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The stream editor (sed) is a useful text parsing and manipulation tool. It can be used to do transformations on text that comes from the standard input or a file. The sed tool edits the text line by line and in a noninteractive way. In this way, you make the decisions as you call the command. These directions are executed automatically. This capability makes sed a powerful and fast tool to transform text.
Basic usage
The sed tool operates on text from the stdin or from a file. This behavior allows you to send the output of another command directly into sed for editing. You can also work from a file that you’ve previously created or edited.
Remember that sed outputs everything to stdout by default. If you want to save the edited text, you’ll need to redirect the output by using the redirect operator (>) as we did with the cat command.
The basic usage of sed is sed [options] commands [file-to-edit].
For example, to use sed directly on the NASA-software-API.txt file, you would run the command as follows:BashCopy
sed '' NASA-software-API.txt
This command will print the content of the file to the stdout. It’s almost equivalent to using the command cat. The single quotation marks contain the editing commands you pass to sed. In this case, you passed it nothing, so sed printed each line it received to standard output.
The sed tool can also use the stdin rather than a file. You can pipe the output of the cat command into sed to produce the same result:BashCopy
cat NASA-software-API.txt | sed ''
Text substitution with sed
Text substitution is perhaps the most well-known use for sed. As we learned before, sed can search for test patterns by using regular expressions. But sed can also replace the matched text with something else.
The basic syntax for text substitution is sed s'/old_text/new_text/', where s is the substitute command and the three forward slashes (/) are used to separate the text fields.
Imagine that you have the URL https://www.nasa.gov/about/sites/index.html, and you want to replace index.html with home. You can make this replacement by using the following command:BashCopy
echo "https://www.nasa.gov/about/sites/index.html" | sed s'/index.html/home/'
The output shows the modified URL:OutputCopy
https://www.nasa.gov/about/sites/homes
Let’s replace the NASA occurrences for National Aerospace Agency in the NASA-software-API.txt file:BashCopy
sed 's/NASA/National Aerospace Agency/' NASA-software-API.txt
You’ll notice that the substitution happened on all matches for NASA, but the command sed printed out all the lines in the file. This behavior is the default for the sed tool.
Let’s say we only want to print the lines in which the pattern was replaced. We can use the flag -n to suppress automatic printing and pass the option p to the replacement flag, like so:BashCopy
sed -n 's/NASA/National Aerospace Agency/p' NASA-software-API.txt
Write to a file
There’s one more flag that can follow the third delimiter. With it, you can specify a file that will receive the modified data. In the following example, we’ll write all lines in which we replaced NASA before:BashCopy
sed -n 's/NASA/National Aerospace Agency/w NASA-replaced.txt' NASA-software-API.txt
You should now have the new file NASA-replaced.txt.
