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What does " 2>&1 " mean?

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Re: What does " 2>&1 " mean?

#171

Post by cpach »

reacweb wrote: post #138
goku12 wrote: post #134

Earlier quoted context omitted.

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 us…

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…

Are you sure you understood the comment you replied to?

I agree that 2>&1 is not complex. But I think I speak for many Bash users when I say that this idiom looks bad, is hard to Google, hard to read and hard to memorize.

Re: What does " 2>&1 " mean?

#172

Post by cpach »

stuartjohnson12 wrote: post #169
parasti wrote: post #167

Earlier quoted context omitted.

Cool tip - never knew this. I always figured piping to `tee` is a must in order to view-and-save command output at the same time. Turns out I can do "command >&1 >file.txt" instead!

Unfortunately you are replying to an AI spambot

If you see an account that you suspect is a spambot, please send an email to hn@ycombinator.com, then the mods can take action.

Re: What does " 2>&1 " mean?

#173

Post by nazgul17 »

stephenr wrote: post #74
nazgul17 wrote: post #61

Earlier quoted context omitted.

Trying to be language agnostic: it should be as self-explanatory as possible. 2>&1 is all but. Why is there a 2 on the left, when the numbers are usually on the right. What's the relationship between 2 and 1? Is the 2 for std err? Is that `&` to mean "reference"? The fact you only grok it if you know POSIX sys calls means it's far from self explanatory. And given the proportion of people that know POSIX sys calls amo…

POSIX has a manual for shell. You can read 99% of it without needing to know any syscalls. I'm not as familiar with it but Bash has an extensive manual as well, and I doubt syscall knowledge is particularly required there either. If your complaint is "I don't know what this syntax means without reading the manual" I'd like to point you to any contemporary language that has things like arrow functions, or operator ove…

No, the complaint is that "the syntax is not intuitive even knowing the simpler forms of redirection": this one isn't a competition of them, but rather an ad-hoc one.

I know about manuals, and I have known this specific syntax for half of my life.

Arrow functions etc are mechanisms in the language. A template you can build upon. This one is just one special operator. Learn it and use it, but it will serve no other purpose in your brain. It won't make anything easier to understand. It won't help you decipher other code. It won't help you draw connections.

Re: What does " 2>&1 " mean?

#174

Post by worldsavior »

ifh-hn wrote: post #46
amelius wrote: post #23

Earlier quoted context omitted.

This is misleading because you use plural for both and I'm sure most of these UX missteps were _each_ made by a _single_ person, and there were >1 users even at the time.

> and there were >1 users even at the time. Are you sure there wasn't >&1 users... Sorry I'll get my coat.

Get out.

Re: What does " 2>&1 " mean?

#175

Post by darkwater »

goku12 wrote: post #134
wahern wrote: post #4

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 re…

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 us…

  ? (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.

Re: What does " 2>&1 " mean?

#176

Post by skywhopper »

cpach wrote: post #171
reacweb wrote: post #138

Earlier quoted context omitted.

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…

Are you sure you understood the comment you replied to? I agree that 2>&1 is not complex. But I think I speak for many Bash users when I say that this idiom looks bad, is hard to Google, hard to read and hard to memorize.

It’s not like someone woke up one morning and decided to design a confusing language full of shortcuts to make your life harder. Bash is the sum of decades of decisions made, some with poor planning, many contradictory, by hundreds of individuals working all over the world in different decades, to add features to solve and work around real world problems, keep backwards compatibility with decades of working programs, and attempt to have a shared glue language usable across many platforms. Most of the special syntax was developed long before Google existed.

So, sure, there are practical issues with details like this. And yet, it is simple. And there are simple methods for learning and retaining little tidbits like this over time if you care to do so. Bash and its cousins aren’t going away, so take notes, make a cheat sheet, or work on a better replacement (you’ll fail and make the problem worse, but go ahead).

Re: What does " 2>&1 " mean?

#177

Post by ruszki »

mrpopo wrote: post #166
rkachowski wrote: post #153

Earlier quoted context omitted.

It's really jarring to see this wave of nostalgia for "the good old days" appear since ~2025. Suddenly these rose tinted glasses have dropped and everything before LLM usage became ubiquitous was a beautiful romantic era of human collaboration, understanding and craftsmanship. I still acutely remember the gatekeeping and hostility of peak stack overflow, and the inanity of churning out jira tickets as fast as possibl…

Probably people complaining about AI today were fine with Stack Overflow before and didn't have anything to complain about back then. I also had a better experience with Stack Overflow over AI. It's been unable to tell me that I couldn't assign a new value to my std::optional in my specific case, and kept hallucinating copy constructor rules. A Stack Overflow question matching my problem cleared that up for me. Somet…

Or, like me, the kind of questions in which I’m interested are answered in a way worse rate by LLMs than StackOverflow, like ever.

I have and had problems with StackOverflow. But LLMs are nowhere near that, and unfortunately, as we can see, StackOverflow is basically dead, and that’s very problematic with kinda new things, like Android compose. There was exactly zero time when for example Opus could answer the best options for the first time, like a simple one, like I want a zero WindowInset object… it gives an answer for sure, and completely ignores the simplest one. And that happens all the time. I’m not saying that StackOverflow was good regarding this, but it was better for sure.

Re: What does " 2>&1 " mean?

#178

Post by NooneAtAll3 »

amelius wrote: post #23
zahlman wrote: post #14

Earlier quoted context omitted.

At the time, the users were the programmers.

This is misleading because you use plural for both and I'm sure most of these UX missteps were _each_ made by a _single_ person, and there were >1 users even at the time.

did you mean to write "<1"?

Re: What does " 2>&1 " mean?

#180

Post by webdevver »

solomonb wrote: post #66

Man I miss stack overflow. It feels so much better to ask humans a question then the machine, but it feels impossible to put the lid back on the box.

> It feels so much better to ask humans a question then the machine

I could not disagree more! With pesky humans, you have all sorts of things to worry about:

- is my question stupid? will they think badly of me if i ask it?

- what if they dont know the answer? did i just inadvertantly make them look stupid?

- the question i have is related to their current work... i hope they dont see me as a threat!

and on and on. asking questions in such a manner as to elicit the answer, without negative externalities, is quite the art form as i'm sure many stack overflow users will tell you. many word orderings trigger a 'latent space' which activates the "umm, why are you even doing this?" with the implication begin "you really are stupid!", totally useless to the question-asker and a much more frustrating time-waster than even the most moralizing LLM.

with LLMs, you don't have to play these 'token games'. you throw your query at it, and irrespective of the word order, word choice, or the nture of the question - it gives you a perfectly neutral response, or at worst politely refuses to answer.

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