Cmd перенаправление вывода ошибок - Решение и исправление самых разных ошибок на TopOshibok.ru

Background info from Microsoft documentation

While the accepted answer to this question is correct, it really doesn’t do much to explain why it works, and since the syntax is not immediately clear I did a quick www search to find out what was actually going on. In the hopes that this information is helpful to others, I’m posting it here.

Taken from the Microsoft documentation page:
Redirecting error messages from Command Prompt: STDERR/STDOUT

Summary

When redirecting output from an application using the > symbol, error messages still print to the screen. This is because error messages are often sent to the Standard Error stream instead of the Standard Out stream.

Output from a console (Command Prompt) application or command is often sent to two separate streams. The regular output is sent to Standard Out (STDOUT) and the error messages are sent to Standard Error (STDERR). When you redirect console output using the > symbol, you are only redirecting STDOUT. In order to redirect STDERR, you have to specify 2> for the redirection symbol. This selects the second output stream that is STDERR.

Example

The command dir file.xxx (where file.xxx does not exist) will display the following output:

Volume in drive F is Candy Cane Volume Serial Number is 34EC-0876
File Not Found

If you redirect the output to the NUL device using dir file.xxx > nul, you will still see the error message:

File Not Found

To redirect the error message to NUL, use the following command:
dir file.xxx 2> nul
Or, you can redirect the output to one place, and the errors to another.
dir file.xxx > output.msg 2> output.err
You can print the errors and standard output to a single file by using the &1 command to redirect the output for STDERR to STDOUT and then sending the output from STDOUT to a file:
dir file.xxx 1> output.msg 2>&1

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Перенаправление ввода-вывода в cmd

Перенаправление стандартных ввода-вывода и ошибок

С помощью переназначения устройств ввода/вывода одна программа может направить свой вывод на вход другой или перехватить вывод другой программы, используя его в качестве своих входных данных. Таким образом, имеется возможность передавать информацию от процесса к процессу при минимальных программных издержках. Практически это означает, что для программ, которые используют стандартные входные и выходные устройства, операционная система позволяет:

выводить сообщения программ не на экран (стандартный выходной поток), а в файл или на принтер (перенаправление вывода);
читать входные данные не с клавиатуры (стандартный входной поток), а из заранее подготовленного файла (перенаправление ввода);
передавать сообщения, выводимые одной программой, в качестве входных данных для другой программы (конвейеризация или композиция команд).

Из командной строки эти возможности реализуются следующим образом. Для того, чтобы перенаправить текстовые сообщения, выводимые какой-либо командой из командной строки, в текстовый файл, нужно использовать конструкцию команда > имя_файла. Если при этом заданный для вывода файл уже существовал, то он перезаписывается (старое содержимое теряется), если не существовал создается. Можно также не создавать файл заново, а дописывать информацию, выводимую командой, в конец существующего файла. Для этого команда перенаправления вывода должна быть задана так: команда >> имя_файла. С помощью символа < можно прочитать входные данные для заданной команды не с клавиатуры, а из определенного (заранее подготовленного) файла: команда < имя_файла

Примеры перенаправления ввода/вывода в командной строке

Приведем несколько примеров перенаправления ввода/вывода.

1. Вывод результатов команды ping в файл ping ya.ru > ping.txt

2. Добавление текста справки для команды XCOPY в файл copy.txt: XCOPY /? >> copy.txt

В случае необходимости сообщения об ошибках (стандартный поток ошибок) можно перенаправить в текстовый файл с помощью конструкции команда 2> имя_файла В этом случае стандартный вывод будет производиться на экран. Также имеется возможность информационные сообщения и сообщения об ошибках выводить в один и тот же файл. Делается это следующим образом: команда > имя_файла 2>&1

Например, в приведенной ниже команде стандартный выходной поток и стандартный поток ошибок перенаправляются в файл copy.txt: XCOPY A:\1.txt C: > copy.txt 2>&1

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Background info from Microsoft documentation

Taken from the Microsoft documentation page:
Redirecting error messages from Command Prompt: STDERR/STDOUT

Summary

When redirecting output from an application using the > symbol, error messages still print to the screen. This is because error messages are often sent to the Standard Error stream instead of the Standard Out stream.

Output from a console (Command Prompt) application or command is often sent to two separate streams. The regular output is sent to Standard Out (STDOUT) and the error messages are sent to Standard Error (STDERR). When you redirect console output using the > symbol, you are only redirecting STDOUT. In order to redirect STDERR, you have to specify 2> for the redirection symbol. This selects the second output stream that is STDERR.

Example

The command dir file.xxx (where file.xxx does not exist) will display the following output:

Volume in drive F is Candy Cane Volume Serial Number is 34EC-0876
File Not Found

If you redirect the output to the NUL device using dir file.xxx > nul, you will still see the error message:

File Not Found

To redirect the error message to NUL, use the following command:
dir file.xxx 2> nul
Or, you can redirect the output to one place, and the errors to another.
dir file.xxx > output.msg 2> output.err
You can print the errors and standard output to a single file by using the &1 command to redirect the output for STDERR to STDOUT and then sending the output from STDOUT to a file:
dir file.xxx 1> output.msg 2>&1

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What to Know

The > redirection operator goes between the command and the file name, like ipconfig > output.txt.
If the file already exists, it’ll be overwritten. If it doesn’t, it will be created.
The >> operator appends the file. Instead of overwriting the file, it appends the command output to the end of it.

Use a redirection operator to redirect the output of a command to a file. All the information displayed in Command Prompt after running a command can be saved to a file, which you can reference later or manipulate however you like.

How to Use Redirection Operators

While there are several redirection operators, two, in particular, are used to output the results of a command to a file: the greater-than sign (>) and the double greater-than sign (>>).

The easiest way to learn how to use these redirection operators is to see some examples:

Export Network Settings to a File

ipconfig /all > networksettings.txt

In this example, all the information normally seen on screen after running ipconfig /all, is saved to a file by the name of networksettings.txt. It’s stored in the folder to the left of the command, the root of the D: drive in this case.

The > redirection operator goes between the command and the filename. If the file already exists, it’ll be overwritten. If it doesn’t already exist, it will be created.

Although a file will be created if it doesn’t already exist, folders will not. To save the command output to a file in a specific folder that doesn’t yet exist, first, create the folder and then run the command. Make folders without leaving Command Prompt with the mkdir command.

Export Ping Results to a File

ping 192.168.86.1 > "C:\Users\jonfi\Desktop\Ping Results.txt"

Here, when the ping command is executed, Command Prompt outputs the results to a file by the name of Ping Results.txt located on the jonfi user’s desktop, at C:\Users\jonfi\Desktop. The entire file path in wrapped in quotes because a space involved.

Remember, when using the > redirection operator, the file specified is created if it doesn’t already exist and is overwritten if it does exist.

The Append Redirection Operator

The double-arrow operator appends, rather than replaces, a file:

ipconfig /all >> \\server\files\officenetsettings.log

This example uses the >> redirection operator which functions in much the same way as the > operator, only instead of overwriting the output file if it exists, it appends the command output to the end of the file.

Here’s an example of what this LOG file might look like after a command has been exported to it:

The >> redirection operator is useful when you’re collecting similar information from different computers or commands, and you’d like all that data in a single file.

The above redirection operator examples are within the context of Command Prompt, but you can also use them in a BAT file. When you use a BAT file to pipe a command’s output to a text file, the exact same commands described above are used, but instead of pressing Enter to run them, you just have to open the BAT file.

Use Redirection Operators in Batch Files

Redirection operators work in batch files by including the command just as you would from the Command Prompt:

tracert yahoo.com > C:\yahootracert.txt

The above is an example of how to make a batch file that uses a redirection operator with the tracert command.

The yahootracert.txt file (shown above) will be created on the C: drive several seconds after executing the sample.bat file. Like the other examples above, the file shows everything Command Prompt would have revealed if the redirection operator wasn’t used.

Export Text Results in Terminal

Terminal provides an easy way to create a text file containing the contents of whatever you see in Command Prompt. Although the same redirection operator described above works in Terminal, too, this method is useful for when you didn’t use it.

To save the results from a Terminal window, right-click the tab you’re working in and select Export Text. You can then choose where to save the TXT file.

Thanks for letting us know!

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Display & Redirect Output

On this page I’ll try to explain how redirection works.
To illustrate my story there are some examples you can try for yourself.

For an overview of redirection and piping, view my original redirection page.

Display text

To display a text on screen we have the ECHO command:

ECHO Hello world

This will show the following text on screen:

Hello world

When I say «on screen», I’m actually referring to the «DOS Prompt», «console» or «command window», or whatever other «alias» is used.

Streams

The output we see in this window may all look alike, but it can actually be the result of 3 different «streams» of text, 3 «processes» that each send their text to thee same window.

Those of you familiar with one of the Unix/Linux shells probably know what these streams are:

Standard Output
Standard Error
Console

Standard Output is the stream where all, well, standard output of commands is being sent to.
The ECHO command sends all its output to Standard Output.

Standard Error is the stream where many (but not all) commands send their error messages.

And some, not many, commands send their output to the screen bypassing Standard Output and Standard Error, they use the Console.
By definition Console isn’t a stream.

There is another stream, Standard Input: many commands accept input at their Standard Input instead of directly from the keyboard.
Probably the most familiar example is MORE:

DIR /S | MORE

where the MORE command accepts DIR‘s Standard Output at its own Standard Input, chops the stream in blocks of 25 lines (or whatever screen size you may use) and sends it to its own Standard Output.

(Since MORE‘s Standard Input is used by DIR, MORE must catch its keyboard presses (the «Any Key») directly from the keyboard buffer instead of from Standard Input.)

Redirection

You may be familiar with «redirection to NUL» to hide command output:

ECHO Hello world>NUL

will show nothing on screen.
That’s because >NUL redirects all Standard Output to the NUL device, which does nothing but discard it.

Now try this (note the typo):

EHCO Hello world>NUL

The result may differ for different operating system versions, but in Windows XP I get the following error message:

'EHCO' is not recognized as an internal or external command, operable program or batch file.

This is a fine demonstration of only Standard Output being redirected to the NUL device, but Standard Error still being displayed.

Redirecting Standard Error in «true» MS-DOS (COMMAND.COM) isn’t possible (actually it is, by using the CTTY command, but that would redirect all output including Console, and input, including keyboard).
In Windows NT 4 and later (CMD.EXE) and in OS/2 (also CMD.EXE) Standard Error can be redirected by using 2> instead of >

A short demonstration. Try this command:

ECHO Hello world 2>NUL

What you should get is:

Hello world

You see? The same result you got with ECHO Hello world without the redirection.
That’s because we redirected the Standard Error stream to the NUL device, but the ECHO command sent its output to the Standard Output stream, which was not redirected.

Now make a typo again:

EHCO Hello world 2>NUL

What did you get?
Nothing
That’s because the error message was sent to the Standard Error stream, which was in turn redirected to the NUL device by 2>NUL

When we use > to redirect Standard Output, CMD.EXE interprets this as 1>, as can be seen by writing and running this one-line batch file «test.bat»:

DIR > NUL

Now run test.bat in CMD.EXE and watch the result:

C:\>test.bat

C:\>DIR  1>NUL

C:\>_

It looks like CMD.EXE uses 1 for Standard Output and 2 for Standard Error.
We’ll see how we can use this later.

Ok, now that we get the idea of this concept of «streams», let’s play with it.
Copy the following code into Notepad and save it as «test.bat»:

@ECHO OFF
ECHO This text goes to Standard Output
ECHO This text goes to Standard Error 1>&2
ECHO This text goes to the Console>CON

Run test.bat in CMD.EXE, and this is what you’ll get:

C:\>test.bat
This text goes to Standard Output
This text goes to Standard Error
This text goes to the Console

C:\>_

Now let’s see if we can separate the streams again.
Run:

test.bat > NUL

and you should see:

C:\>test.bat
This text goes to Standard Error
This text goes to the Console

C:\>_

We redirected Standard Output to the NUL device, and what was left were Standard Error and Console.

Next, run:

test.bat 2> NUL

and you should see:

C:\>test.bat
This text goes to Standard Output
This text goes to the Console

C:\>_

We redirected Standard Error to the NUL device, and what was left were Standard Output and Console.

Nothing new so far. But the next one is new:

test.bat > NUL 2>&1

and you should see:

C:\>test.bat
This text goes to the Console

C:\>_

This time we redirected both Standard Output and Standard Error to the NUL device, and what was left was only Console.
It is said Console cannot be redirected, and I believe that’s true.
I can assure you I did try!

To get rid of screen output sent directly to the Console, either run the program in a separate window (using the START command), or clear the screen immediately afterwards (CLS).

In this case, we could also have used test.bat >NUL 2>NUL
This redirects Standard Output to the NUL device and Standard Error to the same NUL device.
With the NUL device that’s no problem, but when redirecting to a file one of the redirections will lock the file for the other redirection.
What 2>&1 does, is merge Standard Error into the Standard Output stream, so Standard output and Standard Error will continue as a single stream.

Redirect «all» output to a single file:

Run:

test.bat > test.txt 2>&1

and you’ll get this text on screen (we’ll never get rid of this line on screen, as it is sent to the Console and cannot be redirected):

This text goes to the Console

You should also get a file named test.txt with the following content:

This text goes to Standard Output
This text goes to Standard Error

Note: The commands
test.bat > test.txt 2>&1
test.bat 1> test.txt 2>&1
test.bat 2> test.txt 1>&2
all give identical results.

Redirect errors to a separate error log file:

Run:

test.bat > testlog.txt 2> testerrors.txt

and you’ll get this text on screen (we’ll never get rid of this line on screen, as it is sent to the Console and cannot be redirected):

This text goes to the Console

You should also get a file named testlog.txt with the following content:

This text goes to Standard Output

and another file named testerrors.txt with the following content:

This text goes to Standard Error

Nothing is impossible, not even redirecting the Console output.

Unfortunately, it can be done only in the old MS-DOS versions that came with a CTTY command.

The general idea was this:

CTTY NUL
ECHO Echo whatever you want, it won't be displayed on screen no matter what.
ECHO By the way, did I warn you that the keyboard doesn't work either?
ECHO I suppose that's why CTTY is no longer available on Windows systems.
ECHO The only way to get control over the computer again is a cold reboot,
ECHO or the following command:
CTTY CON

A pause or prompt for input before the CTTY CON command meant one had to press the reset button!

Besides being used for redirection to the NUL device, with CTTY COM1 the control could be passed on to a terminal on serial port COM1.

Escaping Redirection (not to be interpreted as «Avoiding Redirection»)

Redirection always uses the main or first command’s streams:

START command > logfile

will redirect START‘s Standard Output to logfile, not command‘s!
The result will be an empty logfile.

A workaround that may look a bit intimidating is grouping the command line and escaping the redirection:

START CMD.EXE /C ^(command ^> logfile^)

What this does is turn the part between parentheses into a «literal» (uninterpreted) string that is passed to the command interpreter of the newly started process, which then in turn does interpret it.
So the interpretation of the parenthesis and redirection is delayed, or deferred.

Note:

Be careful when using workarounds like these, they may be broken in future (or even past) Windows versions.

A safer way to redirect STARTed commands’ output would be to create and run a «wrapper» batch file that handles the redirection.
The batch file would look like this:

command > logfile

and the command line would be:

START batchfile

Some «best practices» when using redirection in batch files:

Use >filename.txt 2>&1 to merge Standard Output and Standard Error and redirect them together to a single file.
Make sure you place the redirection «commands» in this order.
Use >logfile.txt 2>errorlog.txt to redirect success and error messages to separate log files.
Use >CON to send text to the screen, no matter what, even if the batch file’s output is redirected.
This could be useful when prompting for input even if the batch file’s output is being redirected to a file.
Use 1>&2 to send text to Standard Error.
This can be useful for error messages.
It’s ok to use spaces in redirection commands.
Note however, that a space between an ECHO command and a > will be redirected too.
DIR>filename.txt and DIR > filename.txt are identical, ECHO Hello world>filename.txt and ECHO Hello world > filename.txt are not, even though they are both valid.
It is not ok to use spaces in >> or 2> or 2>&1 or 1>&2 (before or after is ok).
In Windows NT 4, early Windows 2000 versions, and OS/2 there used to be some ambiguity with ECHOed lines ending with a 1 or 2, immediately followed by a >:
ECHO Hello world2>file.txt would result in an empty file file.txt and the text Hello world (without the trailing «2») on screen (CMD.EXE would interpret it as ECHO Hello world 2>file.txt).
In Windows XP the result is no text on screen and file.txt containing the line Hello world2, including the trailing «2» (CMD.EXE interprets it as ECHO Hello world2 >file.txt).
To prevent this ambiguity, either use parentheses or insert an extra space yourself:
ECHO Hello World2 >file.txt (ECHO Hello World2)>file.txt
«Merging» Standard Output and Standard Error with 2>&1 can also be used to pipe a command’s output to another command’s Standard Input:
somecommand 2>&1 | someothercommand

Redirection overview page
Use the TEE command to display on screen and simultaneously redirect to file

page last modified: 2016-09-19

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