How to add lines at the beginning of either empty or not file?

I want to add lines at beginning of file, it works with:

sed -i '1s/^/#INFO\tFORMAT\tunknown\n/' file
sed -i '1s/^/##phasing=none\n/' file

However it doesn’t work when my file is empty. I found these commands:

echo > file && sed '1s/^/#INFO\tFORMAT\tunknown\n/' -i file

echo > file && sed '1s/^/##phasing=none\n/' -i file

but the last one erase the first one (and also if file isn’t empty)

I would like to know how to add lines at the beginning of file either if the file is empty or not

I tried a loop with if [ -s file ] but without success

Thanks!

>Solution :

You should use i command in sed:

file='inputFile'
# insert a line break if file is empty
[[ ! -s $file ]] && echo > "$file"

sed -i.bak $'1i\
#INFO\tFORMAT\tunknown
' "$file"

Or you can ditch sed and do it in the shell using printf:

{ printf '#INFO\tFORMAT\tunknown\n'; cat file; } > file.new &&
mv file.new file