When starting a program with the nice command without any additional parameters, which nice level is set for the resulting process?

-10 raises a process above the default priority and requires root privileges; it must be explicitly specified with nice -n -10 and is never applied as a default.

0 is the niceness inherited by a process launched normally without nice - invoking nice applies an increment on top of the existing value, not zero.

The POSIX specification and GNU coreutils implementation both define the default adjustment for nice as +10 when no explicit value is supplied. Niceness ranges from -20 (highest CPU priority) to +19 (lowest), so +10 reduces the process's scheduling priority below the baseline of 0. This makes the process yield CPU time more readily without requiring any privilege escalation.

20 is not the default increment; it is sometimes confused with the upper end of the niceness scale, but the actual default adjustment defined in the standard is +10.