How to convert a bash string into a date?

Question

I have a file which contains a string:

2014-11-22 08:15:00

... which represents Nov 22 2014 @ 8:15 AM.

I want to parse this string and use it with the date function in BASH to compare with the current date. I have code that compares dates but both dates are generated by BASH and it's easy to format and compare. However, I can't use the string (which is collected from another system) to compare.

I've tried stuff like:

$ mydate=$(cat filewithstring);date -d $mydate
$ mydate=$(cat filewithstring);date -d '$mydate'
$ mydate=$(cat filewithstring);date -d $mydate "+%Y-%m-%d %H:%M:%S"

I end up with errors like:

date: the argument ‘08:00:00’ lacks a leading '+'; when using an option to specify date(s), any non-option argument must be a format string beginning with '+'*

...or...

date: extra operand ‘+%Y-%m-%d %H:%M:%S’

I know that if I type in the string explicitly, it works fine:

$ date -d '2014-11-22 08:15:00'
Sat Nov 22 08:15:00 EST 2014

In the end, I'm hoping to do the following:

• capture and collect the date/time string in the file from the "other" server
• read the string in the file
• compare that date/time in the string with the current date/time
• output something like "This event processed 12 minutes ago"

Any ideas? Thanks.

Solution

This is because you are using single quotes, so that the value of the variable is not expanded.

You can say:

mydate=$(<filewithstring)
date -d"$mydate" "+%Y-%m-%d %H:%M:%S"
       ^       ^

Note also mydate=$(<filewithstring) is a more optimal way to read the file into a variable.