#### streams

fd_exists() { (: <&"$1") 2>/dev/null; }

require_fd_not_exists() # usage: require_fd_not_exists fd || return 0

{ ! fd_exists "$1" || type=info die "fd $1 already exists, skipping"; }

set_fds() { echo_func; echo_eval exec "$@"; } # usage: set_fds redirect...

fd_set_default() # usage: fd_set_default 'dest[<>]src'

{

	echo_func

	local dest="${1%%[<>]*}"

	require_fd_not_exists "$dest" || return 0

	set_fds "$1"

}

# convention: use fd 10/11/12 for command-specific alternate stdin/stdout/stderr

set_global_fds()

# allows commands to access global stdin/stdout/stderr using fd 20/21/22

# this works even when /dev/tty isn't available

{

	log++; echo_func; log++

	# ignore errors if a source fd isn't open

	fd_set_default '20<&0' || true

	fd_set_default '21>&1' || true

	fd_set_default '22>&2' || true

}

set_global_fds

err_fd=22 # global stderr

# usage: explicit_errors_only=1 script...

# show only explicitly-displayed errors (which have been redirected to fd 22)

# most of the time this has the same effect as `verbosity=0 script...`,

# which displays everything that isn't explicitly hidden

# this option should only be used for testing the explicit error displaying

if test "$explicit_errors_only"; then disable_logging; fi