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Full Discussion: Find Day of Week
Operating Systems HP-UX Find Day of Week Post 302731167 by rwuerth on Wednesday 14th of November 2012 10:54:49 AM
Old 11-14-2012
Find Day of Week

In HP-UX the date command does not have the "-d" switch like some other *nixes do. I'm working a simple script to tell me, given the day, month and year what day of the week that falls on.

Assuming valid day, month and year input (I'd perform quality checks on the input separately, but not shown here), would the following work for all dates? Or are there some corner cases that may not work correctly? I'm not worried about corner cases like way back in the 1500's when the Gregorian calendar 1st came online and how that might be screwed up.

The output is the output from a call to the 'cal' utility, plus the full name of the day of the week so that the named day of the week can be easily verified.

Code:
#!/bin/sh
day=$1
mth=$2
yr=$3
 
cal $mth $yr
 
set -A _WKDY Saturday Sunday Monday Tuesday Wednesday Thursday Friday
 
print ${_WKDY[$(cal $mth $yr | awk -vDD=$day 'FNR==3 {print ((7-$NF)+DD)%7}')]}

in awk,

Code:
FNR==3

means I skip the headers and am only concerned with the first week of the 'cal' output.

Breaking the math down:

Code:
(7-$NF)

Is an offset to "normalize" partial weeks since the first week of a month may have less than 7 days in it.

Code:
((7-$NF)+DD)

The offset is added to the numerical day of the month (DD)

Code:
((7-$NF)+DD)%7

The offset plus DD is then modded by 7 giving the numerical day of the week, of which, 0 is defined as Saturday, and 1-6 is defined as Sunday through Friday by how I set up the array.

I'm basically just wondering if anyone can break this given a modern date and valid inputs. (no using February 30th! :-) ) I can't break it, but I've a nagging suspicion I'm missing something. Smilie

TIA
 

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DATE(1) 						      General Commands Manual							   DATE(1)

NAME
date - print or set the date and time SYNOPSIS
date [-qsu] [[MMDDYY]hhmm[ss]] [+format] OPTIONS
-q Read the date from stdin -s Set the time (implicit for -q or a date string) -u Print the date as GMT -t Use this number of seconds instead of current time EXAMPLES
date # Print the date and time date 0221921610 # Set date to Feb 21, 1992 at 4:10 p.m. DESCRIPTION
With the -q flag or a numeric argument, date sets the GMT time and date. MMDDYY refers to the month, day, and year; hhmmss refers to the hour, minute and second. Each of the six fields must be exactly two digits, no more and no less. date always display the date and time, with the default format for the system. The -u flag request GMT time instead of local time. A format may be specified with a + followed by a printf-like string with the following options: %% % character %A Name of the day %B Name of the month %D mm/dd/yy %H Decimal hour on 2 digits %I Decimal hour modulo 12 on 2 digits %M Decimal minute on 2 digits %S Decimal seconds on 2 digits %T HH:MM:SS %U Decimal week number, Sunday being first day of week %W Decimal week number, Monday being first day of week %X Same as %T %Y Decimal year on 4 digits %Z Time Zone (if any) %a Abbreviated name of the day %b Abbreviated name of the month %c Appropriate date & time (default format) %d Decimal day of the month on 2 digits %e Same as %d, but a space replaces leading 0 %h Same as %b %j Decimal dey of the year on 3 digits %m Decimal month on 2 digits %n Newline character %p AM or PM %r 12-hour clock time with AM/PM %s Number of seconds since the epoch %t Tab character %w Decimal day of the week (0=Sunday) %x Same as %D %y Decimal year on 2 digits SEE ALSO
time(2), ctime(3), readclock(8). DATE(1)
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