Stutter: the anti-grep

May 1, 2017

I’m going to introduce stutter, a command line tool for generating strings. I’ll first show some examples, then talk a bit about the performance and finally about the implementation (for the Haskell friendly reader).

This short introduction was motivated by a recent post on reddit where the author was looking for something similar (though packaged as a library).


In its essence, stutter does the opposite of what grep does. You pass grep a definition string and it will find all the matches in the input you provide it. You pass stutter a definition string and it will generate an output based on that string:

$ echo 'foo\nbar\nbaz' | grep -E 'foo|bar|baz'

$ stutter 'foo|bar|baz'

The most basic usage would be to use stutter as an echo clone:

$ echo 'foo'

$ stutter 'foo'

Though stutter will go further, and allow you to define the output strings in various ways. For instance, you might want either foo or bar. You can use the sum operator | as in the following snippet:

$ stutter 'foo|bar'

Both foo and bar will appear, but not together. To go back to the first example, let’s simply drop echo so we can better see the parallel between grep and stutter:

$ stutter 'foo|bar|baz' | grep -E 'foo|bar|baz'

There is an equivalent product operator # if you need both foo and bar:

$ stutter 'foo#bar'

In this example you could have simply run stutter 'foobar' to get the same result, but sometimes the # operator can come in handy (see for instance the fold operator {|} in the readme’s examples). Let’s see another important construct, namely the grouping operator (). Say you want either foo and bar or foo and baz:

$ stutter 'foo(bar|baz)'

Two things to notice:

  1. The () operator is optional for groups of one element. It was not necessary to write (foo)(bar|baz). stutter will assume a group is ending on the first operator occurrence. If you need to use operator characters, you can escape them with a backslash (\) character.
  2. The # operator is used by default when no other operator is found. It was not necessary to write foo#(bar|baz).

There are more examples in the stutter readme showcasing things like repetition, sourcing files and stdin, character ranges, and more.


As much as possible stutter tries to only use constant memory. The heap should never grow as long as you stick to “pure” string generations, even if you perform products of infinite lists of words. However the situation is different with “unpure” string generation, for instance when sourcing words from stdin. stutter will do its best to keep the memory usage low. The following for instance will zip stdin with itself:

$ cat some_big_file
$ cat some_big_file | stutter '(@-)$(@-)'

The zip operator $ will zip two outputs together, and the @- group will be replaced by the content read from stdin. The example above will still run in constant space, because stutter realizes that it can discard a line from stdin as soon as it was printed. The situation is different with products:

$ cat some_big_file | stutter '(@-)#(@-)'

Here stutter must keep the entirety of the group to the right of the product operator #, because it will be repeated for each element produced by the group to the left of the operator. The reason is that stutter cannot tell stdin to simply “rewind” and start again from the beginning of the input. However, consider the slightly different example:

$ stutter '(@some_big_file)#(@some_big_file)'

This command produces the exact same output, but reads some_big_file directly. In this case stutter will prefer reading the file from disk over and over again rather than keeping it in memory. This is something you should keep in mind if your operation has to perform many disk reads and that your hard-drive is slow. If the live data is not too big consider threading it through stdin first (I might add an option to enable the behavior when reading from a file).

For most operations stutter should outperform commands composed from several different programs and shell built-ins. The reason is that stutter runs in a single process (which avoids context switches and inter-process communication), and that stutter is optimized for specific kinds of commands. For instance on my machine printing the a character to stdout a million times takes over three seconds with echo while it only takes half a second with stutter:

$ time (stutter 'a+' | head -n 1000000 | wc -l)
( stutter 'a+' | head -n 1000000 | wc -l; ) ⤶
        0.62s user 0.13s system 145% cpu 0.515 total
$ time (while true; do echo 'a'; done | head -n 1000000 | wc -l)
( while true; do; echo 'a'; done | head -n 1000000 | wc -l; ) ⤶
        2.32s user 3.32s system 154% cpu 3.658 total


The stutter code is spread across two tiny modules and is basically a thin wrapper around the excellent conduit library. The first module, Stutter.Parser, is dedicated to parsing the commands. The only exported function is parseGroup:

parseGroup :: Atto.Parser ProducerGroup
parseGroup = ...

From a command input by the user stutter will try to extract a ProducerGroup, which is a tree-like data-structure defined in the second module, Stutter.Producer. The role of ProducerGroup is that of an AST:

data ProducerGroup_ a
  = PSum (ProducerGroup_ a) (ProducerGroup_ a)
  | PProduct (ProducerGroup_ a) (ProducerGroup_ a)
  | PZip (ProducerGroup_ a) (ProducerGroup_ a)
  | PRepeat (ProducerGroup_ a)
  | PRanges [Range]
  | PFile FilePath
  | PStdin a
  | PText T.Text
  deriving (Eq, Show, Functor, Foldable, Traversable)

The meat of stutter is in the produceGroup function, also defined in Stutter.Producer. This turns a parsed ProducerGroup into a conduit Source:

  :: (MonadIO m, MonadResource m)
  => ProducerGroup
  -> Source m T.Text
produceGroup (PZip g g') =
    zipSources (produceGroup g) (produceGroup g')
    .| (uncurry (<>))

If you look through the code you will see that produceGroup doesn’t do anything fancy, it just leverages the conduit capabilities.


I hope you enjoyed this quick introduction to stutter and that, one day, it might help you do your job faster (like for instance recovering a forgotten password). Please read the contributing section of the README, there’s a lot you can do to help the development and improve stutter, the anti-grep.

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