In this line:
tr a A < /etc/hosts | sort -r |pr -d > /etc/hosts
what would be the expected outcome? I know lowercase 'a' would change to uppercase 'A' but what's next? Would the original file be overwritten since the redirect is to the same file name?
The expected outcome is a blank /etc/hosts file.
The redirection > /etc/hosts occurs and truncates the file before the programs start running and tr begins to read from the file.
To write the output into /etc/hosts, you could either work with a copy of the file (or move your output file into place afterwards), or use the sponge command from moreutils, which will soak up standard input and write to a file:
tr a A < /etc/hosts | sort -r | pr -d | sponge /etc/hosts
In that case, each "a" in /etc/hosts will be replaced with "A", all lines will be sorted in reverse according to your locale, a blank line will be inserted between each line, and the result will be put into /etc/hosts.
You may also find useful:
bash: /etc/hosts: Permission deniederror (assuming your shell is bash), unless you're running this asroot./etc/hostsis normally writable by root only. Iff you're running this asroot(which you really shouldn't), you'd get an empty/etc/hostsas Michael Homer explained. – arielf Feb 26 '18 at 18:39