InstalationNDOutils et pb avec cmd: tail-f

Bonjour,
SVP je suis debutant souslinux et j'ai un pb avec la syntaxe tail-f; et je voudrais sortir pour retourner en mode root#. cmt faire?
voici ce que ça affiche:
root@xxxxx-Ubuntu:~# tail -f /usr/local/nagios/var/nagios.log
[1473162902] nerd: Channel opathchecks registered successfully
[1473162902] nerd: Fully initialized and ready to rock!
[1473162902] wproc: Successfully registered manager as @wproc with query handler
[1473162902] wproc: Registry request: name=Core Worker 23658;pid=23658
[1473162902] wproc: Registry request: name=Core Worker 23659;pid=23659
[1473162902] wproc: Registry request: name=Core Worker 23660;pid=23660
[1473162902] wproc: Registry request: name=Core Worker 23661;pid=23661
[1473162902] Error: Could not load module '/usr/local/nagios/bin/ndomod.o' -> /usr/local/nagios/bin/ndomod.o: undefined symbol: hostdependency_list
[1473162902] Error: Failed to load module '/usr/local/nagios/bin/ndomod.o'.
[1473162902] Error: Module loading failed. Aborting.