Odd shape trim path animation

[magdagogo]

Hi! I want to make background animation with a bunch of lines. I wanted to use Trim lines to create smooth animation. Is there any way to change shape of end and start of line? In attachments my shape and stages of animation (shape appears from 0% scale to 100% > then the shape lengthens (without changing angles) white moving > then it is reduced (white moving to) > and disappears (scale 100% to 0%). 
please help! 

[Roei Tzoref]

You could use different layers, one for the head/tail an other for the rest. The trail can be made with repeater operator if it's a shape, or echo effect, or motion trail expression (look it up) 

[Rick Gerard]

I would set up your line like this:

A rectangle for the bar, two duplicates of the rectangle for the left and right side of the bar, to squares rotated 45º to clip off the ends of the bars, a little math to set the shapes in the right position using Anchor Point, a couple of expressions to tie the diamonds (rotated squares) to the left and right bar, and 4 keyframes.  Then all you need to do is add the Merge Paths operator and set the method to Subtract. I've used this setup many times for some very odd shapes. Here's the project file, and here's what that looks like:

The project took about 6 minutes to set up.

If you need more copies set up duplicators add a Repeater or duplicate the shape layer.