data buffers

sed: cycles and data buffers

sed maintains two data buffers: the pattern space and the hold space.

Execution of a sed program

• When sed is started, it initializes the these buffers with «empty» content.
• Then sed iterates over each input line. Each new input line starts a cycle.
• During a cycle, all commands in the script whose addresses match the current input line are executed
• After executing all commands in the script, the content of the pattern space is printed is printed to STDOUT (unless the command line option -n was given)
• The pattern space (but not the hold space) is emptied and the next input line is read, which starts the next cycle

Some commands (or at least one command: D) might start a new cycle without reading a line from STDIN.