Append and prepend a variable string to each line

Use sed to either append or prepend a variable string to each input line.

Append a string to each input line:

$ my_input_string="$(echo -e "a\nb\nc")"
$ echo "${my_input_string}"

a
b
c

$ thing_to_append=" - my appended string"
$ echo "${my_input_string}" | sed -e "s#\$#${thing_to_append}#"

a - my appended string
b - my appended string
c - my appended string

Prepend a string to each input line:

$ my_input_string="$(echo -e "a\nb\nc")"
$ echo "${my_input_string}"

a
b
c

$ thing_to_prepend="my prepended string - "
$ echo "${my_input_string}" | sed -e "s#^#${thing_to_prepend}#"

my prepended string - a
my prepended string - b
my prepended string - c

Hint: The sed command allows using an alternative delimiter for the substitute command. The pipe character | and the hash # can be good choices.