skip to navigation
skip to content

whack 0.6.9

Utility for installing binaries from source with a single command

Latest Version: 0.7.0

Whack allows binaries such as nginx and node.js to be installed with a single command. For instance, to install nginx to ~/apps/nginx:

whack install git+ ~/apps/nginx

On the first installation, the application is compiled and copied to the target directory. On subsequent installations, a cached version of the application is copied to the target directory.


Before you can use Whack, you need to install a utility called whack-run. You can download whack-run from GitHub:

$ curl -L > whack-run-1.0.0.tar.gz
$ tar xzf whack-run-1.0.0.tar.gz
$ cd whack-run-1.0.0
$ make

And as root:

# make install

This installs the binary whack-run to /usr/local/bin. Once whack-run has been installed, you can install Whack as an ordinary Python package:

pip install whack

How does Whack work?

Many Linux applications can be compiled and installed by running the following commands, or similar:

$ ./configure
$ make
$ make install

This usually installs the application under /usr/local. However, sometimes we want to install isolated instances of an application without being root. For instance, if we’re developing a web application that uses Apache, it’s helpful to have an isolated installation of Apache. We can change the installation prefix when running ./configure:

$ ./configure --prefix=/home/user/projects/web-app/apache
$ make
$ make install

While this works, it requires us to re-compile the application whenever we want to install it in a different location. Depending on the application, compilation can take a quite a while.

Whack solves this problem by using unshare and mount to change the filesystem for a single process. Each application is compiled with its prefix set to /usr/local/whack. Before running the binary for an application, Whack uses the unshare syscall to create a private mount namespace. This means that any mount calls only have visible effects within that process. We then mount the directory that the application was installed in onto /usr/local/whack, and exec the binary.

For instance, say we’ve installed nginx to ~/web-app/nginx by running

whack install git+ \
    -p nginx_version=1.2.6 ~/web-app/nginx

The actual nginx binary can be found in ~/web-app/nginx/.sbin (note that the binary is in a directory called .sbin, not sbin). If we try to run ~/web-app/nginx/.sbin/nginx directly, we’ll get an error:

$ ~/web-app/nginx/.sbin/nginx
nginx: [alert] could not open error log file: open() "/usr/local/whack/logs/error.log" failed (2: No such file or directory)
2013/02/18 11:25:17 [emerg] 11586#0: open() "/usr/local/whack/conf/nginx.conf" failed (2: No such file or directory)

nginx expects to be installed under /usr/local/whack, but it’s actually installed under ~/web-app/nginx. To run nginx successfully, we need to use whack-run:

$ whack-run ~/web-app/nginx ~/web-app/nginx/.sbin/nginx

When using whack-run, the following happens:

  1. whack-run calls unshare(CLONE_NEWNS), creating a private mount namespace.
  2. whack-run mounts ~/web-app/nginx onto /usr/local/whack. Since we called unshare earlier, this mount is only visible to this process.
  3. whack-run drops its user and group privileges. whack-run is installed with the setuid bit set so it can call unshare and mount.
  4. whack-run calls exec with the arguments it was passed i.e. exec ~/web-app/nginx/.sbin/nginx

Using whack-run to run nginx is a bit tedious. However, we can call ~/web-app/nginx/sbin/nginx directly (instead of ~/web-app/nginx/.sbin/nginx), which will call whack-run with appropriate arguments.

Although whack-run has the setuid bit set, it only uses root privileges to call unshare and mount. After that, user and group privileges are dropped.

File Type Py Version Uploaded on Size
whack-0.6.9.tar.gz (md5) Source 2013-05-09 12KB