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What does " 2>&1 " mean?

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

#51

Post by jez »

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…

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

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

#52

Post by nodesocket »

I understand how this works, but wouldn’t a more clear syntax be:

command &2>&1

Since the use of & signifies a file descriptor. I get what this ACTUALLY does is run command in the background and then run 2 sending its stout to stdout. That’s completely not obvious by the way.

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

#53

Post by kccqzy »

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…

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.

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

#54

Post by Normal_gaussian »

amelius wrote: post #27
HackerThemAll wrote: post #18

Earlier quoted context omitted.

> bash's syntax is so weird What should be the syntax according to contemporary IT people? JSON? YAML? Or just LLM prompt?

Honestly, Python with the "sh" module is a lot more sane.

Is it more sane, or is it just what you are used to?

Python doesn't really have much that makes it a sensible choice for scripting.

Its got some basic data structures and a std-lib, but it comes at a non-trivial performance cost, a massive barrier to getting out of the single thread, and non-trivial overhead when managing downstream processes. It doesn't protect you from any runtime errors (no types, no compile checks). And I wouldn't call python in practice particularly portable...

Laughably, NodeJS is genuinely a better choice - while you don't get multithreading easily, at least you aren't trivially blocked on IO. NodeJS also has pretty great compatibility for portability; and can be easily compiled/transformed to get your types and compile checks if you want. I'd still rather avoid managing downstream processes with it - but at least you know your JSON parsing and manipulation is trivial.

Go is my goto when I'm reaching for more; but (ba)sh is king. You're scripting on the shell because you're mainly gluing other processes together, and this is what (ba)sh is designed to do. There is a learning curve, and there are footguns.

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

#55

Post by dheera »

nodesocket wrote: post #52

I understand how this works, but wouldn’t a more clear syntax be: command &2>&1 Since the use of & signifies a file descriptor. I get what this ACTUALLY does is run command in the background and then run 2 sending its stout to stdout. That’s completely not obvious by the way.

even clearer syntax:

command &stderr>&stdout

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

#56

Post by nikeee »

So if i happen to know the numbers of other file descriptors of the process (listed in /proc), i can redirect to other files opened in the current process? 2>&1234? Or is it restricted to 0/1/2 by the shell?

Would probably be hard to guess since the process may not have opened any file once it started.

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

#57

Post by ndsipa_pomu »

amelius wrote: post #29
csours wrote: post #25

Earlier quoted context omitted.

The conveniences also mean that there is more than ~one~ ~two~ several ways to do something. Which means that reading someone else's shell script (or awk, or perl, or regex) is INCREDIBLY inconvenient.

Yes. There are many reasons why one shouldn't use sh/bash for scripting. But my main reason is that most scripts break when you call them with filenames that contain spaces. And they break spectacularly.

You're not wrong, but there's fairly easy ways to deal with filenames containing spaces - usually just enclosing any variable use within double quotes will be sufficient. It's tricker to deal with filenames that contain things such as line breaks as that usually involves using null terminated filenames (null being the only character that is not allowed in filenames). e.g find . -type f -print0

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

#58

Post by Normal_gaussian »

I know the underlying call, but I always see the redirect symbols as indicating that "everything" on the big side of the operator fits into a small bit of what is on the small side of the operator. Like a funnel for data. I don't know the origin, but I'm believing my fiction is right regardless. It makes The comment about "why not &2>&1" is probably the best one on the page, with the answer essentially being that it would complicate the parser too much / add an unnecessary byte to scripts.

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

#59

Post by 47282847 »

jez wrote: post #51
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…

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,…

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