Shell – Stable timing for shell scripts

for sh  in bash zsh yash dash mksh ksh
do      printf  "\n%s:\t" "$sh"
        time    "$sh" -c '
                        str="some string"
                        set     "" ""
                        while   ${20001+"break"}
                        do      set "$@$@";done
                        IFS=A;  printf %.100000s\\n "$str$*$*$*$*$*"'|
                wc -c
done

bash:   100001
"$sh" -c   0.15s user 0.01s system 94% cpu 0.176 total
wc -c  0.00s user 0.00s system 1% cpu 0.175 total

zsh:    100001
"$sh" -c   0.03s user 0.01s system 97% cpu 0.034 total
wc -c  0.00s user 0.00s system 9% cpu 0.034 total

yash:   100001
"$sh" -c   0.06s user 0.01s system 94% cpu 0.067 total
wc -c  0.00s user 0.00s system 5% cpu 0.067 total

dash:   100001
"$sh" -c   0.02s user 0.01s system 92% cpu 0.029 total
wc -c  0.00s user 0.00s system 11% cpu 0.028 total

ksh:    100001
"$sh" -c   0.02s user 0.00s system 96% cpu 0.021 total
wc -c  0.00s user 0.00s system 16% cpu 0.021 total

So this benches the various shells set to $sh in the for loop on how quickly they can generate a string of 100,000 characters. The first 11 of those 100,000 chars are some string as is first set to the value of $str, but the tail fill is 999,989 A chars.

The shells get the A chars in $* which substitutes in the first character in the value of the special shell parameter $IFS as a concatenation delimiter between every positional parameter in shell's argument array. Because all of the arguments are "" null, the only chars in $* are the delimiter chars.

The arguments are accrued at an exponential rate for each iteration of the while loop - which only breaks when the $20001 parameter has finally been ${set+}. Until then, basically the while loop does:

### first iteration
while $unset_param; do set "" """" ""; done
### second iteration
while $unset_param; do set "" "" """" "" ""; done
### third iteration
while $unset_param; do set "" "" "" "" """" "" "" "" ""; done

...and so on.

After the while loop completes $IFS is set to A and the special shell parameter $* is concatenated five times to the tail of $str. printf trims the resulting %string argument to a maximum of .100000 bytes before writing it out to its stdout.

One might use the same strategy like:

str='some string'
set "" ""
while ${51+"break"}; do set "$@$@"; done
shift "$((${#}-(51-${#str}))"

...which results in a total argument count of 40 - and so 39 delimiters...

IFS=.; printf %s\\n "$str$*"

some string.......................................

And you can reuse the same arguments you've already set w/ a different $IFS for a different fill:

for IFS in a b c; do printf %s\\n "$str$*"; done

some stringaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
some stringbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
some stringccccccccccccccccccccccccccccccccccccccc

You can also fill in the null arguments with a printf format string rather than using $IFS:

printf "%s m%sy%1ss%st%sr%si%sn%sg" "$str$@"

some string my string my string my string my string my string

A common technique is to use a trap to undo everything when you're done. It needs to be idempotent, i.e. it should fail gracefully if some of the steps cannot be completed.

#!/bin/bash

clean_up=false
errorhandler () {
    umount z || true
    $clean_up && cleanup_thing
    umount y || true
    umount x
}
trap errorhandler ERR EXIT

mount a x
mount b y
setup_thing
clean_up=true
mount c z
do_something

Notice that the trap is also triggered on EXIT so it will execute before the script terminates normally, too; so you don't need to clean up explicitly at all.

The ERR pseudo-signal is a Bash extension, I believe. So this won't work in Ash/Dash/legacy Bourne shell etc.