![]() ![]() *: The first subexpression of the search term contains a group in square brackets.\(: The opening parenthesis enclosing the subexpression, preceded by a backslash ( \). ![]() sed 's/: The sed command and the beginning of the substitution expression.To do this, you would type the following: sed 's/\(*\).*/\1/' /etc/passwd The parentheses also must be preceded by a backward slash ( \) to prevent them from being treated as a normal character. You have to enclose the subexpression in parentheses for this to work. You can then use these numbers in your sed commands to reference specific subexpressions. Each matched item in a search pattern (called subexpressions) can be numbered (up to a maximum of nine items). There are shorter ways to do this (more on that later), but we’ll use the longer way here to demonstrate another concept. Let’s give Coleridge a break and use sed to extract names from the etc/passwd file. We type the following to make our matching pattern the word “after”: sed -n '/after/ s/ay/week/gp' coleridge.txt The string “gonk” is replaced by “geek,” and the new string is printed in the terminal window. sed searches the input text for an occurrence of the first string, and will replace any matches with the second. The echo command sends “howtogonk” into sed, and our simple substitution rule (the “s” stands for substitution) is applied. To do so, we type the following: echo howtogonk | sed 's/gonk/geek/' RELATED: How to Use Regular Expressions (regexes) on Linux A Simple Exampleįirst, we’re going to use echo to send some text to sed through a pipe, and have sed substitute a portion of the text. ![]() You’re going to need some familiarity with these to get the best out of sed. However, the pattern matching and text selection functionalities of sed rely heavily on regular expressions ( regexes). We’ve structured our examples to introduce and demonstrate concepts, not to produce the tersest (and least approachable) sed commands. With sed you can do all of the following: This all works in Bash and other command-line shells. Rather, you provide instructions for it to follow as it works through the text. It doesn’t have an interactive text editor interface, however. Sed is a stream editor that works on piped input or files of text. ![]()
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