背景：
コマンド出力（stdoutとstderr）の一部をcshシェルの下の同じファイルに入れました。
質問：
次のリダイレクトヘッダーに「ターミナルではない」というメッセージが表示されるのはなぜですか？
> which brarchive
/home/ewpadm/brtools/brarchive
>
>
> which brarchive >& t_0
>
>
> cat t_0
Not a terminal
/home/ewpadm/brtools/brarchive
>
>
> ls -l Ac >>& t_0
>
>
> cat t_0
Not a terminal
/home/ewpadm/brtools/brarchive
ls: 0653-341 The file Ac does not exist.
>
>
> (which pwd; ls -l Fc) >>& t_0
>
>
> cat t_0
Not a terminal
/home/ewpadm/brtools/brarchive
ls: 0653-341 The file Ac does not exist.
Not a terminal
/usr/bin/pwd
ls: 0653-341 The file Fc does not exist.
>
>
> (which tar; file /etc/passwd) >>& t_0
>
>
> cat t_0
Not a terminal
/home/ewpadm/brtools/brarchive
ls: 0653-341 The file Ac does not exist.
Not a terminal
/usr/bin/pwd
ls: 0653-341 The file Fc does not exist.
Not a terminal
/usr/bin/tar
/etc/passwd: ascii text
>
>
> (which ntpq; head -1 /etc/ntp.conf) >>& t_0
>
>
> cat t_0
Not a terminal
/home/ewpadm/brtools/brarchive
ls: 0653-341 The file Ac does not exist.
Not a terminal
/usr/bin/pwd
ls: 0653-341 The file Fc does not exist.
Not a terminal
/usr/bin/tar
/etc/passwd: ascii text
Not a terminal
which: 0652-141 There is no ntpq in /oracle/EWP/112_64/bin /usr/bin /etc /usr/sbin /usr/ucb /home/ewpadm/bin /usr/bin/X11 /sbin . /usr/sap/EWP/SYS/exe/uc/rs6000_64 /usr/sap/EWP/SYS/exe/run /home/ewpadm /home/ewpadm/brtools.
# @(#)48 1.2 src/tcpip/etc/ntp.conf, ntp, tcpip610 2/16/96 10:16:34
>
>
> ls -l Ac >>& t_0
>
>
> cat t_0
Not a terminal
/home/ewpadm/brtools/brarchive
ls: 0653-341 The file Ac does not exist.
Not a terminal
/usr/bin/pwd
ls: 0653-341 The file Fc does not exist.
Not a terminal
/usr/bin/tar
/etc/passwd: ascii text
Not a terminal
which: 0652-141 There is no ntpq in /oracle/EWP/112_64/bin /usr/bin /etc /usr/sbin /usr/ucb /home/ewpadm/bin /usr/bin/X11 /sbin . /usr/sap/EWP/SYS/exe/uc/rs6000_64 /usr/sap/EWP/SYS/exe/run /home/ewpadm /home/ewpadm/brtools.
# @(#)48 1.2 src/tcpip/etc/ntp.conf, ntp, tcpip610 2/16/96 10:16:34
ls: 0653-341 The file Ac does not exist.
>
>
> (ls -l Ac; file /etc/passwd) >>& t_0
>
>
> cat t_0
Not a terminal
/home/ewpadm/brtools/brarchive
ls: 0653-341 The file Ac does not exist.
Not a terminal
/usr/bin/pwd
ls: 0653-341 The file Fc does not exist.
Not a terminal
/usr/bin/tar
/etc/passwd: ascii text
Not a terminal
which: 0652-141 There is no ntpq in /oracle/EWP/112_64/bin /usr/bin /etc /usr/sbin /usr/ucb /home/ewpadm/bin /usr/bin/X11 /sbin . /usr/sap/EWP/SYS/exe/uc/rs6000_64 /usr/sap/EWP/SYS/exe/run /home/ewpadm /home/ewpadm/brtools.
# @(#)48 1.2 src/tcpip/etc/ntp.conf, ntp, tcpip610 2/16/96 10:16:34
ls: 0653-341 The file Ac does not exist.
ls: 0653-341 The file Ac does not exist.
/etc/passwd: ascii text
>
>
> (file /etc/passwd; ls -l Ac) >>& t_0
>
>
> cat t_0
Not a terminal
/home/ewpadm/brtools/brarchive
ls: 0653-341 The file Ac does not exist.
Not a terminal
/usr/bin/pwd
ls: 0653-341 The file Fc does not exist.
Not a terminal
/usr/bin/tar
/etc/passwd: ascii text
Not a terminal
which: 0652-141 There is no ntpq in /oracle/EWP/112_64/bin /usr/bin /etc /usr/sbin /usr/ucb /home/ewpadm/bin /usr/bin/X11 /sbin . /usr/sap/EWP/SYS/exe/uc/rs6000_64 /usr/sap/EWP/SYS/exe/run /home/ewpadm /home/ewpadm/brtools.
# @(#)48 1.2 src/tcpip/etc/ntp.conf, ntp, tcpip610 2/16/96 10:16:34
ls: 0653-341 The file Ac does not exist.
ls: 0653-341 The file Ac does not exist.
/etc/passwd: ascii text
/etc/passwd: ascii text
ls: 0653-341 The file Ac does not exist.
>
>
これで、このエラーメッセージは>＆のstderr出力を介してのみ取得できることを確認できます。
>
> which ssh > t_1
>
>
> cat t_1
/usr/bin/ssh
>
>
> (which ssh > t_1) >& t_2
>
>
> cat t_1
/usr/bin/ssh
>
>
> cat t_2
Not a terminal
>
だから私の質問は、「>＆」を介したstderr出力でのみエラーが発生し、「>」を介したstderr出力でエラーが発生しない理由です。