Output redirection in linux

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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How to Redirect Output to a File and Stdout in Linux

In the world of Linux, redirecting output to a file or the standard output (stdout) is a fundamental technique that allows us to capture and manipulate command output efficiently.

Whether you are a beginner or an experienced user, understanding how to redirect output can greatly enhance your productivity and provide you with more control over your command-line operations.

Through this guide, you will learn the process of redirecting output to a file and stdout in Linux.

Redirecting Output to a File in Linux

At some point where you want to save or redirect command output into a specific file for any reason, like debugging. In Linux, to save the output of a file, we use stdout, which is also known as the stream command.

In computing, the stream is something that transfers data. In our case, it is text data. Using stdout, we can stream and save that data into a text file for future use.

Before proceeding ahead, First, you should know what is Redirection and a combination of operators.

A redirection symbol directly redirects the command into a file instead of showing output on a terminal. There is a combination of redirection symbols that you can use like “>” , ”>>” , ”&>” , ”&>>” .

In Linux, what we type is called “stdin”, and the output we receive is known as “stdout”. If the output file does not exist in a specific location, it will recreate automatically and save the file.

Make sure if you have used “>” , then the past data will replace with fresh command output. If you want to redirect both “stdout” and “stderr”, then use “&>” .

Now we will use this redirection symbol to redirect the output into the file.

Redirect Output to File in Linux

When you use “>” redirection operator, it will redirect command output into a specific file. If you used the same file again to redirect, then the last output will be overwritten.

We will demonstrate it using the pwd command and hostnamectl command to show system info with a redirect “>” to save output into demofile.txt.

First, we will save the current working directory output to a file.

Now we will redirect the second output into the file.

Now view the contents of the file.

Appending Output to a File in Linux

When you use this “>>” redirection operator, it will redirect command output into a specific file, and ensure that the last saved data should not get a wiped and append new output into the same file.

For example, we will again use the pwd and hostnamectl commands to show system information with a redirect “>>” to save output into demofile.txt.

$ pwd > demofile.txt $ hostnamectl >> demofile.txt $ cat demofile.txt

From the above output, you can see all the past outputs are still available in the same file.

Discard Output to “/dev/null” in Linux

Sometimes you may want to discard output entirely. In such cases, you can redirect output to the “/dev/null” device file, which acts as a black hole for data.

In this example, the ls command lists the contents of the current directory, but the output is redirected to /dev/null. This effectively discards the output, as /dev/null acts as a black hole for data. No file is created or overwritten, and the output is not visible in the terminal or saved anywhere.

Redirecting Output to Stdout in Linux

Redirecting output to stdout allows you to display command output on the terminal or use it as input for other commands.

Here are some common methods to perform this process on your Linux system.

Using “echo” Command

The echo command is another way to output text or variables to stdout; by using redirection, you can redirect the output to the terminal or a file.

$ echo "Hello, Linux Users!" > output.txt

Using “tee” Command

The “tee” command reads from standard input and simultaneously writes to both stdout and files, helping you to display output on the terminal and save it to a file simultaneously.

$ ls | tee directory_lists.txt

The above command displays the output on the terminal as soon as you execute it and save the information in the “directory_list.txt” file. You can retrieve the information from this file using the cat command later on.

Using “pip” Command

In Linux, you can chain commands together using pipes (|) to redirect output from one command as input to another. This allows you to perform complex operations.

In this example, the ls command lists the contents of the current directory, and the output is then piped (|) to the grep command, which searches for lines containing the word “file” in the output of ls, enabling you to filter the output of one command and pass it as input to another command.

Conclusion

Understanding how to redirect output to a file and stdout in Linux is a crucial skill that can greatly enhance productivity and provide greater control over command-line operations.

By utilizing methods such as using the «>» operator, appending output to a file, discarding output with “/dev/null” and redirecting output to stdout using commands like “echo,” “tee” and piping, you can streamline your workflow and efficiently manage command output in your Linux environment.

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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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