Bash 文件重定向错误？

TL;DR：不是bash，而是apt-cache在搞乱文件描述符。

apt-cache正在做一些非常有趣的事情 - 它往往不会写出以N:用于标准输出的字符开头的行。

考虑一下：

$ apt-cache show nonexistent
N: Unable to locate package nonexistent
E: No packages found

我们看到两行，一行N:以E:. N:行转到标准输出。在您的示例中，您有两N:行。

# apt-cache show php5
N: Can't select versions from package 'php5' as it is purely virtual
N: No packages found

如果您通过跟踪系统调用，strace -e write -f bash -c 'apt-cache show randomtext >&-'您将看到写入E:行发生，但N行不存在：

[pid 12450] write(2, "E", 1E)            = 1
[pid 12450] write(2, ": ", 2: )           = 2
[pid 12450] write(2, "No packages found", 17No packages found) = 17
[pid 12450] write(2, "\n", 1
)           = 1
[pid 12450] +++ exited with 100 +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12450, si_uid=1000, si_status=100, si_utime=5, si_stime=2} ---
+++ exited with 100 +++

所以apt-cache足够聪明来检查重定向的标准输出。但是呢stderr？显然写仍然存在：如果你这样做，strace -e write,openat,dup2 -f bash -c 'apt-cache show randomtext 2>&-你会看到apt-cache打开/dev/null仍然有一些东西存在stderr：

[pid 12543] openat(AT_FDCWD, "/dev/null", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 2
....
[pid 12543] write(2, "N", 1)            = 1
[pid 12543] write(2, ": ", 2)           = 2
[pid 12543] write(2, "Unable to locate package randomt"..., 35) = 35
[pid 12543] write(2, "\n", 1)           = 1
[pid 12543] write(2, "E", 1)            = 1
[pid 12543] write(2, ": ", 2)           = 2
[pid 12543] write(2, "No packages found", 17) = 17
[pid 12543] write(2, "\n", 1)           = 1
[pid 12543] +++ exited with 100 +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12543, si_uid=1000, si_status=100, si_utime=5, si_stime=3} ---
+++ exited with 100 +++

如果您在 bash 中对其他程序执行相同的操作，它会按预期工作：

# stdout closed, stderr not
$ ls -l /proc/self/fd >&-
ls: write error: Bad file descriptor
# stdout open , stderr closed, and it's number is assigned to whatever command is trying to open - in this case /proc/self/fd directory
$ ls -l /proc/self/fd 2>&-
total 0
lrwx------ 1 xie xie 64 Oct  6 11:32 0 -> /dev/pts/1
lrwx------ 1 xie xie 64 Oct  6 11:32 1 -> /dev/pts/1
lr-x------ 1 xie xie 64 Oct  6 11:32 2 -> /proc/12723/fd