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

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

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 3

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

Answer 4

$ 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