Using native libraries in bash scripts

[Tavis Ormandy]

I thought this group might be interested in a fun project 
I worked on a few years ago. It's a module for bash that 
lets scripts load and interact with shared libraries.

https://github.com/taviso/ctypes.sh/wiki

Naturally, everything about this is non-portable, bash-
specific, and probably off-topic... it's just for fun :)

I saw a thread about calculating crc32 in bash, with 
ctypes.sh you could do this:

$ dlopen libz.so
0x7ffff626a860
$ buf="string to check"

$ dlcall -r long crc32 long:0 "$buf" ${#buf}
long:1206832624

Looks correct!

You can create structures and unions too, which lets you 
interact with more complex apis.

The Python ctypes module requires you to define all the 
structs manually, but in most cases ctypes.sh can create 
them for you automatically! This sounds crazy, but it 
usually works.

Here is an example, automatically creating a struct stat:

$ struct -m statbuf stat passwd

Now $statbuf is a pointer that you can pass to native 
APIs. It just looks like this in bash:

$ echo $statbuf
pointer:0x7f6ebb82e9a0

And $passwd is a bash associative array that contains all 
the structure members, so ${passwd[st_ino]} would be the 
inode number.

Here is an example, pass $statbuf to stat() and get it 
filled in (on Linux the name of the symbol is __xstat).

$ dlcall __xstat 0 "/etc/passwd" $statbuf

# This copies the native values into the bash array so you 
# can examine them in your script.
$ unpack $statbuf passwd

$ echo ${passwd[st_size]}
long:1639

I guess most people have used something like the dialog 
command to make interactive shell scripts, but ctypes.sh 
lets you use all of the ncurses API, or even the GTK+ API.

I ported the GTK+3 Hello World to bash, here's the 
original C version:

https://developer.gnome.org/gtk3/stable/gtk-getting-
started.html

And here's my bash port:

http://github.com/taviso/ctypes.sh/blob/master/test/gtk.sh

This also makes some things possible that you normally 
couldn't do in shell. For example, I don't think there's 
any way to service multiple file descriptors efficiently 
in bash, you would have to use `read -u $i -t 0` in a busy 
loop.

With ctypes.sh, you can have access to things like the 
select() system call.

There are more examples here:

https://github.com/taviso/ctypes.sh/tree/master/test

Tavis.

-- 
 _o)
 /\\  _o)  _o)
_\_V _( ) _( ) :wq!