[wahern]

I find it easier to understand in terms of the Unix syscall API. `2>&1` literally translates as `dup2(1, 2)`, and indeed that's exactly how it works. In the classic unix shells that's all that happens; in more modern shells there may be some additional internal bookkeeping to remember state. Understanding it as dup2 means it's easier to understand how successive redirections work, though you also have to know that redirection operators are executed left-to-right, and traditionally each operator was executed immediately as it was parsed, left-to-right. The pipe operator works similarly, though it's a combination of fork and dup'ing, with the command being forked off from the shell as a child before processing the remainder of the line.

Though, understanding it this way makes the direction of the angled bracket a little odd; at least for me it's more natural to understand dup2(2, 1) as 2<1, as in make fd 2 a duplicate of fd 1, but in terms of abstract I/O semantics that would be misleading.

[jez]

Another fun consequence of this is that you can initialize otherwise-unset file descriptors this way:

    $ cat foo.sh
    #!/usr/bin/env bash

    >&1 echo "will print on stdout"
    >&2 echo "will print on stderr"
    >&3 echo "will print on fd 3"

    $ ./foo.sh 3>&1 1>/dev/null 2>/dev/null
    will print on fd 3

It's a trick you can use if you've got a super chatty script or set of scripts, you want to silence or slurp up all of their output, but you still want to allow some mechanism for printing directly to the terminal.

The danger is that if you don't open it before running the script, you'll get an error:

    $ ./foo.sh
    will print on stdout
    will print on stderr
    ./foo.sh: line 5: 3: Bad file descriptor

[hielke]

With exec you can open file descriptors of your current process.

  if [[ ! -e /proc/$$/fd/3 ]]; then
      # check if fd 3 already open and if not open, open it to /dev/null
      exec 3>/dev/null
  fi
  >&3 echo "will print on fd 3"

This will fix the error you are describing while keeping the functionality intact.

Now with that exec trick the fun only gets started. Because you can redirect to subshells and subshells inherit their redirection of the parent:

  set -x # when debugging, print all commands ran prefixed with CMD:
  PID=$$
  BASH_XTRACEFD=7
  LOG_FILE=/some/place/to/your/log/or/just/stdout
  exec 3> >(gawk '!/^RUN \+ echo/{ print strftime("[%Y-%m-%d %H:%M:%S] <PID:'$PID'> "), $0; fflush() }' >> $LOG_FILE)
  exec > >(sed -u 's/^/INFO:  /' >&3)
  exec 2> >(sed -u 's/^/ERROR: /' >&3)
  exec 7> >(sed -u 's/^/CMD:   /' >&3)
  exec 8>&1 #normal stdout with >&8
  exec 9>&2 #normal stderr with >&9

And now your bash script will have a nice log with stdout and stderr prefixed with INFO and ERROR and has timestamps with the PID.

Now the disclaimer is that you will not have gaurantees that the order of stdout and stderr will be correct unfortunately, even though we run it unbuffered (-u and fflush).

[account42]

If you just want to print of the terminal even if normal stdout/stderr is disabled you can also use >/dev/tty but obviously that is less flexible.

[47282847]

Interesting. Is this just literally “fun”, or do you see real world use cases?

[goku12]

This is probably one of the reasons why many find POSIX shell languages to be unpleasant. There are too many syntactical sugars that abstract too much of the underlying mechanisms away, to the level that we don't get it unless someone explains it. Compare this with Lisps, for example. There may be only one branching construct or a looping construct. Yet, they provide more options than regular programming languages using macros. And this fact is not hidden from us. You know that all of them ultimately expand to the limited number of special forms.

The shell syntactical sugars also have some weird gotchas. The &2>&1 question and its answer are a good example of that. You're just trading one complexity (low level knowledge) for another (the long list of syntax rules). Shell languages break the rule of not letting abstractions get in the way of insight and intuitiveness.

I know that people will argue that shell languages are not programming languages, and that terseness is important for the former. And yet, we still have people complaining about it. This is the programmer ego and the sysadmin ego of people clashing with each other. After all, nobody is purely just one of those two.

[skywal_l]

There must be a law of system design about this, because this happens all the time. Every abstraction creates a class of users who are powerful but fragile.

People who build a system or at least know how it works internally want to simplify their life by building abstractions.

As people come later to use the system with the embedded abstractions, they only know the abstractions but have no idea of the underlying implementations. Those abstractions used to make perfect sense for those with prior knowledge but can also carry subtle bias which makes their use error prone for non initiated users.

[reacweb]

make 2>&1 | tee m.log is in my muscle memory, like adding a & at the end of a command to launch a job, or ctrl+z bg when I forget it, or tar cfz (without the minus so that the order is not important). Without this terseness, people would build myriads of personal alias.

This redirection relies on foundational concepts (file descriptors, stdin 0, stdout 1, stderr 2) that need to be well understood when using unix. IMO, this helps to build insight and intuitiveness. A pipe is not magic, it is just a simple operation on file descriptors. Complexity exists (buffering, zombies), but not there.

[miki123211]

Shell is optimized for the minimal number of keystrokes (just like Vim, Amadeus and the Bloomberg Terminal are optimized for the minimum number of keystrokes. Programming languages are primarily optimized for future code readability, with terseness and intuitiveness being second or third (depending on language).

[darkwater]

  ? (defun even(num) (= (mod num 2) 0))
  ? (filter '(6 4 3 5 2) #'even)

I'm zero Lisp expert and I don't feel comfortable at all reading this snippet.

[emmelaich]

Yep, there's a strong unifying feel between the Unix api, C, the shell, and also say Perl.

Which is lost when using more modern or languages foreign to Unix.

[tkcranny]

Python too under the hood, a lot of its core is still from how it started as a quick way to do unixy/C things.

[kccqzy]

And just like dup2 allows you to duplicate into a brand new file descriptor, shells also allow you to specify bigger numbers so you aren’t restricted to 1 and 2. This can be useful for things like communication between different parts of the same shell script.

[momentoftop]

> The pipe operator works similarly, though it's a combination of fork and dup'ing

Any time the shell executes a program it forks, not just for redirections. Redirections will use dup before exec on the child process. Piping will be two forks and obviously the `pipe` syscall, with one process having its stdout dup'd to the input end of the pipe, and the other process having its stdin dup'd to the output end.

Honestly, I find the BASH manual to be excellently written, and it's probably available on your system even without an internet connection. I'd always go there than rely on stack overflow or an LLM.

https://www.gnu.org/software/bash/manual/bash.html#Redirecti...

[ifh-hn]

Haha, I'm even more confused now. I have no idea what dup is...

[jpollock]

There are a couple of ways to figure out.

open a terminal (OSX/Linux) and type:

    man dup

open a browser window and search for:

    man dup

Both will bring up the man page for the function call.

To get recursive, you can try:

    man man unix

(the unix is important, otherwise it gives you manly men)

[ontouchstart]

I did a google search on “dup2(2, 1)” in a fresh private tab on my iPhone Safari and this thread came up the second, between

https://man7.org/linux/man-pages/man2/dup.2.html

and

https://man.archlinux.org/man/dup2.2.en

A lot of bots are reading this. Amazing.

[jolmg]

> Though, understanding it this way makes the direction of the angled bracket a little odd; at least for me it's more natural to understand dup2(2, 1) as 2<1, as in make fd 2 a duplicate of fd 1, but in terms of abstract I/O semantics that would be misleading.

Since they're both just `dup2(1, 2)`, `2>&1` and `2<&1` are the same. However, yes, `2<&1` would be misleading because it looks like you're treating stderr like an input.

[niobe]

I find it very intuitive as is

[manbash]

Respectfully, what was the purpose of this comment, really?

And I also disagree, your suggestion is not easier. The & operator is quite intuitive as it is, and conveys the intention.

[goku12]

Perhaps it is intuitive for you based on how you learned it. But their explanation is more intuitive for anyone dealing with low level stuff like POSIX-style embedded programming, low level unix-y C programming, etc, since it ties into what they already know. There is also a limit to how much you can learn about the underlying system and its unseen potential by learning from the abstractions alone.

> Respectfully, what was the purpose of this comment, really?

Judging by its replies alone, not everyone considers it purposeless. And even though I know enough to use shell redirections correctly, I still found that comment insightful. This is why I still prefer human explanations over AI. It often contains information you didn't think you needed. HN is one of the sources of the gradually dwindling supply of such information. That comment is still on-topic. Please don't discourage such habits.