sed stands for “stream editor.” It accepts standard input and modifies it based on an expression, before displaying it as output data. It is similar to “find and replace.”
Let’s look at the expression 's/snow/rain/':
s: stands for “substitution.” It is always used when usingsedfor substitution.snow: the search string, or the text to find.rain: the replacement string, or the text to add in place.
In this case, sed searches forests.txt for the word “snow” and replaces it with “rain.” Importantly, the above command will only replace the first instance of “snow” on a line.
The above command uses the g expression, meaning “global.” Here sed searches forests.txt for the word “snow” and replaces it with “rain” globally. This means all instances of “snow” on a line will be turned to “rain.”
sed as we’ve used it will only rewrite the command line output and the actual file won’t be changed. In order to rewrite the actual file, we need to use -i at the beginning of the command:
The above command will rewrite forests.txt and replace all instances (since we’re also using g) of “snow” with “rain”.
Instructions
Use cat to display the contents of forests.txt
Use sed on forests.txt to replace just the first appearance of “snow” on each line with “rain”.
Use sed again to replace “snow” with “rain” but this time replace every occurrence of “snow”.
If you use cat to view the contents of forests.txt you’ll see that no changes have been made to the file. Use sed one last time to rewrite the file such that all occurrence of “snow” are permanently replaced by “rain”.
Use cat one more time to see the changes you’ve made to forests.txt.