Inserting a space between each letter

$ echo -e "hello life\nand hello to you" | sed 's/./& /g'
h el l o  l i f e
a n d  h e l l o  t o  y o u

• The problem is that the natural space between each word is doubled. To solve this just add:

$ echo -e "hello life\nand hello to you" | sed 's/./& /g;s/  / /g'
h el l o l i f e
a n d h e l l o t o y o u
$  

• Here is a variant. Compared with the previous replacement and done in a single shot s///

$ echo -e "hello life\nand hello to you" | sed -r 's/([^ ])/\1 /g'
h el l o  l i f e
a n d  h e l l o  t o  y o u

• If you do not want to use the-r option

sed 's/\([^ ]\)/\1 /g'

• [^...] - Is a character class complemented which means "to recognize a non-listed character" , do not sonfuse with "not to recognize a listed character.
• In our case can be translated "to recognize a character that is not space."