$ du -h /var/log/syslog
110G /var/log/syslog
$ sudo truncate -s 0 /var/log/syslog
$ du -h /var/log/syslog
0 /var/log/syslog
$ du -h /var/log/syslog
110G /var/log/syslog
$ sudo truncate -s 0 /var/log/syslog
$ du -h /var/log/syslog
0 /var/log/syslog