Date Format, N Days Ago

In UNIX shell script, it is very difficult to manipulate date/time using standard UNIX commands. Modern programming languages such as Perl, Python, Tcl, ..., are designed to be a general programming/scripting language that can handle this type of task with ease.

Below shows shell functions that wrap around these general scripting languages to determine the date format that is N days ago.

ago_perl()
{
        # localtime return:
        #       Seconds past the minute
        #       Minutes past the hour
        #       Hours past midnight
        #       Day of the month
        #       Months past the start of the year
        #       Number of years since 1900
        #       Number of days since the start of the week (Sunday)
        #       Number of days since the start of the year
        #       Whether or not daylight savings is active
        days=${1:-1}
        perl -e "@t=localtime(time-86400*$days);\
                printf('%4d-%02d-%02d', @t[5]+1900, @t[4]+1, @t[3])"

}
ago_tcl()
{
        days=${1:-1}
        echo "puts [clock format [expr [clock seconds] - $days*86400] \
                -format {%Y-%m-%d}]" | tclsh
}
ago_python()
{
        days=${1:-1}
        python -c "import time, datetime
t=datetime.datetime.fromtimestamp(time.time()-86400*$days)
print '%4d-%02d-%02d' % (t.year,t.month,t.day)"
}

Here is how you can use any of these functions

$ ago=`ago_perl 45`

$ echo "45 days ago is $ago"
45 days ago is 2009-08-12