Reviewing Sed - Part 1

There is a good collection of sed one liners at http://www.student.northpark.edu/pemente/sed/sed1line.txt.

So I have just begun to review few them using sed manpage because of the earthquake in Taiwan.

# Following line will insert a blank space after everyline in temp.txt

sed G temp.txt

G - Append hold space to pattern space.

So hold space holds nothing initially and if you append that to pattern space, you get a new line.

So thats the way to add blank lines after everyline.
Sed goes through each line and loads it to the pattern space.

# Adds only a single blank line to all lines to temp.txt

sed '/^$/d;G' temp.txt

/regexp/ - Match lines matching the regular expression regexp.

d - Delete pattern space. Start next cycle.

There are two things to consider.
For blank lines : 'd' is executed that deletes the pattern space and starts next cycle(loads next line and starts all over again)

For non blank lines : as the pattern doesn't match, d is not executed and next command G gets executed.
We get a blank line after every non blank line.

The command would delete the blank lines and skip the G command for blank lines.
Thing to note is that d command applies only to lines matching the regex /^$/ whereas
G command applies to all lines and is not tied to the regex. However when regex is matched the d command will cause G command to be skipped.