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.
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.
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.
Seems related to the Law of Leaky Abstractions?
It's not necessarily a leaky abstraction. But a lack of _knowledge in the world_.
The abstraction may be great, the problem is the lack of intuitive understanding you can get from super terse, symbol heavy syntax.
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.
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).
? (defun even(num) (= (mod num 2) 0))
? (filter '(6 4 3 5 2) #'even)
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
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 descriptorIf 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.
Interesting. Is this just literally “fun”, or do you see real world use cases?
The aws cli has a set of porcelain for s3 access (aws s3) and plumbing commands for lower level access to advanced controls (aws s3api). The plumbing command aws s3api get-object doesn't support stdout natively, so if you need it and want to use it in a pipeline (e.g. pv), you would naively do something like
$ aws s3api get-object --bucket foo --key bar /dev/stdout | pv ...
You can, though, do the following:
$ aws s3api get-object --bucket foo --key bar /dev/fd/3 3>&1 >/dev/null | pv ...
Would be nice if `curl` had something to dump headers to a third file descriptor while outputting the response on stdout.
This should work?
curl --dump-header /dev/fd/xxx https://google.com
mkfifo headers.out
curl --dump-header headers.out https://google.com
Ah yeah, `/dev/fd/xxx` works :) somehow thought that was Linux only.
I have used this in the past when building shell scripts and Makefiles to orchestrate an existing build system:
https://github.com/jez/symbol/blob/master/scaffold/symbol#L1...
The existing build system I did not have control over, and would produce output on stdout/stderr. I wanted my build scripts to be able to only show the output from the build system if building failed (and there might have been multiple build system invocations leading to that failure). I also wanted the second level to be able to log progress messages that were shown to the user immediately on stdout.
Level 1: create fd=3, capture fd 1/2 (done in one place at the top-level)
Level 2: log progress messages to fd=3 so the user knows what's happening
Level 3: original build system, will log to fd 1/2, but will be captured
One of my use-cases previously has been enforcing ultimate or fully trust of a gpg signature.
tmpfifo="$(mktemp -u -t gpgverifyXXXXXXXXX)"
gpg --status-fd 3 --verify checksums.txt.sig checksums.txt 3>$tmpfifo
grep -Eq '^\[GNUPG:] TRUST_(ULTIMATE|FULLY)' $tmpfifo
You can also redirect specific file descriptors into other commands:
gpg --status-fd 3 --verify checksums.txt.sig checksums.txt 3>(grep -Eq '^\[GNUPG:] TRUST_(ULTIMATE|FULLY)')Red hat and other RPM based distributions recommended kickstart scripts use tty3 using a similar method
Multiple levels of logging, all of which you want to capture but not all in the same place.
Wasn't the idiomatic way the `-v` flag (repeated for verbosity). And then stderr for errors (maybe warning too).
It is, and all logs should ideally go to stderr. But that doesn’t let you pipe them to different places.
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.
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.
> 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.
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.
Haha, I'm even more confused now. I have no idea what dup is...
There are a couple of ways to figure out.
open a terminal (OSX/Linux) and type:
man dup
man dup
To get recursive, you can try:
man man unix
otherwise it gives you manly men
That's only just after midnight [1][2]
[1] - https://www.youtube.com/watch?v=XEjLoHdbVeE
[2] - https://unix.stackexchange.com/questions/405783/why-does-man...
I love that this situation occured.
you may also consider gnu info
info dupI find it very intuitive as is
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.
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.
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 off.
Interestingly, Powershell also uses 2>&1. Given an once-a-lifetime chance to redesign shell, out of all the Unix relics, they chose to keep (borrow) this.
PowerShell actually has 7 streams. Success, Error, Warning, Verbose, Debug, Information, and Progress (though Progress doesn't get a number) https://learn.microsoft.com/en-us/powershell/module/microsof...
Although PowerShell borrows the syntax, it (as usual!) completely screws up the semantics. The examples in the docs [1] show first setting descriptor 2 to descriptor 1 and then setting descriptor 1 to a newly opened file, which of course is backwards and doesn't give the intended result in Unix; e.g. their example 1:
dir C:\, fakepath 2>&1 > .\dir.log
[1] https://learn.microsoft.com/en-us/powershell/module/microsof...
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.
I don't get the confusion.
You redirect stdout with ">" and stderr with "2>" (a two-letter operator).
If you want to redirect to stdout / stderr, you use "&1" or "&2" instead of putting a file name.
Isn't that because of posix?
Powershell is not posix compliant and does not pretend to be. Like conditionals using `()` instead of `[]` is already a clear departure from posix
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 possible for misguided product initiatives. It's just wild yo
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.
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.
I don’t think I’ve ever ask a question on Stack Overflow, but I’ve consulted it several time. Even when I’ve not found my exact use case, there’s always something similar or related that gave me the right direction for research (a book or an article reference, the name of a concept to use as keyword,…)
It’s kinda the same feeling when browsing the faq of a project. It gives you a more complete sense of the domain boundaries.
I still prefer to refer to book or SO instead of asking the AI. Coherency and purposefulness matter more to me then a direct answer that may be wrong.
MSGA: Make Software Great Again? /s
and no ai fluff to start or end the answer, just facts straight to the point.
Perhaps you mean searching for your question first, before asking. :)
It is possible. Many people choose a healthy lifestyle instead of becoming morbidly obese and incapable which is easy to do in our society.
> 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.
That’s a level of paranoia that I can’t really understand. I just do my research, then for information I can’t access, don’t know how to access, or can’t comprehend, I reach out. People have the right to not want to share information. If it’s in a work setting and the situation is blocking, I notify my supervisor.
> 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!"
You may have heard of the XY situation when people asks a Y question only because they have an incorrect answer to X. A question has a goal (unless rethorical) and to the person being asked, it may be confusing. You may have a valid reason to go against common sense, but if the other person is not your tutor or a fellow researcher, he may not be willing to accommodate you and spend his time for a goal he have no context about.
Remember the car wash question for LLMs? Some phrasing have the pattern of a trick question and that’s another thing people watch out for.
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 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.
I think he meant that at that time all users were programmers. Yes, _all_ .
It was a bit of an over-generalization, but yes that's basically what I was going for.
> 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.
I think that's likely to work as a no-op
did you mean to write "<1"?
programmers are people too! bash syntax just sucks
arguably if you're using the CLI they still are
Yeah but now they're using npm to install a million packages to do things like tell if a number is greater than 10000. The chances of the programmer wanting to understand the underlying system they are using is essentially nil.
Yea, they are just much higher level programmers… most programmers don’t know the low level syscall apis.
nah, we have long had other disciplines using the CLI who do not write their own software, e.g. sysadmins
> At least allow us to use names instead of numbers.
You can for the destination. That's the whole reason you need the "&": to tell the shell the destination is not a named file (which itself could be a pipe or socket). And by default you don't need to specify the source fd at all. The intent is that stdout is piped along but stderr goes directly to your tty. That's one reason they are separate.
And for those saying "<" would have been better: that is used to read from the RHS and feed it as input to the LHS so it was taken.
You can do:
2>/dev/stdout
And of course I wish you could use a friendlier name for STDERR instead of `2>`
> You can do: > > 2>/dev/stdout
The situation where this is going to cause confusion is when you do this for multiple commands. It looks like they're all writing to a single file. Of course, that file is not an ordinary file - it's a device file. But even that isn't enough. You have to know that each command sees its own incarnation of /dev/stdout, which refers to its own fd1.
It should be a lesson to learn on how simple, logical and reliable tools can last decades.
… Or how hard it is to replace archaic software that’s extremely prevalent.
Bash syntax is anything but simple or logical. Just look at the insane if-statement syntax. Or how the choice of quotes fundamentally changes behavior. Argument parsing, looping, the list goes on.
if statements are pretty simple
if $command; then <thing> else <thing> fi
You may be complaining about the syntax for the test command specifically or bash’s [[ builtin
Also the choice of quotes changing behavior is a thing in:
1. JavaScript/typescript 2. Python 3. C/C++ 4. Rust
In some cases it’s the same difference, eg: string interpolation in JavaScript with backticks
> Also the choice of quotes changing behavior is a thing in:
In those languages they change what's contained in the string. Not how many strings you get. Or what the strings from that string look like. ($@ being an extreme example)
> $@ being an extreme example
From the bash man page via StackOverflow:
> @ Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" ... If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, "$@" and $@ expand to nothing (i.e., they are removed).
That’s…a lot. I think Bash is interesting in the “I’m glad it works but I detest having to work with it” kind of way. Like, fine if I’m just launching some processes or tail’ing some logs, but I’ve rarely had a time when I had to write an even vaguely complex bash script where I didn’t end up spending most of my time relearning how to do things that should be basic.
Shellcheck was a big game changer at least in terms of learning some of the nuance from a “best practice” standpoint. I also think that the way bash does things is just a little too foreign from the rest of my computing life to be retained.
Are taxes simple?
Why does Bash syntax have to be "simple"? For me, Bash syntax is simple.
It's more like how the need for backwards compatibility prevents bad interfaces from ever getting improved.
I quite like how archaic it is. I am turned off by a lot of modern stuff. My shell is nice and predictable. My scripts from 15 years ago still work just fine. No, I don't want it to get all fancy, thanks.
I've long wanted easy, trivial multichannel i/o with duplication
I want to be able to route x independent input and y independent output trivially from the terminal
Proper i/o routing
It shouldn't be hard, it shouldn't be unsolved, and it shouldn't be esoteric
That's what named pipes do.
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.
Counter reason in favor is that you can always count on it being there and working the same way. Perl is too out of fashion and python has too many versioning/library complexities.
You have to write the crappy sh script once but then you get simple, easy usage every time. (If you're revising the script frequently enough that sh/bash are the bottleneck, then what you have is a dev project and not a script, use a programming language).
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
You're not wrong, but at my place, our main repository does not permit cloning into a directory with spaces in it.
Three factors conspire to make a bug:
1. Someone decides to use a space
2. We use Python
3. macOS
#!/usr/bin/env python3
Since we use Python virtualenvs,
#!/home/bob/src/repo/.venv/bin/python3
#!/home/bob/src/repo with a space/.venv/bin/python3
As I also discovered when I discovered this, the Python tooling checks for this! It will instead emit a polyglot!
#!/bin/bash
# <what follows in a bash/python polyglot>
# the bash will find the right Python interpreter, and then re-exec this
# script using that interpreter. The Python will skip the bash portion,
# b/c of cleverness in the polyglot.
Took me forever to figure out what was going on. So yeah … spaces in paths. Can't recommend them. Stuff breaks, and it breaks in weird and hard to debug ways.
If all of your scripts run in the same venv (for a given user), can you inject that into the PATH and rely on env just finding the right interpreter?
I suppose it would also need env to be able to handle paths that have spaces in them.
What a headache!
My practical view is to avoid spaces in directories and filenames, but to write scripts that handle them just fine (using BASH - I'm guilty of using it when more sane people would be using a proper language).
My ideological view is that unix/POSIX filenames are allowed to use any character except for NULL, so tools should respect that and handle files/dirs correctly.
I suppose for your usage, it'd be better to put the virtualenv directory into your path and then use #!/usr/bin/env python
They're more like capabilities or handles than pointers. There's a reason in Rust land many systems use handles (indices to a table of objects) in absence of pointer arithmetic.
In the C API of course there's symbolic names for these. STDIN_FILENO, STDOUT_FILENO, etc for the defaults and variables for the dynamically assigned ones.
What they point to are capabilities, but the integer handles that user space gets are annoyingly like pointers. In some respects, better, since we don’t do arithmetic on them, but in others, worse: they’re not randomized, and I’ve never come across a sanitizer (in the ASan sense) for them, so they’re vulnerable to worse race condition and use-after-free issues where data can be quietly sent to the entirely wrong place. Unlike raw pointers’ issues, this can’t even be solved at a language level. And maybe worst of all, there’s no bug locality: you can accidentally close the descriptor backing a `FILE*` just by passing the wrong small integer to `close` in an unrelated part of the program, and then it’ll get swapped out at the earliest opportunity.
BITD the one "fd sanitizer" I ever encountered was "try using the code on VxWorks" which at the time was "posix inspired" at best - fds actually were pointers, so effectively random and not small integers. It didn't catch enough things to be worth the trouble, but it did clean up some network code (ISTR I was working on SNTP and Kerberos v4 and Kerberized FTP when I ran into this...)
> At least allow us to use names instead of numbers.
You can use /dev/stdin, /dev/stdout, /dev/stderr in most cases, but it's not perfect.
> You can use /dev/stdin, /dev/stdout, /dev/stderr in most cases
Never ever write code that assumes this. These dev shorthands are Linux specific, and you'll even need a certain minimum Linux version.
I cringe at the amount of shell scripts that assume bash is the system interpreter, and not sh or ksh.
Always assume sh, it's the most portable.
Linux != Unix.
It's a waste of time unless you're specifically targeting and testing mac, all of the BSDs, various descendants of Solaris, and other flavors of Unix. I wrote enough "portable shell" to run into so many quirks and slight differences in flags, in how different tools handle e.g. SIGPIPE.
Adding a new feature in a straightforward way often makes it work only on 4/7 of the operating systems you're trying to support. You then rewrite it in a slightly different way (because it's shell — there's always 50 ways to do the same thing). This gets you to 5/7 working systems, but breaks one that previously worked. You rewrite it yet another way, fixing the new breakage, but another one breaks. Repeat this over and over again, trying to find an implementation that works everywhere, or start adding workarounds for each system. Spend an hour on a feature that should have taken two minutes.
If it's anything remotely complicated, and you need portability, then use perl/python/go.
Actually, while the Actual Nodes are a linux thing, bash itself implements (and documents) them directly (in redirections only), along with /dev/tcp and /dev/udp (you can show with strace that bash doesn't reference the filesystem for these, even if they're present.)
So, you're not wrong, but...
You shouldn't be assuming I'm writing code for Unix.
lol truly informative and clearly something no one here knew. But your terminology is inaccurate. Please change it to GNU/Linux != Unix
Who do you imagine the users were back when it was being developed?
People who were not that one programmer?
Even if you're a programmer, that doesn't mean you magically know what other programmers find easy or logical.
> At least allow us to use names instead of numbers.
Sure. Here's what that looked like:
> bash's syntax is so weird
What should be the syntax according to contemporary IT people? JSON? YAML? Or just LLM prompt?
Nushell, Powershell, Python, Ruby, heck even Perl is better. Shell scripting is literally the worst language I've ever seen in common use. Any realistic alternative is going to be better.
It always exists on any Unix system. Even a busybox root environment. Why do you want to save a few bytes to compromise portability?
But it isn't portable, unless you stick to posix subset which kinda sucks. You'll use some feature that some dude using an ancient shell doesn't have then he'll complain to you. And that list of features is LONG: https://oneuptime.com/blog/post/2026-02-13-posix-shell-compa...
If you're using shell specific features in a tightly controlled environment like a docker container then yeah, go wild. If you're writing a script for personal use, sure. If you're writing something for other people to run then your code will be working around all the missing features posix hasn't been updated to include. You can't use arrays, or arithmetic context, nothing. It sucks to use.
Besides, if you're writing a script it is likely that it will grow, get more complicated, and you will soon bump up against the limitations of the language and have to do truly horrible workarounds.
This is why if I need something for others to run then I just use python from the beginning. The code will be easier to read and more portable. At this point the vast majority of OS's and images have it available anyway so it's not as big a barrier as it used to be.
Nushell! Or powershell, but I much prefer nushell!
There's a movement to write JSON to fd 3, as a machine-parsable alternative to rickety fd 1.
Anything that is infected by UCS-2 / UTF-16 garbage should be revised and reconsidered... Yeah UTF-8 has carve outs for those escape sequences... However JSON is even worse, you _have_ to use UTF-16 escapes. https://en.wikipedia.org/wiki/JSON#Character_encoding
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 among those that use Bash, I think it's a bit of an elitist syntax.
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 overloading, or magic methods, or monkey patching.
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.
Tcl
Haskell
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.
The programmers were the users. They asked. They said it was ok.
Wait until you find out where "tty" comes from!
Redirects are fun but there are way more than I actually routinely use. One thing I do is the file redirects.
diff <(seq 1 20) <(seq 1 10)
Process substitution and calling it file redirect is a bit misleading because it is implemented with named pipes which becomes relevant when the command tries to seek in them which then fails.
Also the reason why Zsh has an additional =(command) construct which uses temporary files instead.
Better: Understanding Linux's File Descriptors: A Deep Dive Into '2>&1' and Redirection https://news.ycombinator.com/item?id=41384919 https://news.ycombinator.com/item?id=39095755
O'Reilly's Essential System Administration [1], I never do a job interview without it.
[1]: https://www.oreilly.com/library/view/essential-system-admini...
I regularly refer to [the unix shell specification][1] to remember the specifics of ${foo%%bar} versus ${foo#bar}, ${parameter:+word} versus ${parameter:-word}, and so on.
It also teaches how && and || work, their relation to [output redirection][3] and [command piping][2], [(...) versus {...}][4], and tricky parts like [word expansion][5], even a full grammar. It's not exciting reading, but it's mostly all there, and works on all POSIXy shells, e.g. sh, bash, ksh, dash, ash, zsh.
[1]: https://pubs.opengroup.org/onlinepubs/7908799/xcu/chap2.html
[2]: https://pubs.opengroup.org/onlinepubs/7908799/xcu/chap2.html...
[3]: https://pubs.opengroup.org/onlinepubs/7908799/xcu/chap2.html...
[4]: https://pubs.opengroup.org/onlinepubs/7908799/xcu/chap2.html...
[5]: https://pubs.opengroup.org/onlinepubs/7908799/xcu/chap2.html...
If you need to know what 2>&1 means, then I would recommend shellcheck
It's very, very easy to get shell scripts wrong; for instance the location of the file redirect operator in a pipeline is easy to get wrong.
As someone who use LLMs to generate, among others, Bash script I recommend shellcheck too. Shellcheck catches lots of things and shall really make your Bash scripts better. And if for whatever reason there's an idiom you use all the time that shellcheck doesn't like, you can simply configure shellcheck to ignore that one.
Not sure why this link and/or question is here, except to say LLMs like this incantation.
It redirects STDERR (2) to where STDOUT is piped already (&1). Good for dealing with random CLI tools if you're not a human.
Humans used this combination extensively for decades too. I'm no aware of any other simple way to grep both stdout and stderr from a process. (grep, or save to file, or pipe in any other way).
"not humans" are using this extensively precisely because humans used this combination extensively for decades. It's muscle-memory for me. And so is it for LLMs.
I found the explanation useful, about "why" it is that way. I didn't realize the & before the 1 means to tell it is the filedescriptor 1 and not a file named 1.
The distinction between file descriptors and regular files trips up many people at first. Recognizing that `&` signifies a file descriptor clears up the confusion about the syntax.
I get the ocassional file named `1` lying around.
It's an operator called ">&", the 1 is the parameter.
Well sure, but surely this takes some inspiration from both `&` as the "address of" operator in C as well as the `>` operator which (apart from being the greater-than operator) very much implies "into" in many circumstances.
So `>&1` is "into the file descriptor pointed to by 1", and at the time any reasonable programmer would have known that fd 1 == STDOUT.
I've also found llms seem to love it when calling out to tools, I suppose for them having stderr interspersed messaged in their input doesn't make much difference
Always wondered how the parser managed the ambiguity between & for file descriptors and & to start background tasks. (And without a good mental model, I kept forgetting where to put the & correctly in redirects)
Treating ">&" as a distinct operator actually makes an elegant solution here. I like the idea.
I've almost never needed any of these, but there's all sorts of weird redirections you can do in GNU Bash: https://www.gnu.org/software/bash/manual/bash.html#Redirecti...
agentic ai tends to use it ALL the time.
rc [1] replaced it with a far more telling >[1=2] and >[1=] for closing.
It means redirect file descriptor 2 to the same destination as file descriptor 1.
Which actually means that an undelrying dup2 operation happens in this direction:
2 <- 1 // dup2(2, 1)
The notation follows I/O redirections: cmd > file actually means that a descriptor [n] is first created for the open file, and then that descriptor's decription is duplicated into [1]:
n <- open("file", O_RDONLY)
1 <- nI 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 <(...) make intuitive sense.
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.
I enjoyed the commenter asking “Why did they pick such arcane stuff as this?” - I don’t think I touch more arcane stuff than shell, so asking why shell used something that is arcane relative to itself is to me arcane squared.
I love myself a little bit of C++. A good proprietary C++ codebase will remind you that people just want to be wizards, solving their key problem with a little bit of magic.
I've only ever been tricked into working on C++...
I am surprised that there still is no built in way to pipe stdout and stderr. *| would be much more ergonomic than 2>&1 |.
Doesn't |& work with bash?
&>
> I am thinking that they are using & like it is used in c style programming languages. As a pointer address-of operator. [...] 2>&1 would represent 'direct file 2 to the address of file 1'.
I had never made the connection of the & symbol in this context. I think I never really understood the operation before, treating it just as a magic incantation but reading this just made it click for me.
No, the shell author needed some way to distinguish file descriptor 1 from a file named "1" (note that 2>1 means to write stderr to the file named "1"), and '&' was one of the few available characters. It's not the address of anything.
To be consistent, it would be &2>&1, but that makes it more verbose than necessary and actually means something else -- the first & means that the command before it runs asynchronously.
It's not inconsistent. The & is attached to the redirection operator, not to the 1 token. The file descriptor being redirected is also attached:
Thus you cannot write:
2 > &1
2 >& 1
2>& 1
A gotcha for me originally and perhaps others is that while using ordering like
$ ./outerr >blah 2>&1
$ ./outerr | 2>&1 cat >blah
err
(where outerr is the following...)
echo out
echo err >&2Useless use of cat error/award
But also | isnt a redirection, it takes stdout and pipes it to another program.
So, if you want stderr to go to stdout, so you can pipe it, you need to do it in order.
bob 2>&1 | prog
You usually dont want to do this though.
The point is that the order in which that is processed is not left to right.
First the | pipe is established as fd [1]. And then 2>&1 duplicates that pipe into [2]. I.e. right to left: opposite to left-to-right processing of redirections.
When you need to capture both standard error and standard output to a file, you must have them in this order:
bob > file 2>&1
bob 2>&1 > file
But if you change > file to | process, then it's fine! process gets the combined error and regular output.
You can pipe the fd directly:
# echo 1 >&2 2>| echo
Why would that second one be expected to work?
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.
> Or is it restricted to 0/1/2 by the shell?
It is not. You can use any arbitrary numbers provided they're initialized properly. These values are just file descriptors.
For Example -> https://gist.github.com/valarauca/71b99af82ccbb156e0601c5df8...
I've used (see: example) to handle applications that just dump pointless noise into stdout/stderr, which is only useful when the binary crashes/fails. Provided the error is marked by a non-zero return code, this will then correctly display the stdout/stderr (provided there is <64KiB of it).
No restrictions. You can create your own beautiful monsters that way.
> Would probably be hard to guess since the process may not have opened any file once it started.
You need to not only inspect the current state, but also race the process before the assignments change.
back when stackoverflow was still good and useful, I asked about some stderr manipulation[0] and learnt a lot from the replies
[0] https://stackoverflow.com/questions/3618078/pipe-only-stderr...
I saw this newer bash syntax for redirecting all output some years ago on irc
foo &> file
foo |& programI didn't know about |&, not sure if it was introduced at the same time. So I'd always use &> for redirection to file and 2>&1 for piping
I think the "|&" is the most intuitive syntax - you can just amend an existing pipe to also include STDERR
It was never fully clear to me why the ordre mattered.
I always wondered if there ever was a standard stream for stdlog which seems useful, and comes up in various places but usually just as an alias to stderr
Powershell has ”stdprogress”
/dev/stderr on Linux
To me it means “I didn’t want to come up with an intelligible syntax for this”. Shell scripts have many dark corners and sharp edges like this is one.
Somewhat off topic, but related: I worked at this place that made internet security software. It ran on Windows, and on various flavors of Unix.
One customer complained about our software corrupting files on their hard disk. Turns out they had modified their systems so that a newly-spawned program was not given a stderr. That is, it was not handed 0, 1, and 2 (file descriptors), but only 0 and 1. So whenever our program wrote something to stderr, it wrote to whatever file had been the first one opened by the program.
We talked about fixing this, briefly. Instead we decided to tell the customer to fix their broken environment.
I first encountered this thirty four years ago and I still hate it. Almost as much as I hate when people ask me to explain it.
Look man, I didn’t invent this stupid shit, and I’m not telling you it’s brilliant, so don’t kill the messenger.
I thought I’d seen somewhere that zsh had a better way to do this but I must have imagined it. Or maybe I’m confusing it with fish.
That’s nothing, try `&>`.
This is one of those places where Bash diverges from POSIX. The standard says `echo &>/dev/null' is two commands, namely `echo &' and `>/dev/null', but Bash interprets it as redirect both stdout and stderr of `echo' to `/dev/null' both in normal and POSIX mode.
Also known as `>&`.
cmd >&out-and-err.txtAwesome. Next week I will forget it again.
A.I. has made the self-important neckbeards of Stack Overflow obsolete.
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
You're not limited to the standard file descriptors.
command 4>&3It means someone did not bother to name their variables properly, reminding you to use a shell from this century.