Displaying seconds as days/hours/mins/seconds?

You can use something like this:

function displaytime {
  local T=$1
  local D=$((T/60/60/24))
  local H=$((T/60/60%24))
  local M=$((T/60%60))
  local S=$((T%60))
  (( $D > 0 )) && printf '%d days ' $D
  (( $H > 0 )) && printf '%d hours ' $H
  (( $M > 0 )) && printf '%d minutes ' $M
  (( $D > 0 || $H > 0 || $M > 0 )) && printf 'and '
  printf '%d seconds\n' $S
}

Examples:

$ displaytime 11617
3 hours 13 minutes and 37 seconds
$ displaytime 42
42 seconds
$ displaytime 666
11 minutes and 6 seconds

Easiest and cleanest way is this one liner (here assuming GNU date):

If the number of seconds is, say:

seconds=123456789 # as in one of the answers above

eval "echo $(date -ud "@$seconds" +'$((%s/3600/24)) days %H hours %M minutes %S seconds')"

--> output: 1428 days 21 hours 33 minutes 09 seconds

Credit goes to Stéphane Gimenez but if someone would like to display seconds only if a period is less than a minute here is my modified version that I use (also with fixed pluralization):

converts()
{
    local t=$1

    local d=$((t/60/60/24))
    local h=$((t/60/60%24))
    local m=$((t/60%60))
    local s=$((t%60))

    if [[ $d > 0 ]]; then
            [[ $d = 1 ]] && echo -n "$d day " || echo -n "$d days "
    fi
    if [[ $h > 0 ]]; then
            [[ $h = 1 ]] && echo -n "$h hour " || echo -n "$h hours "
    fi
    if [[ $m > 0 ]]; then
            [[ $m = 1 ]] && echo -n "$m minute " || echo -n "$m minutes "
    fi
    if [[ $d = 0 && $h = 0 && $m = 0 ]]; then
            [[ $s = 1 ]] && echo -n "$s second" || echo -n "$s seconds"
    fi  
    echo
}

An alternative example in POSIX:

converts(){
    t=$1

    d=$((t/60/60/24))
    h=$((t/60/60%24))
    m=$((t/60%60))
    s=$((t%60))

    if [ $d -gt 0 ]; then
            [ $d = 1 ] && printf "%d day " $d || printf "%d days " $d
    fi
    if [ $h -gt 0 ]; then
            [ $h = 1 ] && printf "%d hour " $h || printf "%d hours " $h
    fi
    if [ $m -gt 0 ]; then
            [ $m = 1 ] && printf "%d minute " $m || printf "%d minutes " $m
    fi
    if [ $d = 0 ] && [ $h = 0 ] && [ $m = 0 ]; then
            [ $s = 1 ] && printf "%d second" $s || printf "%d seconds" $s
    fi
    printf '\n'
}