Bash file redirection bug?

Solution 1:

TL;DR: It's not bash, it's apt-cache that's messing around with file descriptors.

apt-cache is doing something very interesting - it tends to not write out lines starting with N: characters intended for stdout.

Consider this:

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

We see two lines, one starting with N: one starting with E:. N: lines go to stdout. In your example, you have two N: lines.

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

If you trace the system calls via strace -e write -f bash -c 'apt-cache show randomtext >&-' you'll see that writing E: lines happens, but N lines aren't there:

[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 +++

So apt-cache is smart enough to check for redirected stdout. But what about stderr ? Apparently writes are still there: if you do strace -e write,openat,dup2 -f bash -c 'apt-cache show randomtext 2>&- you'll see that apt-cache opens /dev/null to still have something present for 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 +++

If you perform the same with other programs in bash, it works as expected:

# 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