Why is pidof not working?

In the output of ps aux, I can see the process just fine: # ps aux | grep diff root 7787 28.7 0.0 9368 4516 pts/3 D+ 13:56 20:33 diff -qr mnt/mnt/md/ mnt/mnt2/ root 13130 0.0 0.0 6144 876 pts/4 S+ 15:07 0:00 grep diff But pidof claims not to be able to find anything: # pidof diff # echo $? 1 Looking at the man page, there is no info on what to do when lost a process, pidof has. /proc/7787/exe is a symlink to /usr/bin/diff and /usr/bin/diff itself is a regular file and an ELF. According to the man page, this ought to match.