Create a new file with

for i in {1..15}-{a..c} ; do echo this is line $i >> test-file.txt ; done

Add a newline to the beginning and end of the file

sed -e '1i\ ' -e '$a\ ' test-file.txt

Add a newline from the 3rd through to the 10th line

sed -e '3,10i\ ' -e '$a\ ' test-file.txt

Add a newline at the 3rd and 10th line

sed -e '3~10i\ ' -e '$a\ ' test-file.txt

Add to the beginning of lines 1 through 5 with

sed '1,5 s/^/Alice the Goon say'"'"'s\t/' test-file.txt  <<-- Note the quotes for escaping the apostrophe.

Add to the end of lines 6 through 12 with

sed '6,12 s/$/\t  I Love Popeye/' test-file.txt

Select individual lines

sed -e '13 s/^/Lucky\t /' test-file.txt -e '14 s/$/\t\tPopeye/'

Add to the beginning of every third line

sed '0~3 s/^/hello\t/g' < test-file.txt

Add to the end of every third line

sed '0~3 s/$/\tdolly/g' < test-file.txt

change the file’s line numbering now with

sed -i '=' test-file.txt  "=" is a command in sed to print the current line number to the standard output.

append to the lines matching (PATTERN) “10” with

sed '/10/ a\ hello' test-file.txt

append after the line number (ADDRESS) “10” with

sed '10 a\ hello' test-file.txt

in each case

\a  for append (add after match) can be changed for

\i  for insert (add before match)

or  \c  to change the whole matching line.

example: sed '/10/ c\ hello dolly' test-file.txt

Unlike using

sed 's/is/hello dolly/g' test-file.txt  OR  sed 's/10/hello dolly/' test-file.txt

Finally you can re-number your file with

sed = test-file.txt | sed 'N;s/\n/\t/'  OR  nl testfile.txt  OR  cat -n testfile.txt

as always write it to a new file with > newfile-name

or use -i to edit it inline

To delete trailing whitespace from end of each line

cat input.txt | sed 's/[ \t]*$//' > output.txt

Remove all leading and trailing whitespace from each line

cat input.txt | sed 's/^[ \t]*//;s/[ \t]*$//' > output.txt

To change the nth occurrence on each line add a number at the end of the substitute command

sed 's/old/new/2' file