Linux in a Nutshell, 6th Edition

sed Operation

sed operates as follows:

• Each line of input is copied into a pattern space, an internal buffer where editing operations are performed.

• All editing commands in a sed script are applied, in order, to each line of input.

• Editing commands are applied to all lines (globally) unless line addressing restricts the lines affected.

• If a command changes the input, subsequent commands and address tests are applied to the current line in the pattern space, not the original input line.

• The original input file is unchanged because the editing commands modify an in-memory copy of each original input line. The copy is sent to standard output (but can be redirected to a file).

• sed also maintains the hold space, a separate buffer that can be used to save data for later retrieval.