There's a difference in the handling of environment variables between the two implementations.
su provided by login/src:shadow resets HOME LOGNAME PATH SHELL USER su provided by util-linux resets HOME SHELL I changed: th's PATH to "/usr/sbin:/usr/bin:/sbin:/bin" root's PATH to "/root/bin:/usr/sbin:/usr/bin:/sbin:/bin" root's shell to "/bin/mksh" Output: Debian 9 th@localhost ~ $ echo $HOME $LOGNAME $PATH $SHELL $USER /home/th th /usr/sbin:/usr/bin:/sbin:/bin /bin/bash th th@localhost ~ $ su root@localhost /home/th # echo $HOME $LOGNAME $PATH $SHELL $USER /root root /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin /bin/mksh root root@localhost /home/th # grep SUPATH /etc/login.defs # Three items must be defined: MAIL_DIR, ENV_SUPATH, and ENV_PATH. ENV_SUPATH PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin root@localhost /home/th # grep PATH= .profile PATH="/root/bin:/usr/sbin:/usr/bin:/sbin:/bin" RHEL 7 th@localhost ~ $ echo $HOME $LOGNAME $PATH $SHELL $USER /home/th th /usr/sbin:/usr/bin:/sbin:/bin /bin/bash th th@localhost ~ $ su root@localhost /home/th # echo $HOME $LOGNAME $PATH $SHELL $USER /root th /usr/sbin:/usr/bin:/sbin:/bin /bin/mksh th root@localhost /home/th # grep PATH= .profile PATH="/root/bin:/usr/sbin:/usr/bin:/sbin:/bin"
