How is output passed as input in pipes?

Question

I am trying to understand how pipes | work; in particular, in what way the output from the left command is passed as the input to the right command. Is the output from the left command treated as a file that is passed as an argument to the right command, i.e., is

a | b

the same as

a > a_output
b a_output

This would mean that b takes a filename as input and reads it. But this would seem to make things like

cat wonderfulfile | tr 'a-z' 'A-Z'

impossible, since tr does not take a filename as input. Or is the output of the left command treated as standard input for the right command, i.e., if

a

prints

f
g
h

to the screen, is

a | b

the same as

b
f
g
h
^D

or perhaps

b f g h

or

b 'f g h'

This would seem to contradict lines like

who | less

since less, at least without options, requires a filename. Thank you for your help.

• ilkkachu: "A pipe basically just connects the stdout of one process to the stdin of another, effectively piping the data from one process into another." mywiki.wooledge.org/BashGuide/InputAndOutput#Pipes (not sure why it hedges by saying "basically")
• ilkkachu: "A pipeline is a set of processes chained together by their standard streams, so that the output text of each process (stdout) is passed directly as input (stdin) to the next one." en.wikipedia.org/wiki/Pipeline_(Unix)
• Archemar: less and other programs "knows" when input and/or ouput are a terminal, a file or a pipe and can behave accordingly.

Answer 1

It’s the second option: in

a | b

anything that a outputs to its standard output is provided on b’s standard input.

Your perceived contradiction isn’t one: less without options reads from its standard input (unless it’s a terminal, in which case it fails), without requiring a file name.

• Mr. Man: $ less Missing filename ("less --help" for help) $ less f f: No such file or directory
• Stephen Kitt: Try actually piping something into it, e.g. who | less as in your own example.
• Mr. Man: Yes, but I am trying to understand what is the equivalent of piping. Is the output of who taken as an argument to less, like less $(who), which doesn't seem to work, or if it is taken as standard input as if you typed less, pressed Enter, and then starting typing things, why does less fail when I type it on its own?
• Stephen Kitt: less checks whether its standard input is a terminal; if it is, it requires a file name, but if it isn’t, it reads from it. Try less <<<Hello.
• Stephen Kitt: I can’t find it documented, it’s visible in the less source code.

Answer 2

Is the output from the left command treated as a file that is passed as an argument to the right command

No.

The pipe connects the standard output to the standard input. That's really what it does. Just as your > redirects the standard output to a file, | redirects the standard output stream into the stream that is the standard input of the second command.

In other words, whenever program b does something like reading from the input, it reads what a has put onto its output. No file in between – the pipe is the integral "connection" between the output and the input. "standard output" and "standard input" are two streams every program has under a UNIX-style operating system (and also, most other operating systems for PC-style hardware).

• Mr. Man: What about programs like less, which, at least without options specified, require a file as an argument? In who | less, doesn't less expect a filename?
• Marcus Müller: It does not require an argument. less without a file argument just reads from its standard input.
• Paul_Pedant: less - makes it read stdin as a file, and also use the terminal input as command keys (Linux Mint 19.3) with unpleasant results. Needed to Ctrl-Z and kill %1 to end it, as Ctrl-C and Ctrl-D etc are apparently trapped.