tail -f & truncate

tail -f

Example:

tail -f /var/log/syslog

From man tail

-f, --follow[={name|descriptor}]
        output appended data as the file grows;

Seen in https://www.youtube.com/watch?v=NsK7OPlK94U.

truncate

if file.txt is

a
b
c
d
e
f

then after

truncate -s 6 file.txt

it will be

a
b
c