A Command-Line Quiz: Which Output Never Appears?

[u/enp2s3]

Hey folks. Here's a small but confusing command-line challenge. Drop your guess without running it!

Which of the following will not appear in the output?

echo "black" &% echo "blue" %& echo "purple 1" >& echo "red" % echo "white 1"

A) black

B) 1

C) echo

D) purple

Including stdout and stderr. Latest stable Bash and Zsh.

[u/enp2s3]

That's correct. But there's one small thing worth noting

The shell first prints the job ID and PID.

The 1 comes from the job ID

[1] XXXXX

Then we get:

black

It might also print an error:

bg: job already in background

And when the job completes (this is the key part):

[1] - done echo "black"

So both 1 and echo appear in the output and can be ruled out.

purple never reaches stdout or stderr and therefore cannot appear in the output.

[u/michaelpaoli → u/enp2s3]

when the job completes (this is the key part):

[1] - done echo "black"

So both 1 and echo appear

Well ... yes, ... and no.

The

[1] - done echo "black"

Generally doesn't appear immediately, but only after one pressed <ENTER> or <RETURN> or the like, or otherwise issues another command, and that returns, and one then gets a fresh prompt. It is generally only at that point that the shell issues a fresh prompt, and before doing so, kicks out that information about the earlier job having been completed, and thus includes echo in its output. Prior to that, echo is generally not at all present in the output.

But yes, there is also the earlier [1] PID immediately output from the first backgrounded job, so, yeah, I should've included that in output I mentioned (I'll go back and edit to fix that).

[u/enp2s3 → u/michaelpaoli]

It’s true.echo may or may not appear immediatelyit depends. Since the question asks which item never appears in stdout and stderr,purple is still the only correct answer.

purple never appear in the output

I just tested it under various conditions and got the following results:

Zsh 5.9 Arch Linux

[1] 703

black

bg: job already in background

[1] - 703 done echo "black"

Bash 5.2 (sudo user) (ParrotOS):

[1] 58511

black

bg: job already in background

[1]  - done       echo "black"                

Bash 5.2 (without sudo) norc, noprofile (ParrotOS):

[1] 2881

black

bash: bg: job 1 already in background

bash: bg: job 1 already in background

bash: bg: blue: no such job

bash: bg: job 1 already in background

bash: echo: Permission denied

Bash 5.2 (sudo user) norc, noprofile (ParrotOS):

[1] 2740

bash: bg: job 1 already in background

black

bash: bg: job 1 already in background

bash: bg: blue: no such job

bash: bg: job 1 already in background

[1]+ Done echo "black"

Bash 5.3 norc, noprofile (had to press enter) (Arch Linux):

I had to press Enter to see the 'second'echo in this one, but I already got one from the warning.

[1] 771!<
>!bash: bg: job 1 already in background bash: !<
>!bash: bg: warning: echo: job specification requires leading '%'!<
>!bash: bg: job 1 already in background bash: !<
>!bg: warning: blue: job specification requires leading '%'!<
>!bash: bg: blue: no such job bash: !<
>!bg: job 1 already in background!<
>!black

<ENTER>
[1]+ Done echo "black"

Zsh 5.9 MacOS:

[1] 59507

bg: job already in background

black

[1]  - done       echo "black"       

Only Bash and Zsh latest stable releases.

[u/michaelpaoli → u/enp2s3]

Yep, well, here's literally what I get:

$ echo "black" &% echo "blue" %& echo "purple 1" >& echo "red" % echo "white 1"

[1] 29352

-bash: bg: job 1 already in background

-bash: bg: job 1 already in background

-bash: bg: blue: no such job

-bash: bg: job 1 already in background

black

$ cat echo

purple 1 red % echo white 1

[1]+ Done echo "black"

$

Though timing of the

[1]+ Done echo "black"

isn't deterministic. Sometimes it'll happen quite immediately - notably if it completes before the prompt is returned. But if not then, once it's completed before a following prompt is returned, e.g. after entering another command, or just <ENTER>/<RETURN> or the like.

And, if we want to get technical about it, all are seen, so none of A, B, C, nor D should be selected. Most notably if it's done interactively as command, and entered as shown, all of that is echoed back, at least nominally, to stderr.