Question

Is there an easier way to calculate yesterday in Bash? This code (used for incremental tars)

mod_time=""

if [ ! -z ${1} ]; then
    if [ "${1}" = "i" ]; then

        this_month=`date +%m`

        this_year=`date +%y`

        last_day=`date +%d`

        # Subtract one from today's day, to get yesterday.

        if [ "${last_day:0:1}" = "0" ]; then
            if [ "${last_day:1:1}" > "1" ]; then
                last_day=$[$((${last_day:1:1})) - 1]
            fi
        else
            last_day=$[$(($last_day)) - 1]
        fi

        # zero pad if necessary

        if [ 10 -gt $last_day ]; then
            last_day="0$last_day"
        fi

        mod_time=" --newer-mtime $this_month/$last_day/$this_year "
    fi
fi

has a couple of problems like calculating day 0, and also not doing the right thing at the end of the month. I don’t want to build in leap year calculations, and am wondering if there’s an easy way to do this in Bash. If not, I’ll use Clojure or Python.

Answer 1

This might help

 date -d "yesterday" +%Y%m%d

return

 20120202