Sed (short for stream editor) is a non-interactive command line program that takes some text input, performs some command, and prints the results to standard out. Its support for regular expressions makes it extremely powerful, and is often the choice of tool when filtering or transforming text.
To call the sed program, simply use the
sed is such a versatile and powerful command line tool that there are entire books written on it. Most people use
sed for just search and replace operations, but there are quite a few other useful commands.
The steps in processing a sed invocation can be broken down to the following four parts:
There is also another memory area known as the hold buffer or the hold space. Data stored here do not get deleted, and are used for later retrieval. Although
sed clears out the pattern buffer after processing every line, it does not remove the hold buffer until all lines are exhausted.
For this tutorial, let's use the following text titled oneOS.txt. Save the following text to a file called oneOS.txt to follow through our examples.
One OS to rule them all, One OS to find them. One OS to call them all, And in salvation bind them. In the bright land of Linux, Where the hackers play.
Great! Time to try out some
sed, simply invoke the command, with the first argument being a string of commands. As we'll see, this may include line addresses, a sed command (deletion, substitution, printing, etc.), flags, and more.
Before we try out the
sed command, note that it may read from a file, or from read from standard in.
$ sed '' oneOS.txt $ cat oneOS.txt | sed '' $ sed '' < oneOS.txt
We prefer the first method (if possible), as it uses less resources and is easier to manage. The bottom command would fall under form of a Useless Use of Cat
sed out with no commands at all and see what it does.
$ sed '' oneOS.txt One OS to rule them all, One OS to find them. One OS to call them all, And in salvation bind them. In the bright land of Linux, Where the hackers play.
We can see here how sed operates. It reads from the input file, stores the first line in the pattern buffer, then applies the
sed command on the pattern buffer. In this case, there is no command, so no operation is performed. The (un)modified content is then outputted to standard output, and this loop is repeated for the second, third, fourth lines until the end of file.
Great! You're on your way to become a
sed guru. Let's now see how to specify line addresses to print specific lines out.
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