Work the Shell - More Fun with Days and Dates

 in
Figuring out how to calculate the year for a given date and day of week is a task that's not as easy as it sounds.

I received a very interesting note from a reader—a note that referred to a very interesting problem:

Many UNIX commands (for example, last) and log files show brain-dead date strings, such as “Thu Feb 24”. Does anybody out there have a script that will convert that to a year, given a five-year interval and defaulting to the present?

Given a day of the week, a month and a day, is it possible to calculate quickly the most recent year in the past when that particular date occurred on that day of the week? Of course it is!

Various formulas exist for calculating this sort of thing, but I realized pretty quickly that the handy cal utility can do the work for us. If you haven't experimented with it, you'll be surprised at what it can do. Here are two quick, relevant examples:

$ cal
     March 2011
Su Mo Tu We Th Fr Sa
      1  2  3  4  5
6  7  8  9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30 31 

$ cal mar 2007
     March 2007
Su Mo Tu We Th Fr Sa
            1  2  3
4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31

Any light bulb starting to glow for you? If you know the month and day, you simply can go backward looking at that month's day-of-week layout until finally you find a match.

In a rudimentary fashion, the basic idea can be illustrated with a loop, like this:

repeat
  cal $month $year | grep $day
  if day-of-week matches
     echo date $month $day most recently occurred in $year
  else
     year=$(( $year - 1 ))
end repeat

Of course, the problem is a bit more complicated (as they always are), partially because of the complexity of calculating what day a specific date occurs in the cal output. There's another complication too, however; the requested date actually might have occurred in the current year, so it's not as simple as starting with the year 2010 and going backward.

Normalizing Data

The first task is to figure out how to get the information from the user. We'll have only three input parameters and do relatively little testing for misspelled day names and so on:

if [ $# -ne 3 ] ; then
  echo "Usage: $(basename $0) weekday month day"
  echo "  (example: $(basename $0) wed aug 3  )"
  exit 1
fi

That's straightforward and pretty typical, offering a nice usage tip if you forget how to use the script. As is typical of scripts, we return a nonzero result upon error too.

We can't work with completely arbitrary data, however, so when we grab the first few parameters, we'll transliterate them into lowercase and chop off all but the first three letters:

weekday=$(echo $1 | tr '[[:upper:]]' '[[:lower:]]'; | cut -c1-3)
  month=$(echo $2 | tr '[[:upper:]]' '[[:lower:]]'; | cut -c1-3)
    day=$3

Given “Monday, February 8”, it'd be converted automatically to “mon” and “feb” for subsequent testing.

The Current Date

We also need the current date fields for testing, and to do this, I'll demonstrate a very neat trick of date that makes this incredibly efficient:

eval $(date "+thismonth=%m; thisday=%d; thisyear=%Y")

The eval function interprets its argument as if it were a direct script command. More interesting, date can output arbitrary formats (as documented in strftime if you want to read the man page) by using the + prefix, with %m the month number, %d the day of the month and %Y the year. The result is that date creates the string:

thismonth=03; thisday=01; thisyear=2011

which then is interpreted by the shell to create and instantiate the three named variables. Neat, eh?

It turns out that users can specify a month by name or number on the command line. If it's already a number, it'll survive the transforms intact. If it's a name though, we also need the number, so we can figure out whether the date specified could be earlier this year. There are several ways to do this, including a case statement, but that's a lot of work. Instead, I'll lean on sed as I quite frequently do:

monthnum=$(echo $month | sed
's/jan/1/;s/feb/2/;s/mar/3/;s/apr/4/;s/may/5/;s/jun/
↪6/;s/jul/7/;s/aug/8/;s/sep/9/;s/oct/10/;s/
↪nov/11/;s/dec/12/')

Here's where a misspelled month name is a problem, but that's a challenge beyond the scope of this script. For now, however, we'll just roll with it.

______________________

Dave Taylor has been hacking shell scripts for over thirty years. Really. He's the author of the popular "Wicked Cool Shell Scripts" and can be found on Twitter as @DaveTaylor and more generally at www.DaveTaylorOnline.com.

Comments

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

GNU `date' is enough...

zeroxia's picture

As we are talking about Linux, the GNU `date' utility could be far more clever than you might thought, so there is no need to mess with the output of `cal'...

Quick hint:
date -d "Sep 25 2008" +%A
date -d "Sep 25 2008" +%u

But this probably is GNU only, AFAIK, at least the BSD `date' does not have such magic.

Here is an `sh' script listing I just crafted:

#!/bin/sh

export LC_TIME=C

usage()
{
    cat <<!
USAGE:
    ${0##*/} WEEKDAY MONTH_NAME Day
    ${0##*/} WEEKDAY MONTH-DAY
    ${0##*/} WEEKDAY MONTH/DAY
!
}

if [ $# -ne 3 -a $# -ne 2 ]; then
    usage
    exit 1
fi
if [ $# -eq 3 ]; then
    # GNU `date' accepts "Sep 25 2008"
    fmt="$2 $3 %d"
else
    # And also accepts "2011-9-25" or "9/25/2011"
    case "$2" in
        *-*)
            fmt="%d-$2"
            ;;
        */*)
            fmt="$2/%d"
            ;;
        *)
            echo "Uknown date: $2"
            usage
            exit 1
            ;;
    esac
fi

case $(echo $1 | tr '[:upper:]' '[:lower:]' | cut -c1-3) in
    mon) weekday=1 ;;
    tue) weekday=2 ;;
    wed) weekday=3 ;;
    thu) weekday=4 ;;
    fri) weekday=5 ;;
    sat) weekday=6 ;;
    sun) weekday=7 ;;
    *)
        echo "$1: Unknown weekday"
        exit 1
        ;;
esac

MY_DATE_FMT="%Y/%m/%d"
MY_WDAY_FMT="%A"

MAX_TRY=5

y0=$(date +%Y)
i=0
found=0
while [ $i -lt $MAX_TRY ]; do
    y=$((y0 - i))
    str=$(printf "$fmt" $y)
    if ! j=$(date -d "$str" +%u); then
        # `date' will complain, so I keep quiet
        exit 1
    fi
    if [ $j -eq $weekday ]; then
        echo $(date -d "$str" +$MY_DATE_FMT) is \
            $(date -d "$str" +$MY_WDAY_FMT)
        found=1
    fi
    i=$((i + 1))
done

test $found -eq 0 && exit 1
exit 0
Webinar
One Click, Universal Protection: Implementing Centralized Security Policies on Linux Systems

As Linux continues to play an ever increasing role in corporate data centers and institutions, ensuring the integrity and protection of these systems must be a priority. With 60% of the world's websites and an increasing share of organization's mission-critical workloads running on Linux, failing to stop malware and other advanced threats on Linux can increasingly impact an organization's reputation and bottom line.

Learn More

Sponsored by Bit9

Webinar
Linux Backup and Recovery Webinar

Most companies incorporate backup procedures for critical data, which can be restored quickly if a loss occurs. However, fewer companies are prepared for catastrophic system failures, in which they lose all data, the entire operating system, applications, settings, patches and more, reducing their system(s) to “bare metal.” After all, before data can be restored to a system, there must be a system to restore it to.

In this one hour webinar, learn how to enhance your existing backup strategies for better disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible bare-metal recovery solution for UNIX and Linux systems.

Learn More

Sponsored by Storix