Using the sed Command on Linux

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This may sound crazy, but the Linux sed command is a text editor without an interface. You can use it on the command line to manipulate text in files and streams. We show you how to take advantage of its power.

What is Sed?

How to use sed, a special editor for automatic file modification If you want to write a program to make changes to a file, sed is the tool you need to use.

There are a few programs that are the real workhorses of the UNIX Toolbox. These programs are easy to use for simple applications, but have a rich set of commands to perform complex actions. Don’t let the complex potential of a program prevent you from using the simpler aspects. I will start with the simple concepts and then introduce the more advanced topics.
When I first wrote this (1994), most versions of sed did not allow comments to be added to the script. Lines beginning with the “#” characters are comments. More recent versions of sed may also support end-of-line comments.

One can imagine that the old “classic” version was the basis for the GNU, FreeBSD and Solaris versions of sed.

sed is a feed editor. A feed editor is used to perform basic text transformations on an input feed (a file or input from a pipeline). Although it is similar in some ways to an editor that allows editing of scripts (such as ed), sed works by making a single pass at the input(s), and is therefore more efficient. But it is sed’s ability to filter text in a pipeline that makes it particularly different from other types of editors.

Sed Installation

There is not much to say here, as chances are you have already installed sed, as it is used in various system scripts and is an invaluable tool in the life of a Linux user who wants to work efficiently. You can test the version you have in

$ sed --version

On my system, this command tells me that I have installed GNU sed 4.2.1, plus links to the homepage and other useful things. The package is simply called “sed”, regardless of the distribution, but if Gentoo implicitly offers sed, I think that means you can be sure.

Understanding the sed Linux command

The sed command is a non-interactive text editor. The Linux sed command edits data according to the rules you specify, you can use it as follows:

$ sed options file

You are not limited to using sed to manipulate files, you apply it directly to the STDIN :

$ echo "Welcome to LikeGeeks page" | sed 's/page/website/'

The s command replaces the first text with the second text template. In this case, the string “web site” has been replaced by the word “page”, so the result is as shown.

The example above was a very simple example to demonstrate the tool. We can also use the sed Linux command to manipulate files.

Using Multiple sed Linux Commands in The Command Line

To execute multiple sed commands, you can use the -e option as follows:

$ sed -e 's/This/That/; s/test/another test/' ./myfile

The Sed command must be separated by a semicolon with no spaces.

You can also use a simple quote to separate commands like this one:

$ sed -e ‘

s/This/That/

s/test/another test/’ myfile

GNU sed’s Limitations (And Non-Limitations)

For those who want to write portable sed scripts, it should be noted that some implementations are known to limit line length (for template and load spaces) to 4000 bytes maximum. The POSIX standard specifies that compliant sed implementations must support at least 8192 bytes of line length. GNU sed has no built-in limit on line length; as long as it can allocate more (virtual) memory, you can insert or build lines as long as you want.

However, recursion is used to handle subtypes and undefined repetition. This means that the available stack memory may limit the size of the buffer that can be processed by some templates.

https://www.gnu.org/software/sed/manual/html_node/sed-commands-list.html

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