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Conch is a simple but very flexible Clojure library for shelling out to external programs. It is to be used as an alternative to working directly with the java.lang.Process API and as a more flexible alternative to is designed to produce quick one-off processes. There is no way to interact with a running process over time. I've found myself needing to shell out and stream the output of an external process to things in real time and I wasn't able to do that with


In Leiningen:

:dependencies [[conch "0.2.1"]]


Conch is pretty simple. You spin off a process with proc.

user=> (def p (sh/proc "cat"))
user=> p
{:out #<BufferedInputStream>, :in #<BufferedOutputStream>, :err #<DeferredCloseInputStream java.lang.UNIXProcess$DeferredCloseInputStream@58e5f46e>, :process #<UNIXProcess java.lang.UNIXProcess@18f42160>}

When you create a process with proc, you get back a map containing the keys :out, :err, :in, and :proc.

  • :out is the process's stdout.
  • :err is the process's stderr.
  • :in is the process's stdin.
  • :process is the process object itself.

Conch is more flexible than because you have direct access to all of the streams and the process object itself.

So, now we have a cat process. This is a unix tool. If you run cat with no arguments, it echos whatever you type in. This makes it perfect for testing input and output.

Conch defines a few utility functions for streaming output and feeding input. Since we want to make sure that our input is going to the right place, let's set up a way to see the output of our process in realtime:

user=> (future (sh/stream-to-out p :out))
#<core$future_call$reify__5684@77b5c22f: :pending>

The stream-to-out function takes a process and either :out or :err and streams that to System/out. In this case, it has the effect of printing everything we pipe into our cat process, since our cat process just outputs whatever we input.

user=> (sh/feed-from-string p "foo\n")

The feed-from-string function just feeds a string to the process. It automatically flushes (which is why this prints immediately) but you can stop it from doing that by passing :flush false.

I think our cat process has lived long enough. Let's kill it and get its exit code. We can use the exit-code function to get the exit code. However, since exit-code stops the thread and waits for the process to terminate, we should run it in a future until we actually destroy the process.

user=> (def exit (future (sh/exit-code p)))

Now let's kill. R.I.P process.

user=> (sh/destroy p)

And the exit code, which we should be able to obtain now that the process has been terminated:

user=> @exit

Awesome! Let's go back to proc and see what else we can do with it. We can pass multiple strings to proc. The first string will be considered the executable and the rest of them the arguments to that executable.

user=> (sh/proc "ls" "-l")
{:out #<BufferedInputStream>, :in #<BufferedOutputStream>, :err #<DeferredCloseInputStream java.lang.UNIXProcess$DeferredCloseInputStream@5f873eb2>, :process #<UNIXProcess java.lang.UNIXProcess@2825491d>}

Here is an easy way to get the output of a one-off process like this as a string:

user=> (sh/stream-to-string (sh/proc "ls" "-l") :out)
"total 16\n-rw-r--r--  1 anthony  staff  2545 Jan 24 16:37\ndrwxr-xr-x  2 anthony  staff    68 Jan 19 19:23 classes\ndrwxr-xr-x  3 anthony  staff   102 Jan 19 19:23 lib\n-rw-r--r--  1 anthony  staff   120 Jan 20 14:45 project.clj\ndrwxr-xr-x  3 anthony  staff   102 Jan 20 14:45 src\ndrwxr-xr-x  3 anthony  staff   102 Jan 19 16:36 test\n"

Let's print that for readability:

user=> (print (sh/stream-to-string (sh/proc "ls" "-l") :out))
total 16
-rw-r--r--  1 anthony  staff  2545 Jan 24 16:37
drwxr-xr-x  2 anthony  staff    68 Jan 19 19:23 classes
drwxr-xr-x  3 anthony  staff   102 Jan 19 19:23 lib
-rw-r--r--  1 anthony  staff   120 Jan 20 14:45 project.clj
drwxr-xr-x  3 anthony  staff   102 Jan 20 14:45 src
drwxr-xr-x  3 anthony  staff   102 Jan 19 16:36 test

So, that's the ls of the current directory. I ran this REPL in the conch project directory. We can, of course, pass a directory to ls to get it to list the files in that directory, but that isn't any fun. We can pass a directory to proc itself and it'll run in the context of that directory.

user=> (print (sh/stream-to-string (sh/proc "ls" "-l" :dir "lib/") :out))
total 6624
-rw-r--r--  1 anthony  staff  3390414 Jan 19 19:23 clojure-1.3.0.jar

You can also pass a or anything that can be passed to

We can also set environment variables:

user=> (print (sh/stream-to-string (sh/proc "env" :env {"FOO" "bar"}) :out))

The map passed to :env completely replaces any other environment variables that were in place.

Finally, there a couple of low-level functions for streaming from and feeding to a process. They are stream-to and feed-from. These functions are what the utility functions are built off of, and you can probably use them to stream to and feed from your own special places.

You might want to fire off a program that listens for input until EOF. In these cases, you can feed it data for as long as you want and just tell it when you are done. Let's use pygmentize as an example:

user=> (def proc (sh/proc "pygmentize" "-fhtml" "-lclojure"))
user=> (sh/feed-from-string proc "(+ 3 3)")
user=> (sh/done proc)
user=> (sh/stream-to-string proc :out)
"<div class=\"highlight\"><pre><span class=\"p\">(</span><span class=\"nb\">+ </span><span class=\"mi\">3</span> <span class=\"mi\">3</span><span class=\"p\">)</span>\n</pre></div>\n"

When we call done, it closes the process's output stream which is like sending EOF. The process processes its input and then puts it on its input stream where we read it with stream-to-string.

Other options

All of conch's streaming and feeding functions (including the lower level ones) pass all of their keyword options to It can take an :encoding and :buffer-size option. Guess what they do.

Key names

You might notice that the map that proc returns is mapped like so:

  • :in -> output stream
  • :out -> input stream

I did this because swapping them feels counterintuitive. The output stream is what you put :in to and the input stream is what you pull :out from.


Copyright (C) 2012 Anthony Grimes

Distributed under the Eclipse Public License, the same as Clojure.

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