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
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,”27 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.
special file names for accessing the three standard streams.
If the file name matches one of these special names when
(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
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 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
gawk does support them even when
invoked with the --traditional option (see Command-Line Options).
The “tty” in /dev/tty stands for “Teletype,” a serial terminal.