> From: busybox <[email protected]> On Behalf Of Alin Mr
> Sent: Monday, July 19, 2021 9:21 PM
>
> It seems that bash-like pattern substitution is surprisingly slow. Tested
> with busybox
> 1_33_stable, 1_32_stable, master, Debian/buster busybox-static (all having
> the applet-bias
> features configured, i.e. FEATURE_SH_STANDALONE, FEATURE_PREFER_APPLETS,
> STATIC)
>
> == ${} vs built-in sed
>
> x=$(cat /etc/passwd /etc/passwd /etc/passwd | head -30) \
> hyperfine -s basic -w 2 -r 10 -S /tmp/bbox-1.33/sh \
> 'for i in $(seq 1 20); do echo "${x//:/|}"; done' \
> 'for i in $(seq 1 50); do sed -e "s/:/|/g"; done'
>
> Benchmark #1: for i in $(seq 1 20); do echo "${x//:/|}"; done
>
> Time (mean ± σ): 829.9 ms ± 12.8 ms [User: 829.6 ms, System: 0.4 ms]
> Range (min ... max): 820.9 ms ... 863.6 ms 10 runs
>
> Benchmark #2: for i in $(seq 1 50); do sed -e "s/:/|/g"; done
> Time (mean ± σ): 40.6 ms ± 0.3 ms [User: 29.8 ms, System: 12.0 ms]
> Range (min ... max): 39.9 ms ... 40.9 ms 10 runs
>
> Summary
> 'for i in $(seq 1 50); do sed -e "s/:/|/g"; done' ran
> 20.46 ± 0.35 times faster than 'for i in $(seq 1 20); do echo "${x//:/|}";
> done'
>
> == O(n^?)
I think the "sed" command gets no input, so its timing result is not relevant.
Still your observation regarding the time complexity of "${x//:/|}" is valid.
--
Best regards,
Dietmar Schindler
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