How to redirect output to a file and stdout

Question

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?

Answer 1

The command you want is named tee :

foo | tee output.file

For example, if you only care about stdout:

ls -a | tee output.file

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

Answer 2

$ 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 . Eg:

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

Answer 3

Another way that works for me is,

<command> |& tee  <outputFile>

as shown in gnu bash manual

Example:

ls |& tee files.txt

If '|&' is used, command1's standard error , in addition to its standard output , is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error to the standard output is performed after any redirections specified by the command.

For more information, refer redirection

Answer 4

您可以主要使用Zoredache解决方案，但如果您不想覆盖输出文件，您应该使用 -a 选项编写 tee 如下：

ls -lR / | tee -a output.file

Answer 5

Something to add ...

The package unbuffer has support issues with some packages under fedora and redhat unix releases.

Setting aside the troubles

Following worked for me

bash myscript.sh 2>&1 | tee output.log

Thank you ScDF & matthew your inputs saved me lot of time..

Answer 6

使用tail -f output应该可以工作。

Answer 7

You can do that for your entire script by using something like that at the beginning of your script :

#!/usr/bin/env bash

test x$1 = x$'\x00' && shift || { set -o pipefail ; ( exec 2>&1 ; $0 $'\x00' "$@" ) | tee mylogfile ; exit $? ; }

# do whaetever you want

This redirect both stderr and stdout outputs to the file called mylogfile and let everything goes to stdout at the same time .

It is used some stupid tricks :

• use exec without command to setup redirections,
• use tee to duplicates outputs,
• restart the script with the wanted redirections,
• use a special first parameter (a simple NUL character specified by the $'string' special bash notation) to specify that the script is restarted (no equivalent parameter may be used by your original work),
• try to preserve the original exit status when restarting the script using the pipefail option.

Ugly but useful for me in certain situations.

Answer 8

Bonus answer since this use-case brought me here:

In the case where you need to do this as some other user

echo "some output" | sudo -u some_user tee /some/path/some_file

Note that the echo will happen as you and the file write will happen as "some_user" what will NOT work is if you were to run the echo as "some_user" and redirect the output with >> "some_file" because the file redirect will happen as you.

Hint: tee also supports append with the -a flag, if you need to replace a line in a file as another user you could execute sed as the desired user.

Answer 9

< command > |& tee filename # this will create a file "filename" with command status as a content, If a file already exists it will remove existed content and writes the command status.

< command > | tee >> filename < command > | tee >> filename # this will append status to the file but it doesn't print the command status on standard_output (screen).

I want to print something by using "echo" on screen and append that echoed data to a file

echo "hi there, Have to print this on screen and append to a file" 

Answer 10

In my case I had the Java process with output logs. The simplest solution to display output logs and redirect them into the file(named logfile here) was:

my_java_process_run_script.sh |& tee logfile

Result was Java process running with output logs displaying and putting them into the file with name log file

Answer 11

T恤非常适合这个，但这也可以完成这项工作

ls -lr / > output | cat output