Working on Dates in files

[eSmasher]

I have a problem - I have files with rows of data and I need to check if the next row (of the same type) has the NEXT date in it so I need to extract a date in YYYYMMDD format from a row (easy enough) then add one day to it and compare it to the the next date I encounter on a subsequent row.

So if I have
DATE|20100530
VAL|kahskjhd
VAL|lajdlkjad
DATE|20100601
VAL|lah

then I am happy

If I have
DATE|20100530
VAL|kahskjhd
VAL|lajdlkjad
DATE|20100602
VAL|lah

Then I have a problem

I can loop through the file simply enough and compare variables and output warnings but I am entirely stumpted by trying to add a day to a variable.

Graham

[twoHats]

language is important here as most have some utilities to manipulate dates. The typical way is to convert to Julian date - add 1 - and convert back.

Some librarys will have date arithmetic modules (or packages, etc) then it is easy. Almost all will have a way to convert to Julian and back.

[eSmasher]

Ideally I would like to do it in Shell script or Bash so I can do it onsite with my laptop which is an XP tablet running Cygwin - that way I don't need to get back to my office to use my server.

I have seen lots of examples manipulating the system date or file date but nothing using a date from inside a file as the starting point.

Graham

[blinky]

man date

date "--date=tomorrow 20100530" '+%Y%m%d'

will output 20100531.

Store that in a variable and check the next date matches that. If it does use that as the starting point for the following date.

You should be able to implement this in bash:

previous_date=""
while not_eof
{
   current_line=read line
   if current_line is date line
   {
      current_date=extract_date()
      if previous_date is empty
      {
        if previous_date == current_date
        {
          OK
        }
        else
        {
          Fail
        }
      }
      
      previous_date=nextday(current_date)
   }
}

[eSmasher]

The code suggested above to produce the date plus 1 from the file value works:

date "--date=tomorrow $LASTDATE" +%Y%m%d

but when I try to put it into a variable it just displays the value on screen and when I later try to use it the variable is null. I know I am being feeble here and the answer is obvious but I am at a loss.

I have tried all sorts of things but I need something like

NEXTDATE= date "--date=tomorrow $LASTDATE" +%Y%m%d

so I can test it later with

if [ "${LINE:4:8}" != "NEXTDATE" ] ; then

but I have obviously failed to grasp the syntax of the assignment - all help appreciated

Graham

[nmset]

NEXTDATE= $(date "--date=tomorrow $LASTDATE" +%Y%m%d)
echo $NEXTDATE

[eSmasher]

That worked beautifully (I had to take the space out after the = sign - just for any other people like me who struggle with the syntax):

NEXTDATE=$(date "--date=tomorrow $LASTDATE" +%Y%m%d)

Thanks blinky and nmset for your code expertise and twoHats for making me say what I actually needed.

That has saved me looking through another 80,000 lines of data for that defect by eye (I had already looked through 30,000 lines and found two occurances). It found a few more.

Mind you I was being paid by the hour so perhaps this wasn't such a good idea

Graham