When dealing with time synchronization on Unix/Linux systems, you'll primarily encounter two approaches: the ntpd daemon and the ntpdate utility. While both serve the same ultimate purpose, their methodologies differ significantly.
ntpdate is a one-shot time setting tool that queries an NTP server and immediately adjusts the system clock. A typical cron job might look like:
# Run ntpdate every 6 hours
0 */6 * * * /usr/sbin/ntpdate pool.ntp.org
Pros:
- Simple to implement and understand
- Low resource usage (runs only when called)
- Immediate time correction
Cons:
- Causes time jumps which can disrupt applications
- No clock drift compensation between runs
- Not suitable for systems requiring nanosecond precision
The NTP daemon (ntpd) runs continuously and implements complex algorithms to:
# Sample ntpd configuration (/etc/ntp.conf)
server 0.pool.ntp.org
server 1.pool.ntp.org
server 2.pool.ntp.org
server 3.pool.ntp.org
driftfile /var/lib/ntp/ntp.drift
Pros:
- Smooth, gradual time adjustments
- Compensates for clock drift
- Can use multiple servers for redundancy
- Provides better long-term stability
Cons:
- More complex configuration
- Continuous resource usage
- Initial synchronization may be slower
For most modern servers, ntpd (or its newer counterpart chronyd) is the recommended approach. However, ntpdate can still be useful in certain scenarios:
- Initial time setting during system boot
- Virtual machines where continuous time sync might be problematic
- Embedded systems with limited resources
For systems requiring high precision, consider this hybrid approach:
# Initial time set (in rc.local or similar)
/usr/sbin/ntpdate -b pool.ntp.org
# Then start ntpd
/usr/sbin/ntpd -g -x -c /etc/ntp.conf
The -g flag allows large initial corrections, while -x prevents time slewing that might affect some applications.
Regardless of which method you choose, monitoring is crucial. These commands help verify synchronization:
# For ntpd
ntpq -pn
# For ntpdate (check system logs)
grep ntpdate /var/log/syslog
When maintaining accurate system time, Linux administrators have two primary tools at their disposal: NTPD (Network Time Protocol Daemon) and ntpdate. The fundamental distinction lies in their operational approach:
- ntpdate: A one-shot time synchronization tool that executes manual adjustments
- NTPD: A persistent daemon that continuously maintains clock accuracy
NTPD implements a sophisticated algorithm that gradually adjusts the system clock through small, frequent corrections. This prevents sudden time jumps that could disrupt time-sensitive applications. Here's a basic NTPD configuration example:
# /etc/ntp.conf server 0.pool.ntp.org iburst server 1.pool.ntp.org iburst server 2.pool.ntp.org iburst server 3.pool.ntp.org iburst driftfile /var/lib/ntp/ntp.drift restrict default kod nomodify notrap nopeer noquery
In contrast, ntpdate performs immediate clock setting, which can be useful during system startup but may cause problems for running applications. A typical crontab implementation might look like:
0 * * * * /usr/sbin/ntpdate -u pool.ntp.org > /dev/null 2>&1
The NTP pool architecture provides built-in redundancy. NTPD automatically handles server failures by:
- Continuously monitoring all configured servers
- Calculating weighted averages from multiple sources
- Dynamically selecting the most reliable time sources
Modern Linux systems with NTPD can maintain accuracy within milliseconds. The daemon tracks clock drift in the driftfile, allowing it to compensate even when network connectivity is temporarily lost. This is particularly valuable for:
- Distributed systems requiring tight synchronization
- Financial transaction processing
- Scientific computing applications
For production environments, I recommend using NTPD with the following best practices:
# Systemd service management systemctl enable ntpd systemctl start ntpd # Verify synchronization status ntpq -pn # Check system clock accuracy timedatectl status
For legacy systems or specific use cases where ntpdate is preferred, consider adding post-sync hooks to restart critical services:
0 * * * * /usr/sbin/ntpdate -u pool.ntp.org && systemctl restart critical-service