Change stdout for shell to pipe

Question

With

exec >LOG 2>&1

I can change stdout and stderr for the current shell, so all following commands will inherit those file descriptors.

Is there a way to change stdout/stderr for the current shell to a pipe?

I've tried

exec | gpg -aer 0x6aabe354

in the hope to replace current stdout with gnupg, encrypting the output of the remaining script, but I received cleartext output and an empty PGP message instead, so clearly this is not the correct syntax.

Answer 1

Easiest is to do:

{
  rest of your script
} | gpg...

With shells compliant to the 2024 edition of the POSIX specification, you can do:

set -o pipefail
{
  rest of your script
} | gpg...

To make sure that failure of rest of your script or gpg is not lost in the overall exit status.

---

exec | cmd runs 2 commands in parallel in 2 separate processes, the first one (evaluated in a child process in all shells) doing nothing without forking (exec should really have been called nofork), and with its stdout redirected to a pipe and the second (which in some shells is not done in a child) runs cmd (in a child if its external and therefore needs to be executed) with its stdin connected to the other end of a pipe.

In some shells, you can do:

exec > >(cmd)

Using the (non-POSIX) process substitution feature from ksh Which runs cmd in background with its stdin connected to a pipe, and >(cmd) expands to the name of a file (which can be /dev/fd/x or /proc/self/fd/x or a named pipe depending on the system and shell) which once open for writing (as > does) gets you the other end of the pipe.

The exit status of cmd is lost in that case.

That you can do by hand by using a temporary named pipe:

mkfifo fifo || exit
cmd < fifo & cmd_pid=$!
exec > fifo

rest of the script

ret=$?
exec >&-
wait "$cmd_pid" || ret=$?
rm -f fifo
exit "$ret"

(not handling the cleanup of the fifo if the script is killed).

Bearing in mind that there's still no command in POSIX (other than m4) to give you a unique temp file reliably, let alone one of type fifo.