Special Parameters之一

if [ $? != 0 ]; then      HOST_ARCH=x86      log "HOST_ARCH=$HOST_ARCH (32-bit userland detected)"    fi

The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.

?

Expands to the exit status of the most recently executed foreground pipeline.

A pipeline is a sequence of simple commands separated by one of the control operators ‘|’ or ‘|&’.

pipeline其实就是command的集合，通过 ‘|’ or ‘|&’将command串成line