comp.unix.shell FAQ - Answers to Frequently Asked Questions Appendix A and B

comp.unix.shell FAQ - Answers to Frequently Asked Questions Appendix A and B

Post by Joe Halp » Wed, 29 Jun 2005 11:47:50



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Maintainer: Joe Halpin

Please read the introduction in the first section of this document.
This section assumes you have read that introduction.

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Appendix A: Examples

  Web sites:
  -------------------------------------------

    Heiner Steven's Shelldorado site has quite a few example scripts,
    tutorials, and links to other such places.

      http://www.shelldorado.com/

  Arbitrary Date Arithmetic
  -------------------------------------------


    Subject: Re: yesterday's date under the shell
    View: Complete Thread (8 articles)
    Original Format
    Newsgroups: comp.unix.shell
    Date: 2002-02-12 07:45:05 PST


    > To determine the yesterday date i do it so:
    > TZ=PST+24 date +%d
    > it work well but my question is:
    > does this work on all systems and all shells
    > or should i do it otherwise ???

    No, it does not work on all systems at all times.
    In some it will work practically always,
    on others never, on most it works sometimes
    and sometimes not. I would recommend against it.

    Unfortunately there is no short and sweet portable
    solution. If you have or can install Gnu date it
    will do it cleanly, otherwise you can find a number
    of solutions posted in this group in the past.
    For a general solution you could try the following,
    which should work with POSIXy shells (I've only tested
    it with HP's which is essentially ksh88, though):

    #! /usr/bin/sh

    # Date calculations using POSIX shell
    # Tapani Tarvainen July 1998, February 2001 (POSIXified)
    # This code is in the public domain.

    # Julian Day Number from calendar date
    date2julian() #  day month year
    {
      day=$1;  month=$2;  year=$3
      tmpmonth=$((12 * year + month - 3))
      tmpyear=$((tmpmonth / 12))
      print $(( (734 * tmpmonth + 15) / 24 -  2 * tmpyear + \
        tmpyear/4 - tmpyear/100 + tmpyear/400 + day + 1721119 ))
    }

    # Calendar date from Julian Day Number
    julian2date() # julianday
    {
      tmpday=$(($1 - 1721119))            
      centuries=$(( (4 * tmpday - 1) / 146097))  
      tmpday=$((tmpday + centuries - centuries/4))      
      year=$(( (4 * tmpday - 1) / 1461))          
      tmpday=$((tmpday - (1461 * year) / 4))            
      month=$(( (10 * tmpday - 5) / 306))        
      day=$((tmpday - (306 * month + 5) / 10))  
      month=$((month + 2))                              
      year=$((year + month/12))                        
      month=$((month % 12 + 1))
      print $day $month $year
    }

    # Day of week, Monday=1...Sunday=7
    dow() # day month year
    {
      print $(( $(date2julian $1 $2 $3) % 7 + 1))
    }

    ##################### The End ########################

    Those allow rather arbitrary date computations.
    For example, yesterday's date can be computed like this:

    julian2date $(( $(date2julian $(date +"%d %m %Y") ) - 1 ))

    --
    Tapani Tarvainen

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Appendix B: References