In
./binary < file
binary
's stdin is the file open in read-only mode. Note that bash
doesn't read the file at all, it just opens it for reading on the file descriptor 0 (stdin) of the process it executes binary
in.
In:
./binary << EOF
test
EOF
Depending on the shell, binary
's stdin will be either a deleted temporary file (AT&T ksh, zsh, bash...) that contains test\n
as put there by the shell or the reading end of a pipe (dash
, yash
; and the shell writes test\n
in parallel at the other end of the pipe). In your case, if you're using bash
, it would be a temp file.
In:
cat file | ./binary
Depending on the shell, binary
's stdin will be either the reading end of a pipe, or one end of a socket pair where the writing direction has been shut down (ksh93) and cat
is writing the content of file
at the other end.
When stdin is a regular file (temporary or not), it is seekable. binary
may go to the beginning or end, rewind, etc. It can also mmap it, do some ioctl()s
like FIEMAP/FIBMAP (if using <>
instead of <
, it could truncate/punch holes in it, etc).
pipes and socket pairs on the other hand are an inter-process communication means, there's not much binary
can do beside read
ing the data (though there are also some operations like some pipe-specific ioctl()
s that it could do on them and not on regular files).
Most of the times, it's the missing ability to seek
that causes applications to fail/complain when working with pipes, but it could be any of the other system calls that are valid on regular files but not on different types of files (like mmap()
, ftruncate()
, fallocate()
). On Linux, there's also a big difference in behaviour when you open /dev/stdin
while the fd 0 is on a pipe or on a regular file.
There are many commands out there that can only deal with seekable files, but when that's the case, that's generally not for the files open on their stdin.
$ unzip -l file.zip
Archive: file.zip
Length Date Time Name
--------- ---------- ----- ----
11 2016-12-21 14:43 file
--------- -------
11 1 file
$ unzip -l <(cat file.zip)
# more or less the same as cat file.zip | unzip -l /dev/stdin
Archive: /proc/self/fd/11
End-of-central-directory signature not found. Either this file is not
a zipfile, or it constitutes one disk of a multi-part archive. In the
latter case the central directory and zipfile comment will be found on
the last disk(s) of this archive.
unzip: cannot find zipfile directory in one of /proc/self/fd/11 or
/proc/self/fd/11.zip, and cannot find /proc/self/fd/11.ZIP, period.
unzip
needs to read the index stored at the end of the file, and then seek within the file to read the archive members. But here, the file (regular in the first case, pipe in the second) is given as a path argument to unzip
, and unzip
opens it itself (typically on fd other than 0) instead of inheriting a fd already opened by the caller. It doesn't read zip files from its stdin. stdin is mostly used for user interaction.
If you run that binary
of yours without redirection at the prompt of an interactive shell running in a terminal emulator, then binary
's stdin will be inherited from its caller the shell, which itself will have inherited it from its caller the terminal emulator and will be a pty device open in read+write mode (something like /dev/pts/n
).
Those devices are not seekable either. So, if binary
works OK when taking input from the terminal, possibly the issue is not about seeking.
If that 14 is meant to be an errno (an error code set by failing system calls), then on most systems, that would be EFAULT
(Bad address). The read()
system call would fail with that error if asked to read into a memory address that is not writable. That would be independent of whether the fd to read the data from points to a pipe or regular file and would generally indicate a bug1.
binary
possibly determines the type of file open on its stdin (with fstat()
) and runs into a bug when it's neither a regular file nor a tty device.
Hard to tell without knowing more about the application. Running it under strace
(or truss
/tusc
equivalent on your system) could help us see what is the system call if any that is failing here.
1 The scenario envisaged by Matthew Ife in a comment to your question sounds a lot plausible here. Quoting him:
I suspect it is seeking to the end of file to get a buffer size for reading the data, badly handling the fact that seek doesn't work and attempting to allocate a negative size (not handling a bad malloc). Passing the buffer to read which faults given the buffer is not valid.
Simply expanding on your approach:
exec 2> >(tee -a stderr stdall) 1> >(tee -a stdout stdall)
Standard error will be written to the file named stderr
, standard output to stdout
and both standard error and standard output will also be written to the console (or whatever the two file descriptors are pointing at the time exec
is run) and to stdall
.
tee -a
(append) is required to prevent stdall
from being overwritten by the second tee
that starts writing to it.
Note that the order in which redirections are performed is relevant: the second process substitution is affected by the first redirection, i.e. the errors it emitted would be sent to >(tee -a stderr stdall)
. You can, of course, redirect the second process substitution's standard error to /dev/null
to avoid this side effect. Redirecting standard output before standard error would send every error to stdout
and stdall
too.
Since the commands in Bash's process substitutions are executed asynchronously, there is no way to guarantee that their output will be displayed in the order it was generated. Worse, fragments from standard output and standard error are likely to end up appearing on the same line.
Best Answer
It helps a bit if you think the file descriptors as variables that accept a file as a value (or call it an i/o stream) and the order they appear is the order of their evaluation.
What happens in the above example is:
1) The script starts (as per default and unless otherwise inherited) with the following
2) The
exec
command translates to declaring a new variable and assigning a valueSo now, two file descriptors have the value stdout, i.e. both can be used to print to the screen.
3) before
ls
is executed and inherits all open file descriptors, the following setup happensfd/3 has served the purpose of keeping the stdout value long enough to return it to fd/1. So now everything that
ls
sends to fd/1 goes to stdout and notgrep
's stdin.The order is important, e.g. if we'd run
ls -l >&3 2>&1 3>&-
, ls.fd/2 would write to stdout instead ofgrep
's stdin.4) fd/3 for
grep
is closed and not inherited. It would be unused anyway.grep
can only filter error messages fromls
The example provided in ABSG is probably not the most helpful and the comment "Close fd 3 for 'grep' (but not 'ls')" is a bit misleading. You can interpret it as: "for the ls, pass the value of ls.fd/3 to ls.fd/1 before unsetting so it won't get closed".