Universal non-bash `time` benchmark alternative?

Question

For comparing run times of scripts between different shells, some SE answers suggest using bash's built-in time command, like so:

time bash -c 'foo.sh'
time dash -c 'foo.sh'

...etc, for every shell to test. Such benchmarks fail to eliminate the time taken for each shell to load and initialize itself. For example, suppose both of the above commands were stored on a slow device with the read speed of an early floppy disk, (124KB/s), dash (a ~150K executable) would load about 7x faster than bash (~1M), the shell loading time would skew the time numbers -- the pre-loading times of those shells being irrelevant to measuring the run times of foo.sh under each shell after the shells were loaded.

What's the best portable and general util to run for script timing that can be run from within each shell? So the above code would look something like:

bash -c 'general_timer_util foo.sh'
dash -c 'general_timer_util foo.sh'

NB: no shell built-in time commands, since none are portable or general.

---

Better yet if the util is also able to benchmark the time taken by a shell's internal commands and pipelines, without the user having to first wrap them in a script. Artificial syntax like this would help:

general_timer_util "while read x ; do echo x ; done < foo"

Some shells' time can manage this. For example bash -c "time while false ; do : ; done" works. To see what works, (and doesn't), on your system try:

tail +2 /etc/shells | 
while read s ; do 
    echo $s ; $s -c "time while false ; do : ; done" ; echo ----
done

Answer 1

You should note that time is specified by POSIX, and AFAICT the only option that POSIX mentions (-p) is supported correctly by various shells:

$ bash -c 'time -p echo'

real 0.00
user 0.00
sys 0.00
$ dash -c 'time -p echo'

real 0.01
user 0.00
sys 0.00
$ busybox sh -c 'time -p echo'

real 0.00
user 0.00
sys 0.00
$ ksh -c 'time -p echo'       

real 0.00
user 0.00
sys 0.00

Answer 2

I use the GNU date command, which supports a high resolution timer:

START=$(date +%s.%N)
# do something #######################

"$@" &> /dev/null

#######################################
END=$(date +%s.%N)
DIFF=$( echo "scale=3; (${END} - ${START})*1000/1" | bc )
echo "${DIFF}"

And then I call the script like this:

/usr/local/bin/timing dig +short unix.stackexchange.com
141.835

The output unit is in milliseconds.

Answer 3

The time utility is usually built into the shell, as you have noticed, which makes it useless as a "neutral" timer.

However, the utility is usually also available as an external utility, /usr/bin/time, that may well be used to perform the timing experiments that you propose.

$ bash -c '/usr/bin/time foo.sh'

Answer 4

Here is a solution that:

1. eliminate[s] the time taken for each shell to load and initialize itself

2. can be run from within each shell

3. Uses

   no shell built-in time commands, since none are portable or general

4. Works in all POSIX-compatible shells.
5. Works on all POSIX-compatible and XSI-conforming systems with a C compiler, or where you can compile a C executable in advance.
6. Uses the same timing implementation on all shells.

There are two parts: a short C program that wraps up gettimeofday, which is deprecated but still more portable than clock_gettime, and a short shell script that uses that program to get a microsecond-precision clock reading both sides of sourcing a script. The C program is the only portable and minimal-overhead way to get a sub-second precision on a timestamp.

Here is the C program epoch.c:

#include <sys/time.h>
#include <stdio.h>
int main(int argc, char **argv) {
    struct timeval time;
    gettimeofday(&time, NULL);
    printf("%li.%06i", time.tv_sec, time.tv_usec);
}

And the shell script timer:

#!/bin/echo Run this in the shell you want to test

START=$(./epoch)
. "$1"
END=$(./epoch)
echo "$END - $START" | bc

This is standard shell command language and bc and should work as a script under any POSIX-compatible shell.

You can use this as:

$ bash timer ./test.sh
.002052
$ dash timer ./test.sh
.000895
$ zsh timer ./test.sh
.000662

It doesn't measure system or user time, only non-monotonic wall-clock elapsed time. If the system clock changes during the execution of the script, this will give incorrect results. If the system is under load, the result will be unreliable. I don't think anything better can be portable between shells.

A modified timer script could use eval instead to run commands outside of a script.

Answer 5

Multiple times revised solution using /proc/uptime and dc/bc/awk in large parts thanks to the input by agc:

#!/bin/sh

read -r before _ < /proc/uptime

sleep 2s # do something...

read -r after _ < /proc/uptime

duration=$(dc -e "${after} ${before} - n")
# Alternative using bc:
#   duration=$(echo "${after} - ${before}" | bc)
# Alternative using awk:
#   duration=$(echo "${after} ${before}" | awk '{print $1 - $2}')

echo "It took $duration seconds."

Assumes obviously that /proc/uptime exists and has a certain form.