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

Example:

file="123 hello"

How can I edit the string file such that it only contains the numbers and the text part is removed?

So,

echo $file

should print 123 only.

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

You can say:

echo ${file%%[^0-9]*}

However, this runs into problems in certain cases:

$ file="123 file 456"
$ echo ${file%%[^0-9]*}
123

Using tr:

$ file="123 hello 456"
$ new=$(tr -dc '0-9' <<< $file)
$ echo $new
123456