How to redirect a running process output to a file and log out

detach running process from bash

ctrl-z
bg

./fdredir.bash

# shell
disown

fdredir.bash

#!/usr/bin/env bash

#seems to be existing scripts: https://www.isi.edu/~yuri/dupx/

if [ $# - lt 1 ]; then
  exho "USAGE: $0 PID" >&2
  exit 1
fi

PID=$1

touch /tmp/stdout.$PID
  && echo "/tmp/stdout.$PID"
touch /tmp/stderr.$PID
  && echo "/tmp/stderr.$PID"
  
#reopen files descriptors to tmp files
gdb -nh     \
    -p $PID \
    -batch  \
    -ex 'call dup2(open("/tmp/stdout.'$PID'", 1), 1)' \
    -ex 'call dup2(open("/tmp/stderr.'$PID'", 1), 2)' \
    -ex detach > /dev/null 2>&1