Printing current time in milliseconds or nanoseconds with printf builtin

bash v5 and $EPOCHREALTIME

  • EPOCHREALTIME floating point value with micro-second granularity

  • EPOCHSECONDS the number of seconds since the Unix Epoch

Correct way

Simply:

IFS=. read ESEC NSEC <<<$EPOCHREALTIME
printf '%(%F:%T)T.%06.0f\n' $ESEC $NSEC

1. About $EPOCHREALTIME

Please care:

    EPOCHREALTIME
         Each time this parameter is referenced, it expands to the number
         of seconds since the Unix Epoch  (see  time(3))  as  a  floating
         point  value  with  micro-second  granularity.

So, if I ask for same variable two time in same line:

echo $EPOCHREALTIME...  $EPOCHREALTIME 
1572000683.886830... 1572000683.886840

or more clearly:

printf "%s\n" ${EPOCHREALTIME#*.} ${EPOCHREALTIME#*.}
761893
761925

echo $((  -10#${EPOCHREALTIME#*.} + 10#${EPOCHREALTIME#*.} ))
37

Same on my raspberry-pi:

printf "%s\n" ${EPOCHREALTIME#*.} ${EPOCHREALTIME#*.}
801459
801694

echo $((  -10#${EPOCHREALTIME#*.} + 10#${EPOCHREALTIME#*.} ))
246

So inquiring this two time for building interger part and fractional part is separated process could lead to issues: (On same line first access to $ EPOCHREALTIME could give: NNN1.999995, then next: NNN2.000002. Than result will become: NNN1.000002 with 1000000 micro-second error)

2. WARNING! About mixing $EPOCHSECONDS and $EPOCHREALTIME

Using both together not only lead to first mentioned bug!

$EPOCHSECONDS use call to time() which is not updated constantly, while $EPOCHREALTIME use call to gettimeofday()! So results could differ a lot:

I found This answer to time() and gettimeofday() return different seconds with good explanation.

If I try on my host:

epochVariableDiff () {
    local errcnt=0 lasterrcnt v1 v2 v3 us vals line
    while ((errcnt==0)) || ((errcnt>lasterrcnt)); do
        lasterrcnt=$errcnt
        printf -v vals '%(%s)T %s %s' -1 $EPOCHSECONDS $EPOCHREALTIME
        IFS=$' .' read v1 v2 v3 us <<<"$vals"
        [ "$v1" = "$v2" ] && [ "$v2" = "$v3" ] || ((errcnt++))
        [ $errcnt -eq 1 ] && echo "$line"
        printf -v line '%3d %s - %s - %s . %s' $errcnt $v1 $v2 $v3 $us
        printf "%s\r" "$line"
        ((errcnt)) && echo "$line"
        read -t ${1:-.0002}
    done
}

(
Nota: I use read -t instead of sleep, because sleep is not builtin
Nota2: You could play with argument of function to change value of read timeout (sleep)
)

This could render something lile:

$ epochVariableDiff .0002
  0 1586851573 - 1586851573 - 1586851573 . 999894
  1 1586851573 - 1586851573 - 1586851574 . 000277
  2 1586851573 - 1586851573 - 1586851574 . 000686
  3 1586851573 - 1586851573 - 1586851574 . 001087
  4 1586851573 - 1586851573 - 1586851574 . 001502
  5 1586851573 - 1586851573 - 1586851574 . 001910
  6 1586851573 - 1586851573 - 1586851574 . 002309
  7 1586851573 - 1586851573 - 1586851574 . 002701
  8 1586851573 - 1586851573 - 1586851574 . 003108
  9 1586851573 - 1586851573 - 1586851574 . 003495
 10 1586851573 - 1586851573 - 1586851574 . 003899
 11 1586851573 - 1586851573 - 1586851574 . 004400
 12 1586851573 - 1586851573 - 1586851574 . 004898
 13 1586851573 - 1586851573 - 1586851574 . 005324
 14 1586851573 - 1586851573 - 1586851574 . 005720
 15 1586851573 - 1586851573 - 1586851574 . 006113
 16 1586851573 - 1586851573 - 1586851574 . 006526
 17 1586851573 - 1586851573 - 1586851574 . 006932
 18 1586851573 - 1586851573 - 1586851574 . 007324
 19 1586851573 - 1586851573 - 1586851574 . 007733
 19 1586851574 - 1586851574 - 1586851574 . 008144

Where integer part of $EPOCHREALTIME could increase more than 8000 microseconds before $EPOCHSECONDS (on my host).

Nota: This seem to be linked to some bug, result could differ a lot between different hosts or on same host after reboot, and other things... Strangely I could reproduce them on a lot of different hosts (Intel Core, Intel Xeon, Amd64..) but not on raspberry pi!? (Same Debian bash v5.0.3(1)-release), different kernel version.

Correct: This is not a bug! Mixing time() and gettimeofday() is a bug!

So avoid using both together !!!

3. About printf "..%06.0f"

Nota: I use %06.0f instead of %d to ensure $NSEC to be interpreted as a decimal (float), (prevent octal interpretation if variable begin by 0).

Compare:

printf "nn.%06.0f\n" 012345
nn.012345

printf "nn.%06.0f\n" 098765
nn.098765

and

printf "nn.%d\n" 012345
nn.5349

printf "nn.%d\n" 098765
-bash: printf: 098765: invalid octal number
nn.0

Sample run

Little test:

wait until next second, then print current time with micro-seconds

while ! read -t .$((1000000-10#${EPOCHREALTIME#*.})) foo; do
    IFS=. read ESEC NSEC <<< $EPOCHREALTIME
    printf '%(%F:%T)T.%06.0f\n' $ESEC $NSEC
done

You could end this test by pressing Return

2019-10-25:13:16:46.000444
2019-10-25:13:16:47.000489
2019-10-25:13:16:48.000399
2019-10-25:13:16:49.000383
2019-10-25:13:16:50.000508

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