Where can I learn about the default language used in this script?

[elvitin]

vscode-dev-containers/containers/python-3/.devcontainer/library-scripts/common-debian.sh

Line 17 in f28d3ba

if [ "$(id -u)" -ne 0 ]; then

Where can I learn about the default language used in this script?

I can't understand the correct source for the scripts used in the docker, sh or bash operating systems?