Sed - A Stream Editor

sed

Sed is a stream editor. It allows you to transform text when piping it through a series of commands. A common use for sed is to substitue characters. The syntax for substitution in sed is s/old/new/g, where new replaces old. The g means to replace globally on the line. If you leave the g off, sed will only replace the first occurance of old. You can replace g with a number to replace a certain instance of a word. For example, to replace the 2nd occurance of old with new on each line you could use s/old/new/2.

An example of sed would be to take a file with a list of URLs and remove the query strings from the URLs like this:

cat urls.txt | sed 's/\?.*//g'

The backslash is an escape character. Because the question mark has a meaning in regular expressions, the backslash escapes that regular meaning so that it is treated just as a normal question mark. The period indicates any character, and the asterisk means "zero or more of the preceeding character". When put together it means replace the question mark and any characters after it with nothing.

For an overview of what sed can do, type man sed in a terminal, and check out some of these sed resources:

• sed tutorial
• comprehensive sed resource
• sed one-liners
• another sed tutorial
• Some nice sed recipes