I use fragments of this POSIX shell script for cron, to specify that I only want processing on a specific Thursday as defined by GNU date. These were the Thurs

I use fragments of this POSIX shell script for cron, to specify that I only want processing on a specific Thursday as defined by GNU date. These were the Thursdays for July of 2021 (pick any Thursday to see them all). $ ./monthday.sh 7 15 1 8 15 22 29 $ cat monthday.sh #!/bin/dash m="$1" d="$2" w=$(date +%u -d $m/$d) d=$(date +%u -d $m/1) [ $w -lt $d ] && c1=$((8 - d + w)) || c1=$((w - d + 1)) c2=$((c1 + 7)) c3=$((c1 + 14)) c4=$((c1 + 21)) c5=$((c1 + 28)) { date -d $m/$c5 || c5=0; } > /dev/null 2>&1 echo $c1 $c2 $c3 $c4 $c5