84

I saw this line in a script:

DEVICE=`dialog --inputbox "Festplatten-Laufzeit auslesen. Gebe Sie das 
gewünschte Device an: " 0 70 "" 3>&1 1>&2 2>&3`

What is

3>&1 1>&2 2>&3

doing? I know that 1 = stdout and 2 = stderr, but what are the 3 and the & for?

1
97

The numbers are file descriptors and only the first three (starting with zero) have a standardized meaning:

0 - stdin
1 - stdout
2 - stderr

So each of these numbers in your command refer to a file descriptor. You can either redirect a file descriptor to a file with > or redirect it to another file descriptor with >&

The 3>&1 in your command line will create a new file descriptor and redirect it to 1 which is STDOUT. Now 1>&2 will redirect the file descriptor 1 to STDERR and 2>&3 will redirect file descriptor 2 to 3 which is STDOUT.

So basically you switched STDOUT and STDERR, these are the steps:

  1. Create a new fd 3 and point it to the fd 1
  2. Redirect file descriptor 1 to file descriptor 2. If we wouldn't have saved the file descriptor in 3 we would lose the target.
  3. Redirect file descriptor 2 to file descriptor 3. Now file descriptors one and two are switched.

Now if the program prints something to the file descriptor 1, it will be printed to the file descriptor 2 and vice versa.

5
  • You have said "The 3>&1 in your command line will create a new file descriptor and redirect it to 1 which is STDOUT" . But doesn't 1 means STDIN ?
    – sofs1
    Aug 10 '19 at 9:16
  • @sofs1 STDIN is 0; STDOUT is 1; STDERR is 2 I believe.
    – Lou
    Dec 14 '20 at 10:02
  • 2
    My question is - how does a shell script know what to pass to "3"? Say you ran a echo "hello" | 3>&1 - would it pipe "hello" to the 3 file descriptor, and then to STDOUT?
    – Lou
    Dec 14 '20 at 10:04
  • @Lou Normally there would be a command after the | symbol. There is none, and it seems like Bash pipes "hello" to nowhere in that case.
    – user234461
    Aug 9 at 8:57
  • @Lou But if you meant what does echo "hello" 3>&1 do, or similar, then it pipes "hello" to STDOUT, and also directs 3 to stdout. But 3 never receive a stream so the 3>&1 has no effect. It's just like the more common 2>&1, which combines STDERR with STDOUT into a single stream on STDOUT.
    – user234461
    Aug 9 at 9:01
26

It's swapping stdout and stderr.

>name means redirect output to file name.

>&number means redirect output to file descriptor number.

So the & is needed to tell the shell you mean a file descriptor, not a file name.

A file descriptor is a number that refers to an already open file. The standard ones are 0 for standard input, 1 for standard output or 2 for standard error. You can also use any other number, which will create a new file descriptor, just like when you create a new variable with var=value.

By default, both file descriptor 1 and 2 go to /dev/tty, so if you run somecommand 3>&1 1>&2 2>&3 in a new shell, it doesn't change anything (except now you have a file descriptor number 3).

But if somewhere earlier in the script it does a redirection using exec (e.g. exec 2>error.log), or the script is run with a command line including redirection (e.g. ./thescript 2>error.log), then swapping stdout and stderr will do something.

In your specific case, the command that's having its stdout and stderr swapped is dialog. Looking at its man page, I see

Some widgets, e.g., checklist, will write text to dialog's output.
Normally that is the standard error

so perhaps the person who wrote the script wants dialog's output to go to stdout instead of stderr for some reason.

See also Order of redirections

5

The script writer defined fd 3 such as:

exec 3<> File.txt

Open "File.txt" and assign fd 3 to it. Maximum file descriptors: 255

read -n 4 <&3

Read only 4 characters.

echo -n . >&3

Write a decimal point there.

exec 3>&-

Close fd 3.

cat File.txt

==> 1234.67890

2
0

I understood the purpose of the file descriptor and the "3> & 1 1> & 2 2> & 3" command by example below

#!/bin/bash
#it's normal case fd enviroment

echo test 1> afile.txt

echo test1111111111111

cat afile.txt

echo test1111111111111

echo test 2> afile.txt

echo test2222222222222

cat afile.txt

echo test2222222222222


e1cho test 1> afile.txt

echo test333333333333


cat afile.txt

echo test333333333333

ec1ho test 2> afile.txt

echo test4444444444444

cat afile.txt

echo test444444444444

echo > afile.txt

echo ------------------------------------------------------------------------------------------------------

#it's relocate filedescriptor and cause not problem, and cause error(std error)

var=

var=$(echo test 3>&1 1>&2 2>&3)

echo test555555555555

#cat afile.txt

echo $var
echo test555555555555

var=

var=$(ec1ho test 3>&1 1>&2 2>&3)

echo test6666666666666

#cat afile.txt

echo $var

echo test666666666666

var=

var=$(ec1ho test 3>&1 1>&2 2>&3)

echo test77777777777777

#cat afile.txt

echo $var

echo test77777777777777

var=

var=$(echo test 3>&1 1>&2 2>&3)

echo test88888888888888

#cat afile.txt

echo $var

echo test8888888888888

When you run this example, you will see the file descriptors are rearranged and the output changes.

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