nproc - print the number of processing units available
SYNOPSIS
nproc [OPTION]...
DESCRIPTION
Print the number of processing units available to the current process,
which may be less than the number of online processors
--all print the number of installed processors
--ignore=N
if possible, exclude N processing units
--help display this help and exit
--version
output version information and exit
e.g:make -j$(nproc) It can speed up compilation