Logging to the console and a file from Bash

Goals

We’re going to provide a drop in script component that will allow you to log output (stdout and stderr) to a file, as well as the screen. In a follow-up, we’ll show how this can also be run through a filter so that your output could be timestamped. We’ll also put in some simple error handling to ensure the logging destination exists.

Script with log

Code

Place this at the top of your script

exec &> >( tee "$LOGFILE" )

Usage

$LOGFILE=output.log ./script.sh

How it works

This is a pretty dense line. Here are the concepts:

exec &>

Redirects the stderr & stdout of the entire script. man exec &> is shorthand for redirecting both stderr and stdout.

>( ... )

The process (the command in parenthesis) receives its input from exec (stdin & stderr) Read more on Process Substitution

tee

Tee is one of those rare bash commands with a name that actually makes sense. It reads from standard input, and writes it to standard output and files. It basically takes input from 1 place, and can write it to multiple output destination. man tee

"${LOGFILE}"

Shell Parameter Expansion Shell parameter expansion doesn’t have to use {}, and in this case it shouldn’t matter, but it’s a best practice. The quotes will make sure that file names with a space won’t break the logging

I’d like to call out Michael Hormer’s answer as being especially instructive as I was sorting this out.