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What does " 2>&1 " mean?

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

#161

Post by cesaref »

raincole wrote: post #95

The comments on stackoverflow say the words out of my mouth so I'll just copy & paste here: > but then shouldn't it rather be &2>&1? > & is only interpreted to mean "file descriptor" in the context of redirections. Writing command &2>& is parsed as command & and 2>&1 That's where all the confusion comes from. I believe most people can intuitively understand > is redirection, but the asymmetrical use of & throws them…

The way I read it, the prefix to the > indicates which file descriptor to redirect, and there is just a default that means no indicated file descriptor means stdout.

So, >foo is the same as 1>foo

If you want to get really into the weeds, I think 2>>&1 will create a file called 1, append to a file descriptor makes no sense (or maybe, truncate to a file descriptor makes no sense is maybe what I mean), but why this is the case is probably an oversight 50 years ago in sh, although i'd be surprised if this was codified anywhere, or relied upon in scripts.

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

#162

Post by jolmg »

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…

> 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 2Since 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.

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

#163

Post by LatencyKills »

rkachowski wrote: post #153
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'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…

MSGA: Make Software Great Again? /s

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

#165

Post by themafia »

amelius wrote: post #11

It's a reminder of how archaic the systems we use are. File descriptors are like handing pointers to the users of your software. At least allow us to use names instead of numbers. And sh/bash's syntax is so weird because the programmer at the time thought it was convenient to do it like that. Nobody ever asked a user.

> At least allow us to use names instead of numbers.

Sure. Here's what that looked like:

https://en.wikipedia.org/wiki/Job_Control_Language

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

#166

Post by mrpopo »

rkachowski wrote: post #153
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'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.

Sometimes you need someone to tell you no.

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

#168

Post by account42 »

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,…

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.

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

#169

Post by stuartjohnson12 »

parasti wrote: post #167
aichen_dev wrote: post #164

[dead]

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!

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