[Linux] How to redirect output to a file and stdout


Answers

$ 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
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?




You can primarily use Zoredache solution, but If you don't want to overwrite the output file you should write tee with -a option as follow :

ls -lR / | tee -a output.file



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..




< 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 # 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"