Linux std to file

Redirect all output to file in Bash [duplicate]

I know that in Linux, to redirect output from the screen to a file, I can either use the > or tee . However, I’m not sure why part of the output is still output to the screen and not written to the file. Is there a way to redirect all output to file?

9 Answers 9

That part is written to stderr, use 2> to redirect it. For example:

foo > stdout.txt 2> stderr.txt 

or if you want in same file:

Note: this works in (ba)sh, check your shell for proper syntax

well, i found the reference and have deleted my post for having incorrect information. from the bash manual: ‘»ls 2>&1 > dirlist» directs only the standard output to dirlist, because the standard error was duplicated from the standard output before the standard output was redirected to dirlist» 🙂

also from the bash man «There are two formats for redirecting standard output and standard error: &>word and >&word Of the two forms, the first is preferred. This is semantically equivalent to >word 2>&1»

Two important addenda: If you want to pipe both stdout and stderr, you have to write the redirections in the opposite order from what works for files, cmd1 2>&1 | cmd2 ; putting the 2>&1 after the | will redirect stderr for cmd2 instead. If both stdout and stderr are redirected, a program can still access the terminal (if any) by opening /dev/tty ; this is normally done only for password prompts (e.g. by ssh ). If you need to redirect that too, the shell cannot help you, but expect can.

All POSIX operating systems have 3 streams: stdin, stdout, and stderr. stdin is the input, which can accept the stdout or stderr. stdout is the primary output, which is redirected with > , >> , or | . stderr is the error output, which is handled separately so that any exceptions do not get passed to a command or written to a file that it might break; normally, this is sent to a log of some kind, or dumped directly, even when the stdout is redirected. To redirect both to the same place, use:

EDIT: thanks to Zack for pointing out that the above solution is not portable—use instead:

If you want to silence the error, do:

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Redirect Linux Command Output to File

Want to analyze the effect of Linux command for later? Here’s how you can save Linux command output to a file.

There are times when you want to redirect the output of specific output to a file so you can examine the error later.

And in that case, you can redirect the data stream to the file.

So in this tutorial, I will walk you through how to redirect the output to a file.

How to redirect output to file

Before I jump to the how-to part, it is necessary to know the basics of data streams.

There are 3 three types of data streams:

• Input data stream (denoted as 0)
• Output data stream (denoted as 1)
• Error data stream (denoted as 2)

You can not use 0 to redirect the input data stream. For that, you have to use < or you can pipe command.

Now, let’s have a look at redirection symbols and numbers for redirection:

Indicator	Description
0	Input data stream.
1	Output data stream.
2	Error data stream.
>	Redirects std output to a file or command.
	Redirects std input from a file or command.
>>	Appends the std output to a file.
2>	Redirects std error to a file or command.
2>>	Appends std error to a file.

Let’s have a look at examples.

1. Redirecing output to a file (overrides existing data)

As I mentioned above, you have to use the > symbol in the following manner:

For example, here, I used the sudo apt update command and redirected the output to the update.txt file:

sudo apt update > update.txt

And now, if I use the cat command to print the content of the file, it should bring the output of the apt update command:

But wait, if you use to redirect the output in the shown way, it will override the existing content of the file.

2. Redirect output to a file without overriding

As I mentioned earlier, if you don’t want to override the existing file content, then you use the >> instead of > :

For example, here, I will be using a sample text file named Hello.txt which contains the following:

And now, I will use the print command and redirect the output to the Hello.txt file:

echo Lines to be appended >> Hello.txt

Bonus: Redirect output using the tee command

Unlike redirection, using the tee command you can only override the existing file content to redirect its data.

If you don’t like the idea of using the redirection indicators, you can use the tee command in the following manner to have the same effect:

Here I used the file named Hello.txt which already had some text inside:

echo Hello again | tee Hello.txt

More on redirection of data

If you want to learn more about how you redirect the data streams with multiple possibilities, here’s a detailed guide for you:

I hope you will find this guide helpful.

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How to redirect output to a file and stdout

In bash, calling foo would display any output from that command on the stdout. Calling foo > output would redirect any output from that command to the file specified (in this case ‘output’). Is there a way to redirect output to a file and have it display on stdout?

If someone just ended up here looking for capturing error output to file, take a look at — unix.stackexchange.com/questions/132511/…

A note on terminology: when you execute foo > output the data is written to stdout and stdout is the file named output . That is, writing to the file is writing to stdout. You are asking if it is possible to write both to stdout and to the terminal.

@WilliamPursell I’m not sure your clarification improves things 🙂 How about this: OP is asking if it’s possible to direct the called program’s stdout to both a file and the calling program’s stdout (the latter being the stdout that the called program would inherit if nothing special were done; i.e. the terminal, if the calling program is an interactive bash session). And maybe they also want to direct the called program’s stderr similarly («any output from that command» might be reasonably interpreted to mean including stderr).

If we have multiple commands that want to pipe outputs, use ( ) . For example (echo hello; echo world) | tee output.txt

11 Answers 11

The command you want is named tee :

For example, if you only care about stdout:

If you want to include stderr, do:

program [arguments. ] 2>&1 | tee outfile 

2>&1 redirects channel 2 (stderr/standard error) into channel 1 (stdout/standard output), such that both is written as stdout. It is also directed to the given output file as of the tee command.

Furthermore, if you want to append to the log file, use tee -a as:

program [arguments. ] 2>&1 | tee -a outfile 

If OP wants «all output» to be redirected, you’ll also need to grab stderr: «ls -lR / 2>&1 | tee output.file»

@evanmcdonnal The answer is not wrong, it just may not be specific enough, or complete depending on your requirements. There certainly are conditions where you might not want stderr as part of the output being saved to a file. When I answered this 5 years ago I assumed that the OP only wanted stdout, since he mentioned stdout in the subject of the post.

Ah sorry, I might have been a little confused. When I tried it I just got no output, perhaps it was all going to stderr.

Use -a argument on tee to append content to output.file , instead of overwriting it: ls -lR / | tee -a output.file

If you’re using $? afterwards it will return the status code of tee , which is probably not what you want. Instead, you can use $ .

$ program [arguments. ] 2>&1 | tee outfile 

2>&1 dumps the stderr and stdout streams. tee outfile takes the stream it gets and writes it to the screen and to the file «outfile».

This is probably what most people are looking for. The likely situation is some program or script is working hard for a long time and producing a lot of output. The user wants to check it periodically for progress, but also wants the output written to a file.

The problem (especially when mixing stdout and stderr streams) is that there is reliance on the streams being flushed by the program. If, for example, all the writes to stdout are not flushed, but all the writes to stderr are flushed, then they’ll end up out of chronological order in the output file and on the screen.

It’s also bad if the program only outputs 1 or 2 lines every few minutes to report progress. In such a case, if the output was not flushed by the program, the user wouldn’t even see any output on the screen for hours, because none of it would get pushed through the pipe for hours.

Update: The program unbuffer , part of the expect package, will solve the buffering problem. This will cause stdout and stderr to write to the screen and file immediately and keep them in sync when being combined and redirected to tee . E.g.:

$ unbuffer program [arguments. ] 2>&1 | tee outfile 

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