Jaff Ain't Filtering Files
Last updated 6 years ago by aklt .
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Jaff Appends and Filters Files

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Jaff is a small macro language that allows conditional filtering of Files through external programs and Javascript or Coffeescript functions in a way similar to commandline pipes between programs. Jaff scripts can be written to cumulatively apply filters while expanding variables in the text. Here is an example:

#begin coffee
indent = (lines) ->
  return lines.map (line) ->
    return '    ' + line

folder = "/etc"

#if process.env.TEST

Contents of the {folder} folder:

#filter !indent
#  filter nl
#    filter grep rc$
#    run ls -la {folder}
#    end
#  end

Running this will print foo if process.env.TEST is defined and bar otherwise. Next it will list the contents of the /etc folder, filter out files ending with "rc", prepend a line number to each line and finally indent these lines. The output might look like this:

$ jaff example


Contents of the /etc folder:

        1      -rw-r--r--   1 root    root      657 Mar 18  2012 bash.bashrc
        2      -rw-r--r--   1 root    root     3095 Jan  9 21:22 drirc
        3      -rw-r--r--   1 root    root      714 Nov  3  2011 inputrc
        4      drwxr-xr-x   2 root    root     4096 Mar  8  2013 lirc
        5      -rw-r--r--   1 root    root     4243 Dec 28 09:18 mail.rc
        6      -rw-r--r--   1 root    root     1347 Dec  9 21:55 quilt.quiltrc
        7      -rw-r--r--   1 root    root     3312 Jul 14  2011 screenrc
        8      -rw-r--r--   1 root    root     1437 Apr 25  2011 slsh.rc


Using npm:

npm install -g jaff


This program takes a single argument:

$ jaff <filename>

Alternatively a script can be written invoking jaff via the shebang line:

#!/usr/bin/env jaff

#begin coffee
foo = "This is foo"


The following keywords are defined: begin, end, if, elif, else, include, filter and run. Keywords appear following a comment at the beginning of a line. Both Javascript // and Coffeescript comments # are accepted and there may be whitespace between the comment and the keyword.

begin <js|coffee> ... end

Start a block of Javascript or Coffeescript code. Variables and code can be referenced in variable expansions outside of the begin block.

Note that all variable expansions are interpreted as Javascript, even if they have been defined in a begin coffee block. See section variable expansion.

Also note that begin blocks are cumulative meaning that all begin blocks will use the same environment to place functions and variables.

The addition of begin blocks containing javascript code was inspired by PYM - A Macro Preprocessor Based on Python.


The end keyword ends the nearest begin, if or filter block.

if <condition>, elif <condition>, else ... end

Evaluate <condition> as Javascript. If it evaluates to a truthy value the lines immediately following this branch of the conditional up to the next else, elif or end statement are included in the output.

#if 'foo' === 'bar'
    Not shown
#elif 1
    This is shown
    Not shown

include <filename>

Attempt to include the file in <filename>. If the file does not exist try if the file can be found in parent paths up to the root.

  • TODO add include! <filename> to include raw data?

run <command>

Run the external program in <command> almost[1] as if it were invoked on the command line.

filter <command|!function> ... end

Run an external program like the run command and pipe the lines until end through this command:

#filter nl

Note that filtering through an external program is subject to shell escape limitations. [1]

If the <command> is prefixed with an exclamation mark, command is taken to be a function to filter commands through as in the example at the top of this document.

[1] This is not quite correct at the moment because shell expansion is not taken correctly into account.

Variable expansions


Run code as javascript. The code may reference variables and functions declared in begin ... end blocks. This code may reference variables and functions that are globals in node as well as some special variables:

  • __version The version of jaff
  • __argv The arguments passed to the script
  • __quotes Change code reference quoting characters

The default quoting of a reference is {} but may be changed using the __quotes() function in a begin block:

__quotes '[[', ']]'

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