### TL;DR

```
# Turn i2c on
sudo raspi-config nonint do_i2c 0
# Turn i2c off
sudo raspi-config nonint do_i2c 1
```

### Console log to prove it works

```
$ # First see what it looks like with i2c turned off...
$ dmesg | grep i2c
[    2.344954] i2c /dev entries driver
$ lsmod | grep i2c
i2c_dev                 5859  0
$ cat /boot/config.txt | grep i2c
dtparam=i2c_arm=off
$
$ # Now turn it on like this...
$ sudo raspi-config nonint do_i2c 0
$
$ # Check what changed...
$ dmesg | grep i2c
[    2.344954] i2c /dev entries driver
[  231.274035] bcm2708_i2c 3f804000.i2c: BSC1 Controller at 0x3f804000 (irq 83) (baudrate 100000)
$ cat /boot/config.txt | grep i2c
dtparam=i2c_arm=on
$ lsmod | grep i2c
i2c_bcm2708             4834  0
i2c_dev                 5859  0
$
$ # Now turn it back off...
$ sudo raspi-config nonint do_i2c 1
$
$ # and check what changed...
$ dmesg | grep i2c
[    2.344954] i2c /dev entries driver
[  231.274035] bcm2708_i2c 3f804000.i2c: BSC1 Controller at 0x3f804000 (irq 83) (baudrate 100000)
$ lsmod | grep i2c
i2c_bcm2708             4834  0
i2c_dev                 5859  0
$ cat /boot/config.txt | grep i2c
dtparam=i2c_arm=off
```

### Why it works
raspi-config is a big bash script that lives at `/usr/bin/raspi-config`. Basically,
if you start with the `nonint` flag, it will interpret additional arguments as calls
to functions with the script. If you search the script for `do_interface_menu()`, it
should give you an idea of what's possible.

The `0` and `1` values correspond to an `if` block in `do_i2c()` that's checking the
value of `$RET`. When the the `do_i2c` sees that `$INTERACTIVE` is true, it grabs
the next argument after `do_i2c` with `RET=$1`, then that gets used to decide if i2c
should be enabled or disabled.