Eight ways to speed up your shell scripts

sonald

After reading
Shell Scripting Recipes
, I became more interested in the
speed of shell operations
. In his book, Chris says “Command Substitution Is Slow.” He is correct!
$ f() { echo -n }; time for i in {0..100}; do v=$( f ); done
real    0m4.189s
user    0m0.000s
sys     0m4.188s
$ f() { _F="" }; time for i in {0..100}; do f; v=$_F; done
real    0m0.006s
user    0m0.000s
sys     0m0.000s
I found a few other equivalent operations which can be used to speed
up shell scripts to varying degrees (none like the above) depending on
the task at hand.  As Chris says, “the extra few milliseconds … may not
seem significant, but scripts often loop hundred of even thousands of
times.”
${#array[@]} is faster than () when expanding an array (#7)
$ a=(); time for i in {0..1000}; do a=(${a[@]} $i);done; echo ${#a[@]}
real    0m3.545s
user    0m3.544s
sys     0m0.000s
1001
$ a=(); time for i in {0..1000}; do a[${#a[@]}]=$i;done; echo ${#a[@]}
real    0m0.043s
user    0m0.040s
sys     0m0.003s
1001
$ time for i in {0..10000}; do var=`cat out`;done
real    0m9.328s
user    0m2.892s
sys     0m6.436s
$ time for i in {0..10000}; do var=`
real    0m5.930s
user    0m1.412s
sys     0m4.520s
echo is faster than printf (though not nearly as powerful)
$ time for i in {0..100000}; do printf "\n"; done >/dev/null
real    0m4.446s
user    0m4.076s
sys     0m0.236s
$ time for i in {0..100000}; do echo; done >/dev/null
real    0m3.291s
user    0m3.100s
sys     0m0.184s
Arithmetic Evaluation is faster than let
$ i=0; time while :; do let "i = i + 1"; [[ $i -gt 100000 ]] && break;done
real    0m8.211s
user    0m7.900s
sys     0m0.304s
$ i=0; time while :; do ((i++)); [[ $i -gt 100000 ]] && break;done
real    0m5.287s
user    0m4.980s
sys     0m0.304s
UPDATE: This appears to still be true, but by a different margin. See comments.
List expansion is faster than seq and command substitution (though not always available)
$ time for i in $(seq 0 1000000); do :; done
real    0m28.482s
user    0m28.066s
sys     0m0.412s
$ time for i in {0..1000000}; do :; done
real    0m24.563s
user    0m24.402s
sys     0m0.156s
UPDATE: On BSD systems the apparent seq equivalent (jot) is faster than list expansion. See comments.
: is faster than true
$ i=0; time while true; do ((i++)); [[ $i -gt 1000000 ]] && break;done
real    0m57.360s
user    0m53.967s
sys     0m3.392s
$ i=0; time while :; do ((i++)); [[ $i -gt 1000000 ]] && break;done
real    0m54.138s
user    0m50.571s
sys     0m3.560s
               
               
               

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