Using ps -aux reveals current processes, all of which are listed in /proc/ as /proc/(pid)/, by calling cat /proc/(pid)/fd/0 it prints anything that is found in the standard output of that process I think. So perhaps,
/proc/(pid)/fd/0 - Standard Output File
/proc/(pid)/fd/1 - Standard Input File
/proc/(pid)/fd/2 - Standard Error File
for example
But only worked this well for /bin/bash other processes generally had nothing in 0 but many had errors written in 2