EDIT: according Stéphane Chazelas sh language.

When you use ./scriptname.sh it executes with /bin/bash as in the first line with #! but not with sh.

The Shell have no syntax to create arrays, but Bash have the syntax you used.

When you use ./scriptname.sh it executes with /bin/bash as in the first line with #!. But when you use sh scriptname.sh it executes sh, not bash.

The sh shell has no syntax to create arrays, but Bash has the syntax you used.

When you use ./scriptname.sh it executes with /bin/bash as in the first line with #! but not with sh.

The Shell have no syntax to create arrays through array=(values), array could be created by one element through it index, e.g. array[index]=value. But Bash have the syntax you used.

EDIT: according Stéphane Chazelas sh language.

When you use ./scriptname.sh it executes with /bin/bash as in the first line with #! but not with sh.

The Shell have no syntax to create arrays, but Bash have the syntax you used.