Compare two dates in the shell

I am trying to compare two dates in a shell script but I always get a wrong value. This is the code I am using:

currenttime=$(date +%H%M%S);
peak_time=$(date -d "$peak_time" '+%H%M%S');
num1=$(($peak_time-$currenttime))
echo $num1
if [ $num1 < 0 ]
then echo "peak time is bigger than current"
else echo "peak time is smaller than now"
fi

I always get

peak time is bigger than current

Even if it is smaller.

I tried again with static numbers, like this:

 if [ 10 < 0 ] then echo "peak time is bigger than current" else echo "peak time is smaller than now" fi

I always get peak time is bigger than current. So the if statement always prints the first sentence.

1 Answer

You should also get an error message when you run your script:

/home/terdon/scripts/foo.sh: line 7: 0: No such file or directory

This is because you are using < in a [ ], and that doesn't do arithmetic comparison, it's input redirection. So $num1 < 0 means "run the command $num1 and pass it the contents of file 0 as input". What you want is [ $num1 -lt 0 ], -lt means "less than".

Of course, your script doesn't make sense unless you are setting peak_time somewhere. This line:

peak_time=$(date -d "$peak_time" '+%H%M%S');

Will always print 000000 since when you run it, $peak_time has no value:

$ peak_time=$(date -d "$peak_time" '+%H%M%S');
$ echo $peak_time
000000

You need to first set $peak_time and then convert it to a date. for example:

$ peak_time="10:05:31"
$ peak_time=$(date -d "$peak_time" '+%H%M%S');
$ echo $peak_time
100531

So, a working version of your script would be:

#!/bin/bash
currenttime=$(date +%H%M%S);
peak_time="10:05:31"
peak_time=$(date -d "+1 hour" '+%H%M%S');
num1=$(($peak_time-$currenttime))
echo $num1
if [ $num1 -lt 0 ]
then echo "peak time is bigger than current"
else echo "peak time is smaller than now"
fi