How to generate a uuid in shell script

Have you tried

uuidgen

It's installed out-of-the-box on freeBSD systems like MacOS.

On Fedora, CentOS, and RHEL, get it from the util-linux package (CentOS6 has it in util-linux-ng). On debian, get it with sudo apt-get install uuid-runtime. On other linux systems, try looking for the e2fsprogs package.

From this rather comprehensive article.
http://0pointer.de/blog/projects/ids.html
(btw, that's the blog of Lennart Poettering)

Linux offers a kernel interface to generate UUIDs on demand, by reading from /proc/sys/kernel/random/uuid

This is a very simple interface to generate UUIDs. That said, the logic behind UUIDs is unnecessarily complex and often it is a better choice to simply read 16 bytes or so from /dev/urandom