bash - When do we need curly braces around shell variables?

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In shell scripts, when do we use
{} when expanding
variables?

For example, I have seen the
following:

var=10 # Declare
            variable

echo "${var}" # One use of the variable
echo
            "$var" # Another use of the
            variable

Is there a
significant difference, or is it just style? Is one preferred over the
other?

Answer

In this particular example, it makes no
difference. However, the {} in ${} are
useful if you want to expand the variable foo in the
string

"${foo}bar"

since
"$foobar" would instead expand the variable identified by
foobar.

Curly braces
are also unconditionally required
when:

• expanding array
  elements, as in
  ${array[42]}


• using parameter
  expansion operations, as in ${filename%.*} (remove
  extension)


• expanding positional parameters beyond 9:
  "$8 $9 ${10}
${11}"

Doing
this everywhere, instead of just in potentially ambiguous cases,
can be considered good programming practice. This is both for
consistency and to avoid surprises like $foo_$bar.jpg, where
it's not visually obvious that the underscore becomes part of the variable
name.