sed (stream editor)

Add an extra line for each line break in a file:

echo "Line 1\nLine 2" | sed "G"
# --> "\nLine 1\n\nLine 2\n"

Prepend some output to the start of a file:

# Say we wanna add a file.md's last modified time to it's frontmatter
modified=$(date -r file.md); sed -e "1s/^/---\nmodified: "$modified"\n---\n/g" file.md

Update a specific line number

Notice the 1 in previous command? It represents a line number. So, let update the “modified” time of the file on 2nd line:

sed "2 s/.*/modified: \"$(date -r file.md)\"/" file.md

where 2 in the command represents the second line

Or a range of lines

sed -n "1,5p" file.md

To show the first 5 lines of file.md