understanding bash exec {logout}?

Question

What does the following Bash command do?

exec {logout}> >(tee -a "${LOGFILE}")

or

exec {logout}>> "${LOGFILE}"

Answer 1

From the Bash reference about redirections:

Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it to {varname}.

• In the case

  exec {logout}> >(tee -a "${LOGFILE}")
  

  Bash will open a new file descriptor, assign the corresponding integer to the variable logout, and link this file descriptor to the process substitution

  tee -a "${LOGFILE}"
  

  After you enter this command, you can check that the variable logout has been assigned to an integer. Then, each time you redirect a stream to the file descriptor $logout, it will be fed to tee:

  #!/bin/bash
  
  logfile=testlogfile
  
  exec {logout}> >(tee -a "$logfile")
  echo "logout=$logout"
  
  # now output some stuff to fd $logout:
  echo >&"$logout" "Hello file descriptor $logout"
  
  # it will send the message to tee, so that you'll see it on stdout
  # and will also be appended to $logfile
  
  # Check that output really got to $logfile:
  cat "$logfile"
  

• The second case is similar, but this time, output going to file descriptor $logout will only be appended to $logfile (and not passed to tee, so you won't see it on stdout).

After this, you don't really need to close the file descriptor, it will be closed automatically at the end of the script (or of the session); but if you really want to close it anyway:

exec {logout}>&-

You can check the file descriptors that are opened with

ls /dev/fd