Pipes and redirection

A pipe connects standard output of the left command to standard input of the right one, and nothing touches the disk in between. That is why pipes are cheap and why you can chain as many as you like. It is not a comparison operator, which is a different use of the same character in some shells.

uniq compares each line to the one before it and nothing else, so identical lines that are scattered through the file are counted separately. Sorting first brings them together. Leaving the sort out is the single most common mistake in this pattern, and it fails quietly by giving you a wrong number.

Both redirect standard output into a file. A single arrow truncates the file first, so anything that was there is gone. A double arrow appends, leaving the existing contents alone. Getting this the wrong way round is how people lose a log they meant to keep.

Standard error is file descriptor 2, so 2> redirects it. /dev/null is a device that discards whatever is written to it. Together they silence the complaints while leaving the real output alone, which is useful in a script where you expect a command to grumble and do not want that in the report.