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I don't quite understand how the computer reads this command.

cat file1 file2 1> file.txt 2>&1

If I understand, 2>&1 simply redirect Standard Error to Standard Output.

By that logic, the command reads to me as follows:

  1. concatenate files file1 and file2.

  2. send stdout from this operation to file.txt.

  3. send stderr to stdout.

  4. end?

I'm not sure what the computer's doing. By my logic, the command should be

cat file1 file2 2>&1 > file.txt

but this is not correct.

ctrl-alt-delor
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iDontKnowBetter
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4 Answers4

101

I find it easier to think of using assignments.

  • > is like =
  • & is like $

You start out with

1 = /dev/tty
2 = /dev/tty

then your first example, 1> file.txt 2>&1, does

1 = file.txt
2 = $1           # and currently $1 = file.txt

leaving you with

1 = file.txt
2 = file.txt

If you did it the other way, again you start with

1 = /dev/tty
2 = /dev/tty

then 2>&1 > file.txt does

2 = $1           # and currently $1 = /dev/tty
1 = file.txt

so the end result is

1 = file.txt
2 = /dev/tty

and you've only redirected stdout, not stderr.

Mikel
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    i sort of get the analogy, but it's confusing - what does the `$` stand for? – Eliran Malka Jun 24 '19 at 14:28
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    In an assignment like `var=$othervar`, `$` introduces the variable name on the right hand side. In a redirection like `2>&1`, `&` introduces the file descriptor number on the right hand side. I'm saying you can think of it like "file 2 equals file 1". (But there are two types of equals: `<` means "for reading" and `>` means "for writing".) – Mikel Jun 25 '19 at 15:48
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    For anyone who has ever found random files around named `1` or `2` you understand the difference between `2>1` and `2>&1` per @Mikel Also, I appreciate the mental translation, it works for me. `1>&2` becomes `1 = $2` which syntactically "sort of" means something in bash, RegEx, and probably Perl. – razzed Sep 16 '20 at 16:39
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    Another way to think about it is as memory pointers (C/C++) or references (Java): make stream 1 point to the desired output file, and then make stream 2 point to stream 1 (which is now the file). – ralfoide Dec 21 '21 at 22:45
  • Thank you! I never understood why the order matters but it makes sense when you think of ">" as a copy instead of a reference. – laughingbovine May 16 '22 at 15:16
  • A very helpful perspective. – Anthony Labarre Sep 27 '22 at 13:07
  • Answers as beautiful and elegant as this make life worth living. – DryLabRebel Jun 19 '23 at 05:13
  • @DryLabRebel Thank you for the kind words! I'm glad you found my explanation useful. :) – Mikel Jun 20 '23 at 07:01
34

The order of redirection is important, and they should be read left to right.

For example: command 2>&1 >somefile means:

  1. Redirect the descriptor named 2 (bound to stderr) to the current destination of 1 (stdout) which at this point, reading left-ot-right is the terminal.
  2. Then change 1 (stdout) to go to somefile, which is a a file in disk

So in this case, stderr goes to the terminal, and stdout goes to a file, which isn't what you probably want.

On the other hand, command >somefile 2>&1 means:

  1. Redirect stdout to somefile
  2. Then redirect stderr to the same destination as stdout (somefile).

In this last case both stderr and stdout go to somefile, which is probably what you want.

Amelio Vazquez-Reina
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  • in the last line , 2>&1 means: stderr goes to stdout right , where this "somefile" came here ? can you please throw some light... me confused a bit. – BdEngineer Jun 08 '20 at 12:57
  • So it is possible to explain it concisely and on-point. Tip of the hat to you good Sir. – muthuh Feb 03 '22 at 17:13
8
cat file1 file2 1> file.txt 2>&1

>& Actually means duplicate, it uses the dup system call to to map a new file descriptor onto an already opened file.

So, you (bash actually) must first open the new stdout before, saying " and redirect stderr to whatever stdout is currently set."

X Tian
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  • this is awesome! i been wondering about that `&`. could you enclose some references to that syntax, or - better yet - some good resources on the aforementioned dup system? – Eliran Malka Jun 24 '19 at 14:59
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    'man dup' documents the the system call. – X Tian Jun 24 '19 at 15:12
  • makes sense :) thanx – Eliran Malka Jun 25 '19 at 07:39
  • @X Tian , so here file1 & file2 goes to file.txt right ? so what is this "2>&1" ? i.e. stderro goes to stdout ? what to figure out what is duplicated here using ">&" ? – BdEngineer Jun 08 '20 at 13:00
  • The system call is actually called dup, read `man dup`. The thing being duplicated is the file descriptor, in C `dup2(1, 2);` will make fd2 (stderr) a copy of (duplicate) of fd1 (stdout) – X Tian Jun 08 '20 at 13:16
0

The command cat file1 file2 > file.txt 2>&1 sends both stdout and stderr of the cat command to file.txt. Now, this is only useful if somehow either file1 or file2 are not found, in which case error message saying 'file not found' will be saved to file.txt.

This is the same as cat file1 file2 &> file.txt.

Now the order of redirections matter. The following doesn't send stderr of cat command to file.txt but to the terminal.

$ cat file1 file2 2>&1 > file.txt

This is because it is first copying the destination of stdout to stderr, but stdout at this stage is still pointing to the terminal. Then you redirected stdout to file.txt, so only stdout will be saved to file.txt. Error messages will still be printed to the screen and not saved to file.txt.

Logan Lee
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