An applicable use of this is as follows. Say you have the following file file.txt:
1, 2, 6, 7, "p"
We want to replace “p” with 0.
sed 's/"p"/0/g' file.txt
Using the above simply prints the output into command line.
You’d think why not just redirect that text back into the file like this:
sed 's/"p"/0/g' file.txt > file.txt
Unfortunately because of the nature of redirects the above will simply produce a blank file.
Instead a temp file must be created for the output which later overwrites the original file something like this:
sed 's/"p"/0/g' file.txt > tmp.txt && mv tmp.txt file.txt
Instead of doing the long workaround above sed edit in place option with -i allows for a much simpler command:
sed -i 's/"p"/0/g' file.txt