history(s) explicitly. See handler_install()

Access to readline's Custom Completion is now provided.

This module does not work lua -i because that runs its own readline, and the two conflict with each other.


RL.set_options{ histfile='~/.myapp_history', keeplines=100 }

Returns the old options, so they can be restored later. The auto_add option controls whether the line entered will be added to the History List, The default options are:
  auto_add = true,
  histfile = '~/.rl_lua_history',
  keeplines = 500,
  completion = true,
  ignoredups = true,
  minlength = 2,

Lines shorter than the minlength option will not be put on the History List. Tilde expansion is performed on the histfile option. The histfile option must be a string, so don't set it to nil, if you want to avoid reading or writing your History List to the filesystem, set histfile to the empty string. If you want no history behaviour (Up or Down arrows etc.) at all, then set
  set_options{ histfile='', auto_add=false, }

RL.set_readline_name( 'myapp' )

Sets the internal libreadline variable rl_readline_name for use with conditional directives in .inputrc (see the manual).
It should be invoked once, before calling readline(), so that the name is set before .inputrc is sourced.
For example: if, in the initialization before first readline prompt, you

then the call to readline() would execute this conditional in .inputrc
  $if fennel
    set blink-matching-paren On
    set enable-bracketed-paste On

RL.readline( prompt )

Displays the prompt and returns the text of the line the user enters. A blank line returns the empty string. If EOF is encountered while reading a line, and the line is empty, nil is returned; if an EOF is read with a non-empty line, it is treated as a newline.

If the auto_add option is true (which is the default), the line the user enters will be added to the History List, unless it's shorter than minlength, or it's the same as the previous line and the ignoredups option is set.


Normally, you should call this function once, just before your program exits. It saves the lines the user has entered onto the end of the histfile file. Then if necessary it truncates lines off the beginning of the histfile to confine it to keeplines long.

RL.add_history( line )

Adds the line to the History List.
With the Standard Interface, you'll only need this function if you want to assume complete control over the strings that get added, in which case you set:
  RL.set_options{ auto_add=false, }
and then after calling readline(prompt) you can process the line as you wish and call add_history(line) if appropriate.

But with the Alternative Interface, you have to call add_history(line) yourself, even if {auto_add=true}
You should do this in the linehandler function, see below.


Some applications need to interleave keyboard I/O with file, device, or window system I/O, by using a main loop to select() on various file descriptors.
With the Alernate Interface, readline can be invoked as a 'callback' function from an event loop.

The Alternate Interface does not add to the history file, so you will probably want to call RL.add_history(s) explicitly.
You should do this within the linehandler function !
(This constraint is due to what may be an unadvertised quirk of libreadline.)

RL.handler_install( prompt, linehandlerfunction )

This function sets up the terminal, installs a linehandler function that will receive the text of the line as an argument, and displays the string prompt.   A typical linehandler function might be:
  linehandler = function (str)
    line = str   -- line is a global, or an upvalue


Whenever an application determines that keyboard input is available, it should call read_char(), which will read the next character from the current input source. If that character completes the line, read_char will invoke the linehandler function installed by handler_install to process the line.
Before calling the linehandler function, the terminal settings are reset to the values they had before calling handler_install. If the linehandler function returns, and the line handler remains installed, the terminal settings are modified for Readline's use again. EOF is indicated by calling the linehandler handler with a nil line.


Restore the terminal to its initial state and remove the line handler. You may call this function from within the linehandler as well as independently. If the linehandler function does not exit the program, this function should be called before the program exits to reset the terminal settings.


RL.set_complete_list( array_of_strings )

This function sets up custom completion of an array of strings.
For example, the array_of_strings might be the dictionary-words of a language, or the reserved words of a programming language.

RL.set_complete_function( completer_function )

This is the lower-level function on which set_complete_list() is based. Its argument is a function which takes three arguments: the text of the line as it stands, and the indexes from and to, which delimit the segment of the text (for example, the word) which is to be completed. This syntax is the same as string.sub(text, from, to)
The completer_function must return an array of the possible completions.
For example, the completer_function of set_complete_list() is:

  local completer_function = function(text, from, to)
     local incomplete = string.sub(text, from, to)
     local matches = {}
     for i,v in ipairs(array_of_strings) do
        if incomplete == string.sub(v, 1, #incomplete) then
           matches[1 + #matches] = v
     return matches
but the completer_function can also have more complex behaviour. Because it knows the contents of the line so far, it could ask for a date in format 18 Aug 2018 and offer three different completions for the three different fields.
Or if the line so far seems to be in Esperanto it could offer completions in Esperanto, and so on.

By default, after every completion readline appends a space to the string, so you can start the next word. You can change this space to another character by calling set_completion_append_character(s), which sets the append_character to the first byte of the string s. For example, this sets it to the empty string:
It only makes sense to call set_completion_append_character from within a completer_function.
After the completer_function has executed, the readline library resets the append_character to the default space.
Setting the append_character to ',' or ':' or '.' or '-' may not behave as you expect when trying to tab-complete the following word, because readline treats those characters as being part of a 'word', not as a delimiter between words.


This module is available as a LuaRock in so you should be able to install it with the command:
  $ su
  # luarocks install readline

  # luarocks install

If this results in an error message such as:
  Error: Could not find expected file libreadline.a, or,
then you need to find the appropriate directory with:
  find /usr/lib -name 'libreadline.*' -print
and then invoke:
  luarocks install readline\
  READLINE_INCDIR=/usr/local/Cellar/readline/8.1/include \
  READLINE_LIBDIR=/usr/local/Cellar/readline/8.1/lib \
  HISTORY_INCDIR=/usr/local/Cellar/readline/8.1/include \
  HISTORY_LIBDIR=/usr/local/Cellar/readline/8.1/lib # or wherever


It depends on the readline library and its header-files; for example on Debian you may need:
  # aptitude install libreadline6 libreadline6-dev

or on Centos you may need:
  # yum install readline-devel

You can see the source-code in:


 20210418 3.0 pass READLINE_INCDIR and READLINE_LIBDIR to gcc
 20210127 2.9 fix version number again
 20210106 2.8 include string.h
 20200801 2.7 add lua 5.4
 20200409 2.6 jaawerth: added set_readline_name()
 20190110 2.5 fix a lua_rawlen v. lua_objlen bug if using lua 5.1
 20180924 2.2 add set_completion_append_character
 20180912 2.1 C code stack-bug fix in handler_calls_completion_callback
 20180910 2.0 add set_complete_list and set_complete_function
 20180827 1.9 add handler_install read_char and handler_remove
 20151020 1.8 readline() returns nil correctly on EOF
 20150421 1.7 include lua5.3, and move pod and doc back to
 20150416 1.6 readline specified as an external dependency
 20140608 1.5 switch pod and doc over to using moonrocks
 20140519 1.4 installs as readline not Readline under luarocks 2.1.2
 20131031 1.3 readline erases final space if tab-completion is used
 20131020 1.2 set_options{histfile='~/d'} expands the tilde
 20130921 1.1 uses ctermid() (usually /dev/tty) to dialogue with the user
 20130918 1.0 first working version


Peter Billam
Alexander Adler, University of Frankfurt, contributed the Alternate Interface and Custom Completion
Jesse Wertheim, one of the developers of Fennel, contributed the set_readline_name() function.


man readline   Readline Variables
/usr/share/readline/inputrc   ~/.inputrc