Printing current time in milliseconds or nanoseconds with printf builtin

Question

We can print the current time with the builtin printf function, without needing to invoke an external command like date, like this:

Answer 1

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