Is there a POSIX way to get the starting time of a given PID from the shell?

I have a file whose content is PID Tab STARTTIME and I would like to know if that process is still alive.

I can check that a given PID is running with:

kill -0 "$pid" && echo alive

But it lacks the ability to differentiate between the real process and an other one that got the same PID.

So I though of using ps:

ps -o lstart= -p "$pid"

But it errputs:

ps: unknown output format: -o lstart

Is there a POSIX way to get the starting time (in seconds since EPOCH) of a given PID from the shell?

Solution

As @Cyrus pointed out, it's possible to use the output of ps -o etime= to calculate the starting time of a given PID:

_{edit: as @KamilCuk suggested, the functions now use a shell pipe instead of dealing with the ps call from inside awk.}

# usage: starttime PID
starttime() {
    command -p ps -o etime= -p "$1" |
    command -p awk '
        BEGIN { srand(); timeofday = srand() }
        END {
            if (NF) {
                n = split($NF, a, "[:-]")
                print timeofday - a[n-3]*86400 - a[n-2]*3600 - a[n-1]*60 - a[n]
            } else
                exit 1
        }
    '
}

But it's more like an approximation, because when I compare it to the starting "date" provided by ps -o lstart= in Linux, I don't get identical results; as shown by the following script that outputs both values for all the running processes:

#!/bin/bash

starttime() { ... }

while read pid lstart
do
    t1=$(date -d "$lstart" +%s) &&
    t2=$(starttime "$pid") &&
    printf '%s\t%s\n' "$t1" "$t2"
done < <(
    ps -o pid= -o lstart= -e
)

1681318132  1681318133
1681133418  1681133419
1681133418  1681133418
1681318132  1681318132
...

While the results are certainly different, the difference doesn't exceed 1 second; therefore, I think that we can still use ps -o etime= for checking that a process is alive in an environment that doesn't require much precision (for ex. a cron job). Here's a function that "allows" a ±1 second delta:

_{tested on AIX/Solaris/macOS/FreeBSD/Linux}

# usage: is_alive PID STARTTIME
is_alive() {
    command -p ps -o etime= -p "$1" |
    command -p awk -v starttime="$2" '
        BEGIN { srand(); timeofday = srand() }
        END {
            n = split($NF, a, "[:-]")
            d = starttime - timeofday + a[n-3]*86400 + a[n-2]*3600 + a[n-1]*60 + a[n]
            exit( -1 < d || d > 1 )
        }
    '
}