Time

Navigate to the basics/time directory in the examples repo.

Take a look at the get-time.sh bash script.

#!/bin/bash

# Ref: See section on Daytime Protocol
# https://www.nist.gov/pml/time-and-frequency-division/time-distribution/internet-time-service-its

# first arg is frequency, default is 5 seconds
freq=${1:-5}
echo "Fetch UTC(NIST) time every ${freq} seconds..."

while true; do
  if dt=$(cat </dev/tcp/time.nist.gov/13 | tail -n 1); then
    if [[ "$dt" =~ .*"UTC(NIST)".* ]]; then
      d=$(echo "$dt" | cut -d " " -f 2)
      t=$(echo "$dt" | cut -d " " -f 3)
      echo "$d $t"
    fi
  fi
  sleep "${freq}"
done

Using a loop, this program reads a TCP socket on a Linux-based system for fetching the current time from the NIST Internet time service and prints a formatted version of the response to the terminal.

The program accepts an argument specifying the frequency for fetching the time in seconds, defaulting to 5 if not provided.

You can test the program on your own machine.

chmod +x get-time.sh
./get-time.sh 1

Fetch UTC(NIST) time every 1 seconds...
21-09-02 18:15:02
21-09-02 18:15:03
21-09-02 18:15:04
21-09-02 18:15:12
^C

😲 Note

If this doesn’t work on your machine, don’t worry – you’ve just experienced one of the big reasons why containers are so useful!

Continue to the next page to containerize the time app so we can run it in a standard Linux environment that will work everywhere Docker is supported.