How to set pipe of process after process substitution in bash so that process continues after script ends?

Question

How can I set up a process that has had its output redirected via process substitution so that it can survive after the parent script exits?

I am trying to set up a Bash script to monitor the output of a process when it starts up and then release that process so that it can continue normally (I want to call this script in other scripts and have it block further execution until start up is complete).

Right now, I am using the http.server module as my test case. The code below properly starts up the Python module and checks its output looking for a GET request and then exits. However, when it exits, the http.server process also dies. How can I make it so that this process survives after bash exits?

exec 3< <(python3 -u -m http.server 2>&1 )
while true; do
        read -t 2 line
        echo "reading..."
        echo "$line"
        if [[ "$line" =~ .*GET.* ]]; then
            break
        fi
done <&3

I thought the process was dying because the file descriptor 3 is closed when bash exits and Python is exiting when it sees its output file closed. I have tried various redirections of 3, but they didn't help. Maybe something else about the process needs to be modified so that it will survive? Basically I want the equivalent of executing python3 -u -m http.server 2>&1 & but with the ability to monitor the process's output temporarily when it first starts.

Answer 1

From the bash script, you start a subprocess (running a Python program), and create a pipe from it to the bash script.

After the bash script exit, there is no longer any process that has the pipe open for reading. Therefore the next time the Python script writes something to the pipe, it receives a SIGPIPE signal and dies. You would see the resulting error if you'd only redirected standard output to the pipe and not standard error as well.

Even if the Python script ignored the SIGPIPE signal, it would still die when writing to the pipe because the write to the pipe would fail, which would trigger an exception.

If you only want to see the line that tells you the server is ready, or up to the line about the first request, a simple way is to keep a reader running:

exec 3< <(python3 -u -m http.server 2>&1 )
{ while true; do
  …
  done;
  exec cat;
} <&3

You may want to redirect standard error to a log file somewhere rather than blissfully ignoring error messages.

An alternative approach would be to write a few lines of Python, overriding the BaseHTTPRequestHandler.log_request method to emit a log entry the first time only.