Shell script: How to obtain future date

[Michael Hemmingsen]

Hi:

Does anyone have a suggestion to an easy way of generating the following output:

THE DATE AND TIME 3 DAYS LATER THAN THE PRESENT DATE AND TIME.

I need it for producing a script, that creates a user which expires date 3 days later. One approch would be to obtain the date, via the 'date' command, but it would then be nescessary to somehow convert the string '07' (e.g. output of 'date +%m') to the integer 7.

Your help is appreciated!

[Alexander M. Ermes]

Hi there.
Try this :

#
TAG_=`date +'%y%m%d'`; export TAG_


export TAG1="000000"
TAG1=`expr $TAG_ + 1`
echo $TAG1

Rgds
Alexander M. Ermes

[Michael Hemmingsen]

Nope, the expr $TAG... doesn't work. The output echoed to screen is: 010705+1

[federico_3]

Try:
let TAG1=$TAG_+1

[Ralph Grothe]

Micheal,

as date calculations sometimes can get a bit tricky, and since most standard shells (except for bash, zsh etc.) have only limited arithmetic functionality, I would suggest doing it with Perl.
(Perl should be on any decent Unix box ;-)

e.g.

$ perl -e 'printf "%s\n",scalar localtime(time + 3*86400)'

If you need some fancier formatting you would have to invoke localtime() in list context.
See the POD for what localtime returns (i.e. $ perldoc -f localtime)

[federico_3]

or
TAG1=`expr $TAG_ + 1`

[Michael Hemmingsen]

Nope, both let TAG1=$TAG_+1 and
TAG1=`expr $TAG_ + 1` result in syntax errors.

[Andreas Voss · Accepted solution]

Hi,

put you a C-Source at attachment.
Save it ie: under /usr/local/src/fdate.c
Compile it ie:
cc /usr/local/src/fdate.c -o /usr/local/bin/fdate

Use it ie: 3 days forward:
fdate 3

You can also compute backwards with ie:

fdate -3

Regards

[A. Clay Stephenson]

Hi Micheal:

I has a technique that you will find very useful. It involves converting a date into a truncated Julian Day (more or less the number of days since Jan 1, 4712 BCE; it's used by astronomers to make life simple). In your case, it takes care of month,year,leap year boundaries with ease.

The script I've attached will convert a calender date into a Julian Day and also do the inverse. It also has an argument to calculate the day of the week. Invoke the script as caljd.sh -u for full usage.

In your case I would do it like this:

INC=3
JD=`caljd.sh`
JD=$((${JD} + ${INC}))
CAL_DATE=`caljd.sh ${JD}`

This will calculate 3 days after the current date. If you supply the first call to caljd.sh
with MM DD YYYY it will calculate 3 days after that date. The number of args is how caljd.sh knows to convert from calendar to julian day or julian day to calendar date.

Regards, Clay

[federico_3]

Andrea, your c script does not work. I get the following:
cc: "fdate.c", line 2: error 1000: Unexpected symbol: "i".
cc: "fdate.c", line 2: error 1002: Unexpected character: 'u'.
cc: "fdate.c", line 2: error 1000: Unexpected symbol: "<".
cc: "fdate.c", line 2: error 1000: Unexpected symbol: ".".
cc: "fdate.c", line 2: error 1000: Unexpected symbol: ">".
cc: "fdate.c", line 16: error 1588: "stdout" undefined.


How come?

[Andreas Voss]

Hi,

this may be a bug when putting something at attachment.
The first line of the file is inserted with tabs.

So delete the inserted tabs so that it looks like:

#include
#include
.
.
.
.

Regards

[Michael Hemmingsen]

The perl thing does work (however a bit elaborate). I created the below script to obtain the date 5 days later in the format dd/mm/yy. It itsn't wery elegant, but does work:

#!/usr/bin/sh
#
# futuredate.sh
#
###############################################################################

NINETH=`perl -e 'printf "%s",scalar localtime(time + 5*86400)'|cut -c9`

if [ "$NINETH" = " " ]
then
NEWDATE=0`perl -e 'printf "%s",scalar localtime(time + 5*86400)'|cut -c10`
else
NEWDATE=`perl -e 'printf "%s",scalar localtime(time + 5*86400)'|cut -c9-10`
fi

MONTH=`perl -e 'printf "%s",scalar localtime(time + 5*86400)'|cut -c5-7`

case $MONTH in
Jan) NEWMONTH=01 ;;
Feb) NEWMONTH=02 ;;
Mar) NEWMONTH=03 ;;
Apr) NEWMONTH=04 ;;
May) NEWMONTH=05 ;;
Jun) NEWMONTH=06 ;;
Jul) NEWMONTH=07 ;;
Aug) NEWMONTH=08 ;;
Sep) NEWMONTH=09 ;;
Oct) NEWMONTH=10 ;;
Now) NEWMONTH=11 ;;
Dec) NEWMONTH=12 ;;
esac

NEWYEAR=`perl -e 'printf "%s",scalar localtime(time + 5*86400)'|cut -c23-24`

echo ${NEWDATE}/${NEWMONTH}/${NEWYEAR}