Debian :: Create A Script For Screenshots?

I want to create a script with the command

sleep 3; xwd -root | xwdtopnm | pnmtopng > capture.png

for creating screenshots. But I'm having problems with creating its structure. I'd like it to:

1- Check if there's a capture.png file on /home

2- If there is, check if there is a capture1.png file (and so on, until it reaches a captureNUMBER.png file that does not exist)
If there is not, proceed

3- Take the snapshot after three seconds (so that I can go to the window I want)

4- Create a captureNUMBER.png file at /home

Here's my attempt (I don't know how to automatically change the name of the file, so my first idea was to create keep this structure until capture10.png)

#!/bin/bash
cd /home
if[ -f capture.png ]

[code]....