请问一个 Bash 的 String Operator 的问题

做习题的时候遇到这个问题，script是

#filename is "highest"
filename=$1
filename=${filename:?"missing."}
howmany=$2
sort -nr $filename | head -${howmany:=10}
echo "$howmany books have been selected"

filename中的输入文件中myflie的内容是

#filename is myfile
5 Depeche Mode
2 Split Enz
3 Simple Minds
1 Vivaldi, Antonio

输入

./highest myfile 3

返回内容为

5 Depeche Mode
3 Simple Minds
2 Split Enz
3 books have been selected

我设想是如果输入的是

./highest myfile

返回内容应该是

5 Depeche Mode
3 Simple Minds
2 Split Enz
1 Vivaldi, Antonio
10 books have been selected

但是实际上返回的是

5 Depeche Mode
3 Simple Minds
2 Split Enz
1 Vivaldi, Antonio
books have been selected

也就是说 sort -nr $filename | head -${howmany:=10} 执行完之后 $howmany 依然是null而不是10。

求解各位BASH大神，这是怎么回事？