Shorter way to assign a default value to the standard output stream

37 33 32 bytes

tr \  \\n|sort -n|sed s/^$/0/\;q

Edit: Saved 4 bytes thanks to @DigitalTrauma.

34 bytes

a=($(tr \  \\n|sort -n))
echo $[a]

The first line saves the sorted input values in an array a.

This avoids using head -1, since referencing the array as $a will yield its first value.

The second line uses a in the arithmetic expansion $[a]. In this context, an empty string is interpreted as 0.

For a different default value, parameter substitution could be used instead. For example, the last line could become

echo ${a:-puppies}