Calculate time needed to send 1 byte back and forth within an SSH connection

sshpingpong

#!/bin/bash
#
me=sshpingpong
#
# Measure close to minimal packet latency (RTT) of an SSH connection
#
prgversion="$me * 2022-07-27 (c) Andreas Schamanek"
#
# @author     Andreas Schamanek <https://andreas.schamanek.net>
# @license    GPL <https://www.gnu.org/licenses/gpl.html>
# @copyright  (c) 2022 Andreas Schamanek
#
usage="$prgversion

   $me sshpingpong [-L] [-t] [-c N] [-i S] {sshlogin@host}

      -c N ... quit after N measurements
      -i S ... wait S seconds between pings; can be fractional, default 5s
      -t   ... terse/compact mode using _, 1, 2, ... for hundreds of ms
               where 1 means 100<=RTT<200, 2 means 200<=RTT<300, ...
      -L   ... do not write to logfile

Cf. https://serverfault.com/q/807910/23900
"

while [[ $1 == -* ]] ; do
case $1 in
	-t) compactmode=yes; shift;;
	-c) [[ $2 != *[!0-9]* ]] || { echo "invalid count" >&2; exit 1; }
		count="$2"; shift 2;;
	-i) [[ $2 != *[!0-9.-]* ]] || { echo "invalid interval" >&2; exit 1; }
		sleep="$2"; shift 2;;
	-L) logfile=/dev/null; shift;;
	-\?|-h|--help) echo "$usage"; exit 0;;
esac
done

ssh="${1:?$me error: missing argument}"

if [[ -z $logfile ]] ; then
	logfile="${ssh#*@}"; logfile="spp_${logfile//[: .]/_}".log
	echo "Logfile is $logfile"
elif [[ $logfile == /dev/null ]] ; then
	logfile=
fi

declare -i c=-1 now=0 sent=0 rcvd=0 rtt=0 count="${count:-0}"
declare rttms=0.0

: "${sleep:=5}"
: "${initialsleep:=2}"
# compact mode factor 1 or 10: cf=10 will make _, 1, 2, 3, ... indicate RTTs
# of <10, <20, <30, ...; w/ cf=1 it will be <100, <200, <300, ...
: "${cf:=1}"

: "${trapsigs:="INT TERM EXIT"}"

if [[ -d $XDG_RUNTIME_DIR ]] ; then
	mkdir -p "$XDG_RUNTIME_DIR/$me"
	fifo="$XDG_RUNTIME_DIR/$me/$$"
else
	fifo="$HOME/fifo$$"
fi

set_now() { now=$(date +%s%N); now="${now%???}"; } # microseconds
compactmodeoutput() {
	if ((cf*rtt<100000)) ; then echo -n _ >&2 ; return; fi
	if ((cf*rtt>999999)) ; then echo -n "#" >&2 ; return; fi
	echo -n "$((cf*rtt/100000))" >&2
}
# logline() also printing "seq=$c" at the time it was sent
#logline() { printf '%(%Y-%m-%d %T)T  seq=%d  %.3f\n' "${sent%??????}" "$c" "$rttms" ; }
# logline() just printing the RTT at the time the data was received
logline() { printf '%(%Y-%m-%d %T)T  %.3f\n' "${rcvd%??????}" "$rttms" ; }

trap "trapexit" $trapsigs
trapexit() {
	trap '' $trapsigs
	[[ ! -e $fifo ]] || rm "$fifo"
	if [[ -z "$logfile" ]] ; then
		printf '\n' >&2
	else
		printf '\nLogfile is %s\n' "$logfile" >&2
	fi
	exit
}

[[ ! -e $fifo ]] || rm "$fifo"
mkfifo "$fifo"

# compact mode header and output redirection
if [[ -z $compactmode ]] ; then
	[[ -z "$logfile" ]] || exec 1> >(tee -a "$logfile")
else
	# in compact mode, if no logging is requested we need to redirect to null
	# so that logline() is silenced
	if [[ -z "$logfile" ]] ; then
		exec 1>>/dev/null
	else
		exec 1>>"$logfile"
	fi
	if ((cf==10)) ; then
		echo "0 _ 10 1 20 2 30 3 40 4 50 5 60 6 70 7 80 8 90 9 99 # ..." >&2
	else
		echo "0 _ 100 1 200 2 300 3 400 4 500 5 600 6 700 7 800 8 900 9 999 # ..." >&2
	fi
fi

mepid="$$"

( sleep "$initialsleep"; echo "ignore initial ping" >"$fifo"; ) &

cat 0<> "$fifo" | ssh $sshargs $ssh cat \
| while read -r R ; do
	set_now; rcvd="$now"
	rtt="$((rcvd-sent))"; rttms="${rtt%???}.${rtt: -3}"
	if [[ $R == o ]] ; then
		logline
		[[ -z $compactmode ]] || compactmodeoutput
		sleep "$sleep"
	fi
	if ((count>0 && c>=count)) ; then
		# found no nicer way to make sure $ssh ends
		# and w/o kill the subshell would never exit
		pkill -2 --parent "$mepid"
		exit
	fi
	c=$((c+1))
	echo 'o' >"$fifo"
	set_now; sent="$now"
done

@gullevek I have no macOS available nor any experience with it. My script is written for Bash on Linux only, I am afraid. I know that %N is not available everywhere, though it's trending ;) However, the %()T format for printf is a Bashism. It was introduced with Bash 4.4 and should be available on platforms.

@xrat The best thing is to use "#!/usr/bin/env bash" because I have a bash 5.5 installed from macports, and bash doesn't even exist in macOS anymore.

For the date, that is fine, for mac users with macPorts and the coreutils installed they just need to replace "date" with "gdate"