34.1 Two Things You Must Know About sed

If you are already familiar with global edits in other editors like vi or ex, you know most of what you need to know to begin to use sed. There are two things, though, that make it very different:

1. It doesn't change the file it edits. It is just what its name says: a "stream ed;;itor"-designed to take a stream of data from standard input (13.1) or a file, transform it, and pass it to standard output (13.1). If you want to edit a file, you have to write a shell wrapper (34.3) to capture standard output and write it back into your original file.

2. sed commands are implicitly global. In an editor like ex, the command:

   s/old/new/

   will change "old" to "new" only on the current line unless you use the global command or various addressing symbols to apply it to additional lines. In sed, exactly the opposite is true. A command like the one above will be applied to all lines in a file. Addressing symbols are used to limit the extent of the match. (However, like ex, only the first occurrence of a pattern on a given line will be changed unless the g flag is added to the end of the substitution command.)

If all you want to do is make simple substitutions, you're ready to go. If you want to do more than that, sed has some unique and powerful commands.

This chapter makes no attempt to cover everything there is to know about sed. Article 34.24 contains a complete quick reference to sed commands, with many examples, because we use so many sed scripts elsewhere in this book, and we need a "dictionary" so beginners can interpret them. But for the most part, this chapter contains advice on working with sed and extended explanations of how to use some of its more difficult commands.