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.
$ 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 descriptorhttps://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
Which is lost when using more modern or languages foreign to Unix.
open a terminal (OSX/Linux) and type:
man dup
man dup
To get recursive, you can try:
man man unix
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...
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.
Are you sure there wasn't >&1 users... Sorry I'll get my coat.
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.
Which means that reading someone else's shell script (or awk, or perl, or regex) is INCREDIBLY inconvenient.
But my main reason is that most scripts break when you call them with filenames that contain spaces. And they break spectacularly.
Even if you're a programmer, that doesn't mean you magically know what other programmers find easy or logical.
What should be the syntax according to contemporary IT people? JSON? YAML? Or just LLM prompt?
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.
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.
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.
diff <(seq 1 20) <(seq 1 10)
Also the reason why Zsh has an additional =(command) construct which uses temporary files instead.
It redirects STDERR (2) to where STDOUT is piped already (&1). Good for dealing with random CLI tools if you're not a human.
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.
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.
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 <- nWould probably be hard to guess since the process may not have opened any file once it started.
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.
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.
Thus you cannot write:
2 > &1
2 >& 1
2>& 1
[0] https://stackoverflow.com/questions/3618078/pipe-only-stderr...
I've only ever been tricked into working on C++...
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.
foo &> file
foo |& program $ ./outerr >blah 2>&1
$ ./outerr | 2>&1 cat >blah
err
(where outerr is the following...)
echo out
echo err >&2But 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.
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.
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.
command &stderr>&stdout