sed command

To replace all occurrences of "day" with "night" and write to stdout

sed 's/day/night/g' <file>

To replace all occurrences of "day" with "night" within

sed -i 's/day/night/g' <file>

To replace all occurrences of "day" with "night" on stdin

echo 'It is daytime' | sed 's/day/night/g'

To remove leading spaces

sed -i -r 's/^\s+//g' <file>

To remove empty lines and print results to stdout

sed '/^$/d' <file>

To replace newlines in multiple lines

sed ':a;N;$!ba;s/\n//g' <file>

To insert a line before a matching pattern

sed '/Once upon a time/i\Chapter 1'

To add a line after a matching pattern

sed '/happily ever after/a\The end.'