Pad all lines with spaces to a fixed width in Vim or using sed, awk, etc

Question

How can I pad each line of a file to a certain width (say, 63 characters wide), padding with spaces if need be?

For now, let’s assume all lines are guaranteed to be less

Answer 1

With sed, without a loop:

$ sed -i '/.\{63\}/!{s/$/                                                                /;s/^\(.\{63\}\).*/\1/}' file

Be sure to have enough spaces in the 1st substitution to match the number of space you want to add.