Bash: add an empty row after each line of each file in a directory

Question

I have a directory with a series of text files and want to add a blank row after each line.

If I just wanted to alter one file, I could do this.

sed -i $' \n ' 00000BD7_ar.txt

However, how can I alter all files in the directory. Kinda like the following.

for file in *; do sed -i $' \n ' ${file}

Answer 1

sed can process multiple files.

sed -i $' \n ' *