How does linux determine the current user?

In linux, how does one determine whether the current user is root in linux? Determine whether the current user is the root www.2cto.com Shell code # ------------------------------ root user check ------------------- start if ['id-U'-ne 0]; then echo "Please re-run $ {this_file} as root. "exit 1 fi # ------------------------------ root user check ------------------- end if [x" $ USER "= x" "]; then if id | grep" ^ uid = 0 (root) ">/dev/null; then: else echo" Please rerun 'basename $ 0' as root. "exit 1 fi else if [! X "$ USER" = x "root"]; then echo "Please rerun 'basename $ 0' as root. "exit 1 fi #==================== check current user start ======================== = if [! $ USER = root]; then echo "Please rerun this script as root. "exit 1 fi #===================== check current user end ============================ #! /Bin/sh if id | grep "^ uid = 0 (root)">/dev/null; then echo is root else echo not root fi check_user () {if [$ UID-ne 0]; then echo "Current user is not ROOT. "echo" Please rerun the installer as Root user. "exit 11 fi}