在UNIX系統(tǒng)中，Shell程序（ ）實現(xiàn)顯示用戶主目錄以及當(dāng)前命令的進(jìn)程標(biāo)識符。
A、echo UserHome directory: $LOGNAME
echo Current shell’s PID: $$
B、echo UserHome directory: $HOME
echo Current shell’s PID: $@
C、echo UserHome directory: $LOGNAME
echo Current shell’s PID: $@
D、echo UserHome directory: $HOME
echo Current shell’s PID: $$