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.
Usually if I want to print the output of a command and in addition capture that output in a file, tee is the solution. But I'm making a script using a utility which seems to have a special behaviour. It's the wps wireless assessment tool bully.
If I run a bully command as normal (without tee), the output is shown in a standard way, step by step. But If I put the pipe at the end to log like this | tee "/path/to/my/logfile" the output on the screen freezes. It shows nothing until the command ends. And after ending, it shows everything in one shot (not step by step) and of course it puts the output also in the log tee file.
An example of bully command: bully wlan0mon -b 00:11:22:33:44:55 -c 8 -L -F -B -v 3 -p 12345670 | tee /root/Desktop/log.txt
Why? Not sure if it only happens with bully or if there are other programs with the same behaviour.
Is there another way to capture the output into a file having the output on screen in real time?
What you're seeing is full buffering vs line buffering. By default, when stdout is writing to a tty (i.e. interactive) you'll have line buffering, vs the default of being fully buffered otherwise. You can see the setvbuf(3) man page for a more detailed explanation.
Some commands offer an option to force line buffering (e.g. GNU grep has --line-buffered). But that sort of option is not widely available.
Another option is to use something like expect's unbuffer command if you want to be able to see the output more interactively (at the cost of depending on expect, of course).
I want to run gdb (GNU debugger) in Screen virtual terminal and grep the output in real time on adjacent Screen view.
How do I implement this arrangement? Normal pipe just redirects the output. I'm also curious how to bind Screen view (^A + c) to an existing process for IO.
EDIT:
I came up with the following solution. Created a named pipe with mkfifo pipe and executed gdb program | tee pipe in pty1. This will duplicate the output to a pipe. In pty2 I executed less -f pipe | grep foo to print the lines of interest.
I'm sure there have to be simpler way for such a trivial task though.
EDIT2:
The method mentioned above seems somewhat buggy. Gdb doesn't print anything to it's console unless something actually reads from the FIFO. Why is that? Also, when I try this method with my own program, which simply printf HelloWorld to stdout, nothing is printed in neither view.
EDIT3:
I figured out it's intentional that Tee blocks if nobody actually reads from the pipe. A matter of synchronization. Still I wonder, how is the original program able to read the input from keyboard even Tee controls now the terminal window. Or is it so that terminal input goes to stdin of original program and output to stdout of Tee?
You don't have to start your program out of gdb. Just start it in one screen pane, and determine the pid (use top, pgrep, ps).
In the other pane you start the gdb session:
gdb <path_to_program> <pid>
This way you have a terminal to control gdb and a terminal to use the program you are debugging, both have their own input and outputs.
The only condition is that the program runs long enough for you to attach the debugger to the process. An easy way to do that is to make it wait for input at the beginning. You could also make it print its pid.
I'd like to log standard output form a script of mine to a file, but also have it display to me on screen for realtime monitoring. The script outputs something about 10 times every second.
I tried to redirect stdout to a file and then tail -f that file from another terminal, but for some reason tail is updating the screen significantly slower than the script is writing to the file.
What's causing this lag? Is there an alternate method of getting one standard output stream both on my terminal and into a file for later examination?
I can't say why tail lags, but you can use tee:
Redirect output to multiple files, copies standard input to standard output and also to any files given as arguments. This is useful when you want not only to send some data down a pipe, but also to save a copy.
Example: <command> | tee <outputFile>
How much of a lag do you see? A few hundred characters? A few seconds? Minutes? Hours?
What you are seeing is buffering. Almost all file reads and writes are buffered. This includes input and output and there is also some buffering taking place within pipes. It's just more efficient to pass a packet of data around rather than a byte at a time. I believe data on HFS+ file systems are stored in UTF-16 while Mac OS X normally use UTF-8 as a default. (NTFS also stores data using UTF-16 while Windows uses code pages for character data by default).
So, if you run tail -f from another terminal, you may be seeing buffering from tail, but when you use a pipe and then tee, you may have a buffer in the pipe, and in the tee command which maybe why you see the lag.
By the way, how do you know there's a lag? How do you know how quickly your program is writing to the disk? Do you print out something in your program to help track the writes to the file?
In that case, you might not be lagging as much as you think. File writes are also buffered. So, it is very possible that the lag isn't from the tail -f, but from your script writing to the file.
Use tee command:
tail -f /path/logFile | tee outfile
I want to put my program's output into a file. I keyed in the following :
./prog > log 2>&1
But there is nothing in the file "log". I am using the Ubuntu 11.10 and the default shell is bash.
Anybody know the cause of this AND how I can debug this?
There are many possible causes:
The program reads the input from log file while you try to redirect into it with truncation (see Why doesn't "sort file1 > file1" work?)
The output is buffered so that you don't see data in the file until the output buffer is flushed. You can manually call fflush or output std::flush if using C++ I/O stream etc.
The program is smart enough and disables output if the output stream is not a terminal.
You look at the wrong file (i.e. in another directory).
You try to dump file's contents incorrectly.
Your program outputs '\0' as the first character so the output appears to be empty, even though there is some data.
Name your own.
Your best bet is to run this application under a debugger (like gdb) or use strace or ptrace (or both) and see what the program is doing. I mean, really, output redirection works for the last like 40 years, so the problem must be somewhere else.