If you want to see what executable the process 1234 is running:
ls -l /proc/1234/exe
If you want to see what files that process has open:
ls -l /proc/1234/fd
If you want to see the full command line with arguments:
tr '\0' '\n' </proc/1234/cmdline
If you prefer to use commands, lsof
-p1234
shows all the files the process has open. ps
uww 1234
shows various pieces of information about process 1234 including the full command line.