'expr' and 'let' performances

[hakermania]

Hehehe, here you are:


alex@MaD-pc:~$ a=0; time while [ $a -lt 10000 ]; do let a=$a+1; done

real 0m0.217s
user 0m0.208s
sys 0m0.008s
alex@MaD-pc:~$ echo $a
10000
alex@MaD-pc:~$ a=0; time while [ $a -lt 10000 ]; do a=$(expr $a + 1); done

real 0m30.448s
user 0m1.540s
sys 0m5.088s
When I first started bash scripting I was told to use 'expr' so as to do a small addition to a variable but now I can see that it's much slower than 'let'.

From the above executions you can see that expr is 172 times slower (!) ;)

[Lars Noodén]

Nice tip and an excellent test.

[sisco311]

Hehehe, here you are:


alex@MaD-pc:~$ a=0; time while [ $a -lt 10000 ]; do let a=$a+1; done

real 0m0.217s
user 0m0.208s
sys 0m0.008s
alex@MaD-pc:~$ echo $a
10000
alex@MaD-pc:~$ a=0; time while [ $a -lt 10000 ]; do a=$(expr $a + 1); done

real 0m30.448s
user 0m1.540s
sys 0m5.088s
When I first started bash scripting I was told to use 'expr' so as to do a small addition to a variable but now I can see that it's much slower than 'let'.

From the above executions you can see that expr is 172 times slower (!) ;)

It's because let and (( are BASH builtin commands. Builtins are faster than external commands.

let and (( aren't standard. The POSIX equivalent would be:

a=0; time while [ $a -lt 10000 ]; do a=$(($a+1)); done

http://mywiki.wooledge.org/BashGuide/CompoundCommands#Arithmetic_Evaluation

[Lars Noodén]

let and (( aren't standard.

Which means that they won't run in dash (http://manpages.ubuntu.com/manpages/oneiric/en/man1/dash.1.html) which is where /bin/sh links to in Ubuntu.