I have a program that accepts input on one FIFO and emits output to another FIFO. I want to write a small script to control this program. The script needs to listen both to standard input (so I can input commands to adjust things in real time) and the program's output FIFO (so it can respond to events happening there as well).
Essentially my control program needs to select between standard input and a file (my FIFO).
I like learning how to figure out how to develop simple and elegant bash-based solutions to complex problems, and after a little headscratching I remembered that that tail -f will happily select on multiple files and tell you when one of them changes in real time, so I initially tried
tail -f <(od -An -vtd1 -w1) <(cat fifo)
to read both standard input (I'd previously run stty icanon min 1; this od invocation shows each stdin character on a separate line alongside its ASCII code, and is great for escape sequence parsing) and my FIFO. This failed epically (as does cat <(cat)): od gets run here as a backgrounded task, so it doesn't get access to the controlling TTY, and fails with a cryptic "I/O error" that was explained incredibly well here.
So now I'm a bit stumped. I realize that I can use any scripting language like Perl/Python/Ruby/Tcl to solve this; my compsci/engineering question is whether/how I might be able to solve this using (Linux) shell scripting.
I have written my code for writing a number to pipe in linux. it is as under,but it is showing errors,can anyone help me on this.
Basically the problem statement for the program is as below:-
One program will open a pipe, write a number to pipe.
- Other program will open the same pipe, will read the number and print them.
- Close both the pipes
int main()
{
int number;
FILE *fout;
fout = popen(" ","w");
pclose(fout);
return 0;
}
Now my question is what command should i give in the popen command option (as shown blank above) so as i can proceed further and write a number to pipe.
First, create a named pipe using the mknod command. mknod pipe p. Then read and write using functions as if they are files. A node can be created using code also, using the mknod function. Look for error code EEXIST. More information here. http://linux.die.net/man/2/mknod
You don't understand well how IPC and pipes work; please read a good book: Advanced Linux Programming has several chapters on these issues.
We don't have hours to explain difficult concepts covered by such good books. Take several hours to read them!
The library function popen(3) runs a command. Very probably, you don't have a p command on your system. I guess fp is NULL and errno is set.
popen is using pipe(2), fork(2), dup2(2), execve(2) and /bin/sh -c etc
Let's say I have 10 programs (in terminals) working in tandem: {p1,p2,p3,...,p10}.
It's hard to keep track of all STDOUT debug statements in their respective terminal. I plan to create a GUI to keep track of each STDOUT such that, if I do:
-- Click on p1 would "tail" program 1's output.
-- Click on p3 would "tail" program 4's output.
It's a decent approach but there may be better ideas out there? It's just overwhelming to have 10 terminals; I'd rather have 1 super terminal that keeps track of this.
And unfortunately, linux "screen" is not an option. RESTRICTIONS: I only have the ability to either: redirect STDOUT to a file. (or read directly from STDOUT).
If you are looking for a creative alternative, I would suggest that you look at sockets.
If each program writes to the socket (rather than STDOUT), then your master terminal can act as a server and organize the output.
Now from what you described, it seems as though you are relatively constrained to STDOUT, however it could be possible to do something like this:
# (use netcat (or nc on some systems) to write to a socket on the provided port)
./prog1 | netcat localhost 12312
I'm not sure if this fits in the requirements of what you are doing (and it might be more effort than it is worth!), but it could provide a very stable solution.
EDIT: As was pointed out in the comments, netcat does exactly what you would need to make this work.
I am rather confused with the purpose of these three files. If my understanding is correct, stdin is the file in which a program writes into its requests to run a task in the process, stdout is the file into which the kernel writes its output and the process requesting it accesses the information from, and stderr is the file into which all the exceptions are entered. On opening these files to check whether these actually do occur, I found nothing seem to suggest so!
What I would want to know is what exactly is the purpose of these files, absolutely dumbed down answer with very little tech jargon!
Standard input - this is the file handle that your process reads to get information from you.
Standard output - your process writes conventional output to this file handle.
Standard error - your process writes diagnostic output to this file handle.
That's about as dumbed-down as I can make it :-)
Of course, that's mostly by convention. There's nothing stopping you from writing your diagnostic information to standard output if you wish. You can even close the three file handles totally and open your own files for I/O.
When your process starts, it should already have these handles open and it can just read from and/or write to them.
By default, they're probably connected to your terminal device (e.g., /dev/tty) but shells will allow you to set up connections between these handles and specific files and/or devices (or even pipelines to other processes) before your process starts (some of the manipulations possible are rather clever).
An example being:
my_prog <inputfile 2>errorfile | grep XYZ
which will:
create a process for my_prog.
open inputfile as your standard input (file handle 0).
open errorfile as your standard error (file handle 2).
create another process for grep.
attach the standard output of my_prog to the standard input of grep.
Re your comment:
When I open these files in /dev folder, how come I never get to see the output of a process running?
It's because they're not normal files. While UNIX presents everything as a file in a file system somewhere, that doesn't make it so at the lowest levels. Most files in the /dev hierarchy are either character or block devices, effectively a device driver. They don't have a size but they do have a major and minor device number.
When you open them, you're connected to the device driver rather than a physical file, and the device driver is smart enough to know that separate processes should be handled separately.
The same is true for the Linux /proc filesystem. Those aren't real files, just tightly controlled gateways to kernel information.
It would be more correct to say that stdin, stdout, and stderr are "I/O streams" rather
than files. As you've noticed, these entities do not live in the filesystem. But the
Unix philosophy, as far as I/O is concerned, is "everything is a file". In practice,
that really means that you can use the same library functions and interfaces (printf,
scanf, read, write, select, etc.) without worrying about whether the I/O stream
is connected to a keyboard, a disk file, a socket, a pipe, or some other I/O abstraction.
Most programs need to read input, write output, and log errors, so stdin, stdout,
and stderr are predefined for you, as a programming convenience. This is only
a convention, and is not enforced by the operating system.
As a complement of the answers above, here is a sum up about Redirections:
EDIT: This graphic is not entirely correct.
The first example does not use stdin at all, it's passing "hello" as an argument to the echo command.
The graphic also says 2>&1 has the same effect as &> however
ls Documents ABC > dirlist 2>&1
#does not give the same output as
ls Documents ABC > dirlist &>
This is because &> requires a file to redirect to, and 2>&1 is simply sending stderr into stdout
I'm afraid your understanding is completely backwards. :)
Think of "standard in", "standard out", and "standard error" from the program's perspective, not from the kernel's perspective.
When a program needs to print output, it normally prints to "standard out". A program typically prints output to standard out with printf, which prints ONLY to standard out.
When a program needs to print error information (not necessarily exceptions, those are a programming-language construct, imposed at a much higher level), it normally prints to "standard error". It normally does so with fprintf, which accepts a file stream to use when printing. The file stream could be any file opened for writing: standard out, standard error, or any other file that has been opened with fopen or fdopen.
"standard in" is used when the file needs to read input, using fread or fgets, or getchar.
Any of these files can be easily redirected from the shell, like this:
cat /etc/passwd > /tmp/out # redirect cat's standard out to /tmp/foo
cat /nonexistant 2> /tmp/err # redirect cat's standard error to /tmp/error
cat < /etc/passwd # redirect cat's standard input to /etc/passwd
Or, the whole enchilada:
cat < /etc/passwd > /tmp/out 2> /tmp/err
There are two important caveats: First, "standard in", "standard out", and "standard error" are just a convention. They are a very strong convention, but it's all just an agreement that it is very nice to be able to run programs like this: grep echo /etc/services | awk '{print $2;}' | sort and have the standard outputs of each program hooked into the standard input of the next program in the pipeline.
Second, I've given the standard ISO C functions for working with file streams (FILE * objects) -- at the kernel level, it is all file descriptors (int references to the file table) and much lower-level operations like read and write, which do not do the happy buffering of the ISO C functions. I figured to keep it simple and use the easier functions, but I thought all the same you should know the alternatives. :)
I think people saying stderr should be used only for error messages is misleading.
It should also be used for informative messages that are meant for the user running the command and not for any potential downstream consumers of the data (i.e. if you run a shell pipe chaining several commands you do not want informative messages like "getting item 30 of 42424" to appear on stdout as they will confuse the consumer, but you might still want the user to see them.
See this for historical rationale:
"All programs placed diagnostics on the standard output. This had
always caused trouble when the output was redirected into a file, but
became intolerable when the output was sent to an unsuspecting
process. Nevertheless, unwilling to violate the simplicity of the
standard-input-standard-output model, people tolerated this state of
affairs through v6. Shortly thereafter Dennis Ritchie cut the Gordian
knot by introducing the standard error file. That was not quite enough.
With pipelines diagnostics could come from any of several programs
running simultaneously. Diagnostics needed to identify themselves."
stdin
Reads input through the console (e.g. Keyboard input).
Used in C with scanf
scanf(<formatstring>,<pointer to storage> ...);
stdout
Produces output to the console.
Used in C with printf
printf(<string>, <values to print> ...);
stderr
Produces 'error' output to the console.
Used in C with fprintf
fprintf(stderr, <string>, <values to print> ...);
Redirection
The source for stdin can be redirected. For example, instead of coming from keyboard input, it can come from a file (echo < file.txt ), or another program ( ps | grep <userid>).
The destinations for stdout, stderr can also be redirected. For example stdout can be redirected to a file: ls . > ls-output.txt, in this case the output is written to the file ls-output.txt. Stderr can be redirected with 2>.
Using ps -aux reveals current processes, all of which are listed in /proc/ as /proc/(pid)/, by calling cat /proc/(pid)/fd/0 it prints anything that is found in the standard output of that process I think. So perhaps,
/proc/(pid)/fd/0 - Standard Output File
/proc/(pid)/fd/1 - Standard Input File
/proc/(pid)/fd/2 - Standard Error File
for example
But only worked this well for /bin/bash other processes generally had nothing in 0 but many had errors written in 2
For authoritative information about these files, check out the man pages, run the command on your terminal.
$ man stdout
But for a simple answer, each file is for:
stdout for a stream out
stdin for a stream input
stderr for printing errors or log messages.
Each unix program has each one of those streams.
stderr will not do IO Cache buffering so if our application need to print critical message info (some errors ,exceptions) to console or to file use it where as use stdout to print general log info as it use IO Cache buffering there is a chance that before writing our messages to file application may close ,leaving debugging complex
A file with associated buffering is called a stream and is declared to be a pointer to a defined type FILE. The fopen() function creates certain descriptive data for a stream and returns a pointer to designate the stream in all further transactions. Normally there are three open streams with constant pointers declared in the header and associated with the standard open files.
At program startup three streams are predefined and need not be opened explicitly: standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). When opened the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device
https://www.mkssoftware.com/docs/man5/stdio.5.asp
Here is a lengthy article on stdin, stdout and stderr:
What Are stdin, stdout, and stderr on Linux?
To summarize:
Streams Are Handled Like Files
Streams in Linux—like almost everything else—are treated as though
they were files. You can read text from a file, and you can write text
into a file. Both of these actions involve a stream of data. So the
concept of handling a stream of data as a file isn’t that much of a
stretch.
Each file associated with a process is allocated a unique number to
identify it. This is known as the file descriptor. Whenever an action
is required to be performed on a file, the file descriptor is used to
identify the file.
These values are always used for stdin, stdout, and stderr:
0: stdin
1: stdout
2: stderr
Ironically I found this question on stack overflow and the article above because I was searching for information on abnormal / non-standard streams. So my search continues.
Greetings,
while porting old Solaris 2.4 code to CentOS 5.3 I came across an invocation like
/usr/bin/xterm -S%s%d ...
where %s is a two-character digit sequence XX like 00, 01 and %d is a numeric file descriptor. This was apparently a way to tell xterm to use /dev/ttypXX (a pseudo terminal slave) but the code does not seem to bother with opening the corresponding master, calling pipe(2) instead and passing the write fd as the %d substitution above. On Solaris, writing to this write fd from the spawner causes output to appear in the xterm child. In a strace(1) I saw no attempt to open anything under /dev, by the way.
According to the solaris manpage, the pipe system call creates two bidirectional pipes. So on solaris you can use both fds for reading and writing:
The files associated with fildes[0] and fildes1 are streams and are both
opened for reading and writing.
However according to the pipe(2) manpage on linux:
pipe() creates a pipe, a unidirectional data channel that can be used
for interprocess communication.
Note also the following from pipe(7):
On some systems (but not Linux), pipes are bidirectional: data can be
transmitted in both directions between the pipe ends. According to
POSIX.1-2001, pipes only need to be unidirectional. Portable applications
should avoid reliance on bidirectional pipe semantics.
So, on linux you can't pass pipefd1, the write end, to xterm since it expects an fd for bidirectional communication. To make it work, you'd have to use openpty() and pass the slave fd down to xterm.
AFAIK, openpty is not available on Solaris; that seems be the reason your code doesn't use it.