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 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 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
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
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,”28 which on modern systems is a keyboard
and screen, not a serial console.)
This usually 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 provides special file names for accessing the three standard
streams. (c.e.). It also provides syntax for accessing
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
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
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.
Finally, using the
close() function on a file name of the
"/dev/fd/N", for file descriptor numbers
above two, does actually close the given file descriptor.
The /dev/stdin, /dev/stdout, and /dev/stderr
special files are also recognized internally by several other
The “tty” in /dev/tty stands for “Teletype,” a serial terminal.