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5.7 Special Files for Standard Preopened Data Streams
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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 open streams (and any other open files or pipes) are often
referred to by the technical term "file descriptors".

   These streams are, by default, connected to your keyboard and screen,
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 traditional 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 also requires a separate process.  So people
writing 'awk' programs often don't do this.  Instead, they send the
error messages to the screen, like this:

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

('/dev/tty' is a special file supplied by the operating system that is
connected to your keyboard and screen.  It represents the "terminal,"(1)
which on modern systems is a keyboard and screen, not a serial console.)
This generally has the same effect, but not always: although the
standard error stream is usually the screen, it can be redirected; when
that happens, writing to the screen 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', BWK 'awk', and 'mawk' provide special file names for
accessing the three standard streams.  If the file name matches one of
these special names when 'gawk' (or one of the others) redirects input
or output, then it directly uses the descriptor 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:

'/dev/stdin'
     The standard input (file descriptor 0).

'/dev/stdout'
     The standard output (file descriptor 1).

'/dev/stderr'
     The standard error output (file descriptor 2).

   With these facilities, the proper way to write an error message then
becomes:

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

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

   'gawk' does not treat these file names as special when in
POSIX-compatibility mode.  However, because BWK 'awk' supports them,
'gawk' does support them even when invoked with the '--traditional'
option (*note Options::).

   ---------- Footnotes ----------

   (1) The "tty" in '/dev/tty' stands for "Teletype," a serial terminal.