Explain the bash command "exec > >(tee $LOG_FILE) 2>&1"

Question

My intent was to have all the output of my bash script displayed on the console and logged to a file.

Here is my script that works as expected.

#!/bin/bash

LOG_FILE="test_log.log"
touch $LOG_FILE

# output to console and to logfile
exec > >(tee $LOG_FILE) 2>&1

echo "Starting command ls"
ls -al
echo "End of script"

However I do not understand why it works that way.

I expected to have exec >>(tee $LOG_FILE) 2>&1 work but it fails although exec >>$LOG_FILE 2>&1 indeed works.

I could not find the reason for the construction exec > >(command ) in the bash manual nor in advanced bash scripting. Can you explain the logic behind it ?

Answer 1

The >(tee $LOG_FILE) is an example of Process substitution, you might wish to search for that. Advanced Shell Scriptng and Bash manual

Using the syntax, <(program) for capturing output and >(program) for feeding input, we can pass data just one record at a time. It is more powerful than command substitution (backticks, or $( )) because it substitutes for a filename, not text. Therefore anywhere a file is normally specified we can substitute a program's standard output or input (although process substitution on input is not all that common). This is particularly useful where a program does not use standard streams for what you want.

Note that in your example you are missing a space, exec >>(tee $LOG_FILE) 2>&1 is wrong (you will get a syntax error). Rather,

exec > >(tee $LOG_FILE) 2>&1

is correct, that space is critical.

So, the exec > part changes file descriptor 1 (the default), also known as stdout or standard output, to refer to "whatever comes next", in this case it is the process substitution, although normally it would be a filename.

2>&1 redirects file descriptor 2 (stderr or standard error) to refer to the same place as file descriptor 1 (stdout or standard out). Important: if you omit the & you end-up with a file called 1 rather than successful redirection.

Once you have called the exec line above, then you have changed the current process's standard output, so output from the commands which follow go to that tee process instead of to regular stdout.