bash script to check if the current git branch = “x”

Use git rev-parse --abbrev-ref HEAD to get the name of the current branch.

Then it’s only a matter of simply comparing values in your script:

BRANCH="$(git rev-parse --abbrev-ref HEAD)"
if [[ "$BRANCH" != "x" ]]; then
  echo 'Aborting script';
  exit 1;
fi

echo 'Do stuff';