Some minor changes to Bash

[jkstill]

Some simple stuff; directory check, existence of perf.

Also, use python2. I am not sure if this will work everywhere, though on Oracle Linux I found it necessary.

Added Query-Examples.md

These are changes made in Dec 2020, and I promptly forgot about them for 4 months.

Hopefully they do not cause conflicts.

[tanelpoder]

Let's not hardcode to python2 as many systems don't have python2 anymore. It's the role of /usr/bin/env to find out whatever is the installed version of python in the system.

[tanelpoder]

This ./xcapture call assumes that user has changed their working dir to where xcapture binary is located, this won't always be the case in reality when people run it from cmdline.

[tanelpoder]

There was actually a reason why I used hardcoded /usr/bin/perf - on systems with different kernels (like Oracle linux kernel on RHEL clones), sometimes perf report used the wrong perf and used a legacy text UI output. However, in this script, we use perf for just sampling the kernel, so it might be fine. I will do some tests to make sure.