stdout - Bash redirecting synchronization dilemma

Question

In order to automate some system update functionality I have written some BASH scripts.

What I want to achieve is to redirect stdout and stderr to a log file and stderr to the terminal running the script.

This is what I have come up with so far: ((./cmd >logfile) 2>&1) | tee logfile

Note ./cmd is a placeholder here, in my script it is actually an ssh call into one of my machines.

The unfortunate thing is that writing to the same file from two different processes is not ideal. My update.log obviously shows signs of the two writes stomping on each other. I want the log to interleave so the errors thrown are close to the relevant standard output, thus writing to separate files and merging them afterwards is not really an option.

Any suggestions would be greatly appreciated.

Answer 1

Have tee copy its stdin to the controlling terminal, pipe ./cmd's stderr to tee's stdin, and redirect both programs' stdout to a file descriptor connected to logfile. tee does not buffer its output, so make sure ./cmd doesn't either, and hope for the best.

{ stdbuf -o 0 ./cmd 2>&1 >&3 | tee /dev/tty >&3; } 3>logfile