How to remove all non-numeric characters from a string in Bash?

Question

This is one way with sed:

$ echo $file | sed 's/[^0-9]*//g' 
123
$ echo "123 he23llo" | sed 's/[^0-9]*//g'
12323

Or with pure bash:

$ echo "${file//[!0-9]/}" 
123
$ file="123 hello 12345 aaa"
$ echo "${file//[!0-9]/}" 
12312345

To save the result into the variable itself, do

$ file=$(echo $file | sed 's/[^0-9]*//g')
$ echo $file
123

$ file=${file//[!0-9]/}
$ echo $file
123