[pbhjpbhj]

What's the line to test `yes` with `pv`?

    pv < /usr/bin/yes > /dev/null

doesn't seem to work properly. FWIW I get 330MiB/s vs 8.4GiB/s for /dev/zero.

[Incidentally first I've heard of pv but I've known about dd for a decade or two].

[masklinn]

< and > are for file redirection, yes is a binary so you want to pipe its stdout into pv:

    yes | pv > /dev/null

[pbhjpbhj]

So how did that redirect even work; should we be doing a `mknod` to make a "yes" device to make the comparison work (can we, does it help other than in my naive imagination).

[BenjiWiebe]

The reason that redirect worked is because it was using the contents of the yes program instead of its output.

[kevindqc]

I know this works, but how come we can see the output of pv when it is redirected to /dev/null? Maybe I just don't understand how pipes and redirection works since I rarely use Linux :(

[masklinn]

> I know this works, but how come we can see the output of pv when it is redirected to /dev/null?

From pv's man page:

> Its standard input will be passed through to its standard output and progress will be shown on standard error.

> Maybe I just don't understand how pipes and redirection works since I rarely use Linux :(

The Windows/DOS command line has the same concepts[0], though it's probably used less often: by default a process has 3 FDs for STDIN (0), STDOUT (1) and STDERR (2).

At a shell

* you can feed a file to STDIN via <$FILE (so `</dev/zero pv` will feed the data of /dev/zero to pv), the 0 is optional

* you can pipe an output to a command (other | command) or the output of a command to an other (command | other)

* you can redirect the STDOUT or STDERR to files via 1> and 2> (the "1" is optional so 1> and > are the same thing) (you can redirect both independently)

* you can "merge" either output stream to the other by redirecting them to &FD so 1>&2 will redirect STDOUT (1) to STDERR (2) and 2>&1 will redirect STDERR (2) to STDOUT (1), you can combine that with a regular redirection so e.g. `command >foo 2>&1 ` or with a pipe (`command 2>&1 | other`)

And you can actually create more FDs to use in your pipelines[1] though I don't remember ever seeing

[0] https://support.microsoft.com/en-us/help/110930/redirecting-...

[1] http://tldp.org/LDP/abs/html/io-redirection.html

[wutbrodo]

You're redirecting stdout to /dev/null and pv is writing to stderr. If you use &> instead, stderr and stdout will both be redirected to /dev/null and you will see no output at all.

[moosingin3space]

Maybe this?

    yes | pv > /dev/null

[wutbrodo]

That's not quite apples-to-apples though:

cat /dev/zero | pv > /dev/null

uses a pipe like the 'yes' line above, but runs substantially slower on my computer than the redirect-only version.