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Special Files for Standard Descriptors

   Running programs conventionally have three input and output streams
already available to them for reading and writing.  These are known as
the "standard input", "standard output", and "standard error output".
These streams are, by default, connected to your terminal, but they are
often redirected with the shell, via the `<', `<<', `>', `>>', `>&',
and `|' operators.  Standard error is typically used for writing error
messages; the reason there are two separate streams, standard output,
and standard error, is so that they can be redirected separately.

   In other implementations of `awk', the only way to write an error
message to standard error in an `awk' program is as follows:

     print "Serious error detected!" | "cat 1>&2"

This works by opening a pipeline to a shell command that can access the
standard error stream that it inherits from the `awk' process.  This is
far from elegant, and it is also inefficient, because it requires a
separate process.  So people writing `awk' programs often don't do
this.  Instead, they send the error messages to the terminal, like this:

     print "Serious error detected!" > "/dev/tty"

This usually has the same effect but not always: although the standard
error stream is usually the terminal, it can be redirected; when that
happens, writing to the terminal is not correct.  In fact, if `awk' is
run from a background job, it may not have a terminal at all.  Then
opening `/dev/tty' fails.

   `gawk' provides special file names for accessing the three standard
streams, as well as any other inherited open files.  If the file name
matches one of these special names when `gawk' redirects input or
output, then it directly uses the stream that the file name stands for.
(These special file names work for all operating systems that `gawk'
has been ported to, not just those that are POSIX-compliant.):

     The standard input (file descriptor 0).

     The standard output (file descriptor 1).

     The standard error output (file descriptor 2).

     The file associated with file descriptor N.  Such a file must be
     opened by the program initiating the `awk' execution (typically
     the shell).  Unless special pains are taken in the shell from which
     `gawk' is invoked, only descriptors 0, 1, and 2 are available.

   The file names `/dev/stdin', `/dev/stdout', and `/dev/stderr' are
aliases for `/dev/fd/0', `/dev/fd/1', and `/dev/fd/2', respectively.
However, they are more self-explanatory.  The proper way to write an
error message in a `gawk' program is to use `/dev/stderr', like this:

     print "Serious error detected!" > "/dev/stderr"

   Note the use of quotes around the file name.  Like any other
redirection, the value must be a string.  It is a common error to omit
the quotes, which leads to confusing results.

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