Converting integer to day-of-week using date(1)

Question

Using date, I can convert days of weeks to integers ranging from 1 to 7, for example:

$ date -d "Thursday" +%u
4

But what if I want to perform the reverse operation? Something like,

$ date [options] [+format]
Thursday

It's okay if it's not possible with date, I can use other programs as well. Aiming for brevity here.

Answer 1

You could get the date for next Sunday, add N days and print the day name for that particular date:

getdayname () {
nextplus=$(date -d "next Sunday +$1 days")
date -d "${nextplus}" '+%A'
}

and run getdayname 2, getdayname 7 etc (assuming gnu date).
Or simply use an array and do without the date e.g.

getdayname () {
local days=( Sunday Monday Tuesday Wednesday Thursday Friday Saturday )
printf %s\\n ${days[$1]}
}

though with bash/ksh you need printf %s\\n "${days[$1-1]}" (as indexing starts at 0), and quotes are needed to avoid split+glob (also with yash).

Answer 2

Based on don_crissti's answer, since his first answer gives me incorrect results:

getdayname() {
        date -d "$(( 7-$(date '+%u')+$1 )) days" '+%A'
}

Answer 3

POSIXly (-d is not a standard date option. When available, it means something different to different implementations), to give the day name in the user's locale, Sunday being either 0 or 7:

getdayname() (
  IFS=';'
  set -o noglob
  days=$(locale day) || exit
  for day do
    set -- $days $days
    shift "$day" &&
      printf '%s\n' "$1"
  done
)

Examples:

$ getdayname 0 4 7
Sunday
Thursday
Sunday
$ LANG=es_VE.UTF-8 getdayname 0 4 7
domingo
jueves
domingo

With GNU or busybox date (whose -d ... doesn't support relative dates), or ast-open date (whose -d ... supports relative dates but in a different way from GNU's date):

getdayname() (
  for day do
    date -ud "1970-01-$((day + 11))" +%A
  done
)

With zsh without external requirements:

$ zmodload zsh/langinfo
$ day=0
$ printf '%s\n' $langinfo[DAY_$((day+1))]
Sunday

Or:

$ zmodload zsh/datetime
$ day=0
$ TZ=UTC0 strftime %A $((86400 * (3+day)))
Sunday