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0 pointsmmastrac6y ago0 comments

It might be easiest to view it like so:

  $ echo <(echo 1) <(echo 2)
  /dev/fd/63 /dev/fd/62

The shell spawns two commands connected to pipes, then replaces those with a file that represents the other (read) side of that pipe.

The command above with >(grep something) does the same thing, just with the other end of a pipe.

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8 comments · 2 top-level

deaddodo6y ago· 4 in thread

It's not actually a pipe. >, < and | are IO redirection commands; but of those only | is a pipe. Pipes specifically "pipe" the STDOUT of one command as the STDIN of another. The other two instead point a command to the file descriptor of the output of another.

This is explained in more detail here:

https://askubuntu.com/questions/172982/what-is-the-differenc...

TheDong6y ago

It is a pipe, just for a different definition of pipe.

| is the pipe operator in sh for doing a shell "pipeline". However, the parent comment is referring to a linux pipe as in pipe(7) [0].

One easy way to see this is with the following:

    $ ls -l <(echo foo)
    lr-x------ 1 user group Jan 12 01:23 /proc/self/fd/16 -> 'pipe:[2937585]'

As you can see, that command created a fd (16) which referred to a pipe (pipe:[2937585]).

Those file descriptors were created using the pipe(2)[1] call by the shell, so it seems fine to refer to them to pipes.

I'll also note that <() / >() are _not_ using the "redirection operator". They're actually distinct operators for "process substitution"[2] in bash terminology. They look similar to redirects, but they're not the same operator, so that stack overflow answer isn't really relevant.

[0]: http://man7.org/linux/man-pages/man7/pipe.7.html

[1]: http://man7.org/linux/man-pages/man2/pipe.2.html

[2]: https://tldp.org/LDP/abs/html/process-sub.html

0x8BADF00D6y ago

On POSIX systems, the line between file and pipe doesn’t exist. If you create a named pipe w/ mknod or mkfifo, you would interact with the named pipe as you would a normal file. Or block/character device. Etc.

In case of unnamed pipe the fds just exist as long as the process is running, then it disappears. Point is it’s irrelevant what you refer to as pipe, at the end of the day some file-like object is either written to or read from. And really, that file-like object is actually just a buffer.

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mmastracOP6y ago

On linux at least, these two constructions are effectively identical from the view of the "piped" process.

  # cat <(ls -l /proc/self/fd/)
  total 0
  lrwx------ 1 root root 64 Jan 12 15:02 0 -> /dev/pts/0
  l-wx------ 1 root root 64 Jan 12 15:02 1 -> pipe:[20519634]
  lrwx------ 1 root root 64 Jan 12 15:02 2 -> /dev/pts/0
  lr-x------ 1 root root 64 Jan 12 15:02 3 -> /proc/23611/fd/

  # ls -l /proc/self/fd/ | cat -
  total 0
  lrwx------ 1 root root 64 Jan 12 15:02 0 -> /dev/pts/0
  l-wx------ 1 root root 64 Jan 12 15:02 1 -> pipe:[20518265]
  lrwx------ 1 root root 64 Jan 12 15:02 2 -> /dev/pts/0
  lr-x------ 1 root root 64 Jan 12 15:02 3 -> /proc/23621/fd/

deaddodo6y ago

Yes, because “cat” can use either STDIN or a file as input:

https://github.com/coreutils/coreutils/blob/master/src/cat.c...

It’s an intentional design decision.

3 more replies

lostmsu6y ago· 2 in thread

Does anybody know if this can be done in Windows (not WSL)? E.g. is it possible to open a named pipe (?) through its name, and work with it as a sequential file?

mbreese6y ago

This isn’t a Linux thing, it’s a bash thing. The <() syntax isn’t using a named pipe — it is creating an anonymous pipe. But you can replicate the same functions with named pipes.

A quick search says that Windows has support for named pipes as an IPC mechanism [1], but I’ve never used them. But this is a Windows API. I am not aware for any way to do this with plain cmd.exe. You’re not the first to ask though... [2]

[1] https://docs.microsoft.com/en-us/windows/win32/ipc/named-pip...

[2] https://superuser.com/questions/430466/in-windows-can-i-redi...

lostmsu6y ago

Are you sure? How does diff know to read from those two pipes?

Even the echo example hints, that the created pipe has a name, and that name is then simply passed as the argument to the command.

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8 comments · 2 top-level

deaddodo6y ago· 4 in thread

This is explained in more detail here:

https://askubuntu.com/questions/172982/what-is-the-differenc...

TheDong6y ago

It is a pipe, just for a different definition of pipe.

| is the pipe operator in sh for doing a shell "pipeline". However, the parent comment is referring to a linux pipe as in pipe(7) [0].

One easy way to see this is with the following:

    $ ls -l <(echo foo)
    lr-x------ 1 user group Jan 12 01:23 /proc/self/fd/16 -> 'pipe:[2937585]'

As you can see, that command created a fd (16) which referred to a pipe (pipe:[2937585]).

Those file descriptors were created using the pipe(2)[1] call by the shell, so it seems fine to refer to them to pipes.

[0]: http://man7.org/linux/man-pages/man7/pipe.7.html

[1]: http://man7.org/linux/man-pages/man2/pipe.2.html

[2]: https://tldp.org/LDP/abs/html/process-sub.html

0x8BADF00D6y ago

1 more reply

mmastracOP6y ago

On linux at least, these two constructions are effectively identical from the view of the "piped" process.

  # cat <(ls -l /proc/self/fd/)
  total 0
  lrwx------ 1 root root 64 Jan 12 15:02 0 -> /dev/pts/0
  l-wx------ 1 root root 64 Jan 12 15:02 1 -> pipe:[20519634]
  lrwx------ 1 root root 64 Jan 12 15:02 2 -> /dev/pts/0
  lr-x------ 1 root root 64 Jan 12 15:02 3 -> /proc/23611/fd/

  # ls -l /proc/self/fd/ | cat -
  total 0
  lrwx------ 1 root root 64 Jan 12 15:02 0 -> /dev/pts/0
  l-wx------ 1 root root 64 Jan 12 15:02 1 -> pipe:[20518265]
  lrwx------ 1 root root 64 Jan 12 15:02 2 -> /dev/pts/0
  lr-x------ 1 root root 64 Jan 12 15:02 3 -> /proc/23621/fd/

deaddodo6y ago

Yes, because “cat” can use either STDIN or a file as input:

https://github.com/coreutils/coreutils/blob/master/src/cat.c...

It’s an intentional design decision.

3 more replies

lostmsu6y ago· 2 in thread

Does anybody know if this can be done in Windows (not WSL)? E.g. is it possible to open a named pipe (?) through its name, and work with it as a sequential file?

mbreese6y ago

This isn’t a Linux thing, it’s a bash thing. The <() syntax isn’t using a named pipe — it is creating an anonymous pipe. But you can replicate the same functions with named pipes.

[1] https://docs.microsoft.com/en-us/windows/win32/ipc/named-pip...

[2] https://superuser.com/questions/430466/in-windows-can-i-redi...

lostmsu6y ago

Are you sure? How does diff know to read from those two pipes?

Even the echo example hints, that the created pipe has a name, and that name is then simply passed as the argument to the command.

1 more reply

j / k navigate · click thread line to collapse