Re: How case statement is tested?

[Eric Blake]

On 3/11/20 11:47 AM, Peng Yu wrote:
> Hi,
>
> I am wondering if there is any optimization in testing the conditions
> in a case statement?

What optimization(s) do you have in mind?  Bash has to perform 
glob-matching for each potential pattern.  Anything you can do to speed 
up globbing (such as recognizing when a glob can only match a literal 
string) might help, but since side effects are possible in specifying a 
glob pattern, there's very little you can do to parallelize things by 
testing multiple glob patterns at once.  For example:

x= y=
case 1 in
  $((x=0)) ) ;;
  $((y=1)) ) ;;
  $((x=1)) ) ;;
esac
echo "$x.$y"

must output 0.1, which means you cannot precompute the pattern for the 
third clause bounded by $((x=1)) (which is really the glob '1' plus the 
side-effect of setting x=1) until after you have already ascertained 
whether the first two clauses did not match.

> Or it is just like a series of if-statement and each condition must be
> tested until one condition is true? Thanks.

This, with the additional complication that you can skip conditions 
(with ';&') or evaluate more than one condition as true (with ';;&'). 
See if you can figure out what this will output prior to running it:

for var in a b c d; do
  case $var in
    a) echo 1 ;;&
    b) echo 2 ;&
    c) echo 3 ;;
    *) echo 4
  esac
done

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org