Trying to subtract two times down to the nanosecond in shell.

declare start

function handler() {
count=$1

if [ "$1" -le 0 ]; then
  echo "Not a valid entry"
  exit
  fi

  read -p "Enter in R or N: " VAR1
  VAR2="R"
  VAR3="r"
  VAR4="N"
  VAR5="n"

  date  "%A %B %d, %Y %T.%N"
  start=$(date  "%T.%N")
  if [ "$VAR1" = "$VAR2" ]; then
    recFib "$1" "$start"
  elif [ "$VAR1" = "$VAR3" ]; then
    recFib "$1" "$start"
  elif [ "$VAR1" = "$VAR4" ]; then
    itFib "$1" "$start"
  elif [ "$VAR1" = "$VAR5" ]; then
    itFib "$1" "$start"
  else
    echo "Not a valid input"
  fi

}

function recFib() {

  if [ $1 -le 0 ]; then
    echo 0
  elif [ $1 -eq 1 ]; then
    echo 1
  else
    stop=$(date  "%T.%N")
    echo $((date $stop-$start))
    echo $(($(recFib $(($1 - 2)))   $(recFib $(($1 - 1)))))
  fi

}

I have tried this a number of different ways however I am not sure how to perform the subtraction on the two times. Since it is recursive I am trying to grab the time after each generation.

CodePudding user response：

Use

start=$(date  %s.%N)
stop=$(date  %s.%N)

Using only %s and %N is timezone independent (like -u). So hazards on summer time switch are suppressed.

To get the difference, use:

duration=$( bc <<<"$stop-$start" )

CodePudding user response：

Why not move

stop=$(date  "%T.%N")
echo $((date $stop-$start))

to the end of function handler() body.

CodePudding user response：

You must use something that supports floating point math. You can use bc or you can use awk. For example, let's say you have a start and end time of:

01:39:47.109964128
01:43:07.280513101

To get the time difference, you can simply pipe the two times to awk on separate lines. Below with the start time first and stop time second, e.g.:

$ echo -e "01:39:47.109964128\n01:43:07.280513101" |
awk -F: '
  FNR==1 {h1=$1; m1=$2; s1=$3; next} 
  {h2=$1; m2=$2; s2=$3; exit} 
  END { 
    if (s2 < s1) {s2  = 60; m2 -= 1} 
    printf "d:d:
2.9f\n", h2-h1, m2-m1, s2-s1 }
'
00:03:20.170548973

Where h1, m1, s1 are hour-start, minute-start, seconds-start, etc....

For a solution with date, you have to convert both start time and stop time to seconds since epoch, e.g. ' %s.%N' and then pipe the subtraction of seconds.nanoseconds to bc and then you need to convert back to H:M:S.N format.