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

Question

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 1

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.