Bash script to iterate through a specific time range, one second at a time - bash

Question

I am trying to create some sort of loop in bash that will iterate through a specific time range, one second at a time.
At each interval, it will perform a duty with that timestamp I.e. "Wed Mar 2 12:00:03 CDT 2018"
I am having a hard time wrapping my head around how to make a loop that will iterate every second of time, and when it hits 60 seconds, update the minute, etc.
Thoughts? This seems obvious but the right syntax escapes me.

Answer 1

this is one way of doing it, delegate time computations to date
$ for i in {1..10}; do date -d "+$i seconds"; done
Sun Mar 11 20:40:57 UTC 2018
Sun Mar 11 20:40:58 UTC 2018
Sun Mar 11 20:40:59 UTC 2018
Sun Mar 11 20:41:00 UTC 2018
Sun Mar 11 20:41:01 UTC 2018
Sun Mar 11 20:41:02 UTC 2018
Sun Mar 11 20:41:03 UTC 2018
Sun Mar 11 20:41:04 UTC 2018
Sun Mar 11 20:41:05 UTC 2018
Sun Mar 11 20:41:06 UTC 2018
Sun Mar 11 20:41:07 UTC 2018
if you want to iterate up to a certain time, change the for loop to a while loop and a counter.

Answer 2

To make a loop with time you need to format date/time in such a way that can be comparable. The most easy trick is to transform date to something that will look like an integer and you then use lt,le,eq,gt,ge operators of bash.
Consider this :
$ date
Mon Mar 12 00:16:29 EET 2018 #this format/data type is not comparable by bash
$ date +%Y%m%d%H%M%S
20180312001629 #this is an integer representing the current time
This is a sample loop from now up to a specific time point:
stop=$(date +%Y%m%d%H%M%S --date "2018-03-12 00:20:55")
while [[ $(date +%Y%m%d%H%M%S) -le $stop ]];do #comparing current date & time with stop date & time
echo "date now is $(date)";
sleep 1; #sleep 1 second
done
date now is Mon Mar 12 00:20:51 EET 2018
date now is Mon Mar 12 00:20:52 EET 2018
date now is Mon Mar 12 00:20:53 EET 2018
date now is Mon Mar 12 00:20:54 EET 2018
date now is Mon Mar 12 00:20:55 EET 2018
The trick here is that the command
stop=$(date +%Y%m%d%H%M%S --date "2018-03-12 00:20:55")
converts given date (using the --date flag) to a kind of integer format.
Then the while loop keeps comparing current date in the same integer format with the stop date.
This is a similar script to start/stop at a specific time, but it is a bit "resources hungry" since it keeps comparing current time to start time:
date; #just print the current date & time
compl=false;
start=$(date +%Y%m%d%H%M%S --date "2018-03-12 01:45:50");
stop=$(date +%Y%m%d%H%M%S --date "2018-03-12 01:45:55");
while true;do
while [[ $(date +%Y%m%d%H%M%S) -ge $start ]] && [[ $(date +%Y%m%d%H%M%S) -le $stop ]];do
echo "date now is $(date)";
sleep 1;
compl=true;
done;
($compl) && break;
done
Mon Mar 12 01:45:37 EET 2018
date now is Mon Mar 12 01:45:50 EET 2018
date now is Mon Mar 12 01:45:51 EET 2018
date now is Mon Mar 12 01:45:52 EET 2018
date now is Mon Mar 12 01:45:53 EET 2018
date now is Mon Mar 12 01:45:54 EET 2018
date now is Mon Mar 12 01:45:55 EET 2018
An alternative to start/stop a script at specific time would be to use epoch date / epoch seconds.
Epoch date is measured in seconds since the epoch (1970-01-01 UTC).
Every date can be expressed as epoch seconds, using the format date +%s.
date #just print the current date & time
start="2018-03-12 02:17:52"
stop="2018-03-12 02:17:57"
timerequired=$(( $(date +%s --date "$start") - $(date +%s) ))
sleep $(($timerequired)) #sleep till the starting time
while [[ $(date +%s) -le $(date +%s --date "$stop") ]];do
echo "date now is $(date)";
sleep 1;
done
Mon Mar 12 02:17:39 EET 2018
date now is Mon Mar 12 02:17:52 EET 2018
date now is Mon Mar 12 02:17:53 EET 2018
date now is Mon Mar 12 02:17:54 EET 2018
date now is Mon Mar 12 02:17:55 EET 2018
date now is Mon Mar 12 02:17:56 EET 2018
date now is Mon Mar 12 02:17:57 EET 2018
Finally, you can use external tools to automate things based on date & time like cron, anacron , at, etc

Answer 3

Echo a date for every second in a range:
from=$(date -d "Wed Mar 2 12:59:58 CDT 2018" +%s)
to=$(date -d "Wed Mar 2 13:00:04 CDT 2018" +%s)
for s in $(seq $from $to); do echo $(date -d #$s); done
The date -d takes a well formed date as input. +%s formats it into seconds since epoch. So you get two big values like from=1520013598, to=...3604. The date -d #1520013598 transforms the timestamp back to a human readable date.
Your range might be in a variable like start and stop, and you might like to foobar ($time) instead of echoing it:
start="Wed Mar 2 12:59:58 CDT 2018"
stop="Wed Mar 2 13:00:04 CDT 2018"
from=$(date -d "$start" +%s)
to=$(date -d "$stop" +%s)
for s in $(seq $from $to)
do
timestamp=$(date -d #$s)
foobar $timestamp
done