Shell loops are slow and bash's are the slowest.
Shells aren't meant to do heavy work in loops. Shells are meant to launch a few external, optimized processes on batches of data.
Anyway, I was curious how shell loops compare so I made a little benchmark:
#!/bin/bash
export IT=$((10**6))
echo POSIX:
for sh in dash bash ksh zsh; do
TIMEFORMAT="%RR %UU %SS $sh"
time $sh -c 'i=0; while [ "$IT" -gt "$i" ]; do i=$((i+1)); done'
done
echo C-LIKE:
for sh in bash ksh zsh; do
TIMEFORMAT="%RR %UU %SS $sh"
time $sh -c 'for ((i=0;i<IT;i++)); do :; done'
done
G=$((10**9))
TIMEFORMAT="%RR %UU %SS 1000*C"
echo 'int main(){ int i,sum; for(i=0;i<IT;i++) sum+=i; printf("%d\n", sum); return 0; }' |
gcc -include stdio.h -O3 -x c -DIT=$G -
time ./a.out
The (abbreviated) results (time per iteration) are:
POSIX:
5.8 µs dash
8.5 µs ksh
14.6 µs zsh
22.6 µs bash
C-LIKE:
2.7 µs ksh
5.8 µs zsh
11.7 µs bash
C:
0.4 ns C
From the results:
If you want a slightly faster shell loop, then if you have the [[ syntax and you want a fast shell loop, you're in an advanced shell and you have the C-like for loop too. Use the C like for loop, then. They can be about 2 times as fast as while [-loops in the same shell.
- ksh has the fastest
for ( loop at about 2.7µs per iteration
- dash has the fastest
while [ loop at about 5.8µs per iteration
C for loops can be 3-4 decimal orders of magnitude faster. (I heard the Torvalds love C).
The optimized C for loop is 56500 times faster than bash's while [ loop (the slowest shell loop) and 6750 times faster than ksh's for ( loop (the fastest shell loop).
Again, the slowness of shells shouldn't matter much though, because the typical pattern with shells is to offload to a few processes of external, optimized programs.
With this pattern, shells often make it much easier to write scripts with performance superior to python scripts (last time I checked, creating process pipelines in python was rather clumsy).
Another thing to consider is startup time.
time python3 -c ' '
takes 30 to 40 ms on my PC whereas shells take around 3ms. If you launch a lot of scripts, this quickly adds up and you can do very very much in the extra 27-37 ms that python takes just to start (small scripts can be finished several times over in that time frame).
