5.3. How Does sed Work?

The sed editor processes a file (or input) one line at a time and sends its output to the screen. Its commands are those you may recognize from the vi and ed/ex editors. Sed stores the line it is currently processing in a temporary buffer called a pattern space or temporary buffer. Once sed is finished processing the line in the pattern space (i.e., executing sed commands on that line), the line in the pattern space is sent to the screen (unless the command was to delete the line or suppress its printing). After the line has been processed, it is removed from the pattern space and the next line is then read into the pattern space, processed, and displayed. Sed ends when the last line of the input file has been processed. By storing each line in a temporary buffer and performing edits on that line, the original file is never altered or destroyed.