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,”29 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:
The standard input (file descriptor 0).
The standard output (file descriptor 1).
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 double quotes around the file name. Like with any other redirection, the value must be a string. It is a common error to omit the double 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 (see Command-Line Options).