manpagez: man pages & more
info gawk
Home | html | info | man

gawk: Special FD

 
 5.7 Special Files for Standard Preopened Data Streams
 =====================================================
 
 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 (⇒Options).
 
    ---------- Footnotes ----------
 
    (1) The "tty" in '/dev/tty' stands for "Teletype," a serial terminal.
 
© manpagez.com 2000-2018
Individual documents may contain additional copyright information.