Date calculation using GNU date

Ugly, but one line:

date -d "20110405 -2 days -$(date -d '20110405' '+%w') days" "+%Y%m%d"

EDIT: See comments.

date -d "20110405 -$(date -d "20110405 +2 days" +%u) days" "+%Y%m%d"

Explanation:

• %w returns day of the week. Friday = 5 so take off 2 more days to get the right offset.
• Works out as "20110405 -x days", where x is the number of days back to last Friday.

I don't like that it repeats the date string, but hopefully it goes some way to helping.

Script example (based on the accepted answer)

DT="20170601"# get the Friday before $DT# expected value is 20170526date -d "$DT -`date -d "$DT +2 days" +%u` days" "+%Y%m%d"

Further examples, using undocumented features of GNU date (from unix.com)

# assign a value to the variable DT for the examples belowDT="2006-10-01 06:55:55"echo $DT# add 2 days, one hour and 5 sec to any datedate --date "$DT  2 days 1 hour 5 sec"# subtract from any datedate --date "$DT 3 days 5 hours 10 sec ago"date --date "$DT -3 days -5 hours -10 sec"# or any mix of +/-. What will be the date in 3 months less 5 days?date --date "now +3 months -5 days"