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
3>&1 in your command line will create a new file descriptor and redirect it to
1 which is
1>&2 will redirect the file descriptor 1 to
2>&3 will redirect file descriptor 2 to 3 which is
So basically you switched
STDERR, these are the steps:
- Create a new fd 3 and point it to the fd 1
- Redirect file descriptor 1 to file descriptor 2. If we wouldn’t have saved the file descriptor in 3 we would lose the target.
- 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.
>name means redirect output to file
>&number means redirect output to file descriptor
& 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
By default, both file descriptor
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
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.
Close fd 3.