Confused about stdin stdout and stderr

Question

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

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stdin  Reads input through the console  e g  Keyboard input   Used in C with scanf  scanf  lt formatstring gt   lt pointer to storage gt          stdout  Produces output to the console  Used in C with printf  printf  lt string gt    lt values to print gt          stderr  Produces  error  output to the console  Used in C with fprintf  fprintf stderr   lt string gt    lt values to print gt            Redirection  The source for stdin can be redirected  For example  instead of coming from keyboard input  it can come from a file  echo  lt  file txt    or another program   ps   grep  lt userid gt       The destinations for stdout  stderr can also be redirected   For example stdout can be redirected to a file  ls    gt  ls-output txt  in this case the output is written to the file ls-output txt   Stderr can be redirected with 2 gt

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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

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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

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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

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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

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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  quot hello quot  as an argument to the echo command  The graphic also says 2 gt  amp 1 has the same effect as  amp  gt  however ls Documents ABC  gt  dirlist 2 gt  amp 1  does not give the same output as  ls Documents ABC  gt  dirlist  amp  gt   This is because  amp  gt  requires a file to redirect to  and 2 gt  amp 1 is simply sending stderr into stdout

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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  quot getting item 30 of 42424 quot  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    quot All programs placed diagnostics on the standard output  This had always caused trouble when the output was redirected into a  le  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  le  That was not quite enough  With pipelines diagnostics could come from any of several programs running simultaneously  Diagnostics needed to identify themselves  quot

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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  lt inputfile 2 gt 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

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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

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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  gt   tmp out       redirect cat s standard out to  tmp foo cat  nonexistant 2 gt   tmp err     redirect cat s standard error to  tmp error cat  lt   etc passwd                redirect cat s standard input to  etc passwd   Or  the whole enchilada   cat  lt   etc passwd  gt   tmp out 2 gt   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

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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

[linux] Confused about stdin, stdout and stderr?

stdin

stdout

stderr

Redirection

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