10.1. Conceptual Overview

The stream editor, sed, is a noninteractive editor. It interprets a script and performs the actions in the script. sed is stream-oriented because, like many Unix programs, input flows through the program and is directed to standard output. For example, sort is stream-oriented; vi is not. sed's input typically comes from a file or pipe but it can also be taken from the keyboard. Output goes to the screen by default but can be captured in a file or sent through a pipe instead. GNU sed can edit files that use multibyte character sets.

10.1.1. Typical Uses of sed

• Editing one or more files automatically.

• Simplifying repetitive edits to multiple files.

• Writing conversion programs.

10.1.2. 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.