tr a A < /etc/hosts | sort -r |pr -d > /etc/hosts

Question

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?

You should get a `bash: /etc/hosts: Permission denied` error (assuming your shell is bash), unless you're running this as `root`. `/etc/hosts` is normally writable by root only. Iff you're running this as `root` (which you really shouldn't), you'd get an empty `/etc/hosts` as Michael Homer explained. — arielf, Feb 26 '18 at 18:39

score 17 · Answer 1 · edited Feb 22 '18 at 06:34

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.

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Thank you very much for clarifying what I should expect with the way it was written and how to could be written to gain the expected outcome. — tdharrison, Feb 22 '18 at 01:47
[moreutils](https://joeyh.name/code/moreutils/) is a really great tool-suite — Robert Riedl, Feb 22 '18 at 08:01

tr a A < /etc/hosts | sort -r |pr -d > /etc/hosts

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