Cmd 2>&1 > Log VS Cmd > Log 2>&1

cmd 2&1 log vs cmd log 2&1

Order matters. The way to reason about redirections is to read them from left to right and realize that redirections make streams point at the same place. They don't make streams point at each other.

What does that mean? If you say 2>&1 then you are redirecting stderr to wherever stdout is currently redirected to. If stdout is going to the console then stderr is, too. If stdout is going to a file then stderr is as well. If you follow this up by then redirecting stdout, stderr still points to what stdout used to point to. It does not "follow" stdout to the new location.

Right

cmd > log 2>&1

This redirects stdout to log and then redirects stderr to wherever stdout is now being redirected, which is log.

End result: both stdout and stderr are redirected to log.

Wrong

cmd 2>&1 > log

This redirects stderr to wherever stdout is currently being redirected, which is typically the console. Then stdout is redirected to log. Remember that stderr does not "follow" stdout, so it continues to redirect to the console.

End result: stdout is redirected to the log file and stderr is (still) sent to the console. This is almost certainly not what you want.

What does 2&1 in a Windows Command Do?

Decomposing the line

ping -n 40 127.0.0.1

Send 40 ping packets to local host. If there is not any problem the default behaviour is to wait 1 second between packets, so it generates a 39 second delay

>nul   or   1>nul

Redirects anything written to the standard output stream (stream number 1) to the nul device. Anything sent to this device is discarded. The effect is that all the normal output of the ping command is hidden.

2>&1

This redirects anything written to the standard error stream (stream number 2). As in the previous case, this is done to hide output (errors in this case), but instead of directly requesting to write to the nul device (we could have done 2>nul), this syntax requests to send data in the standard error stream to a copy of the handle used in the standard output stream.

What is 2 in command, while redirecting both System.out , System.err Java messages to file

2 is a standard error file descriptor.

File descriptor 1 is the standard output (stdout).
File descriptor 2 is the standard error (stderr).

2>error.log 

is used to redirect the standard error logs to a file named error.log

In Windows, what's the difference between 2&1 and 2&1?

Some examples should show what happens:

c:\YourDir> cd FolderNotHere > nul
The system cannot find the path specified. 

You get the error stream

c:\YourDir>cd FolderNotHere > nul  2>&1

You get nothing, the error stream goes to the std output stream which goes to null.

c:\YourDir>cd > nul

You get nothing, the output stream goes to null.

c:\YourDir>cd > nul 1>&2
c:\YourDir

You get the std outout which has been sent to the error stream so it doesn't get redirected.

c:\YourDir>cd > nul 1<&2

This seams to do the same as 1>&2

How to redirect Windows cmd stdout and stderr to a single file?

You want:

dir > a.txt 2>&1

The syntax 2>&1 will redirect 2 (stderr) to 1 (stdout). You can also hide messages by redirecting to NUL. More explanation and examples are on the Microsoft documentation page Redirecting error messages from Command Prompt: STDERR/STDOUT.

What does 2&1 mean?

File descriptor 1 is the standard output (stdout).

File descriptor 2 is the standard error (stderr).

At first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as "redirect stderr to a file named 1".

& indicates that what follows and precedes is a file descriptor, and not a filename. Thus, we use 2>&1. Consider >& to be a redirect merger operator.

Windows command prompt log to a file

You can redirect the output of a cmd prompt to a file using > or >> to append to a file.

i.e.

echo Hello World >C:\output.txt
echo Hello again! >>C:\output.txt

or

mybatchfile.bat >C:\output.txt

Note that using > will automatically overwrite the file if it already exists.

You also have the option of redirecting stdin, stdout and stderr.

See here for a complete list of options.

What does 2&1 mean?

File descriptor 1 is the standard output (stdout).

File descriptor 2 is the standard error (stderr).

At first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as "redirect stderr to a file named 1".

& indicates that what follows and precedes is a file descriptor, and not a filename. Thus, we use 2>&1. Consider >& to be a redirect merger operator.

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