Special File Names in gawk

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 terminal, but they are often redirected with the shell, via the `<', `<<', `>', `>>', `>&' and `|' operators. Standard error is typically used for writing error messages; the reason we have 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 which can access the standard error stream which it inherits from the awk process. This is far from elegant, and is also inefficient, since it requires a separate process. So people writing awk programs often neglect to do this. Instead, they send the error messages to the terminal, like this:

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

This usually has the same effect, but not always: although the standard error stream is usually the terminal, it can be redirected, and when that happens, writing to the terminal 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' will fail.

gawk provides special file names for accessing the three standard streams. When you redirect input or output in gawk, if the file name matches one of these special names, then gawk directly uses the stream it stands for.

`/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).

`/dev/fd/N'

The file associated with file descriptor N. Such a file must have been opened by the program initiating the awk execution (typically the shell). Unless you take special pains in the shell from which you invoke gawk, 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, but they are more self-explanatory.

The proper way to write an error message in a gawk program is to use `/dev/stderr', like this:

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

gawk also provides special file names that give access to information about the running gawk process. Each of these "files" provides a single record of information. To read them more than once, you must first close them with the close function (see section Closing Input and Output Files and Pipes). The filenames are:

`/dev/pid'

Reading this file returns the process ID of the current process, in decimal, terminated with a newline.

`/dev/ppid'

Reading this file returns the parent process ID of the current process, in decimal, terminated with a newline.

`/dev/pgrpid'

Reading this file returns the process group ID of the current process, in decimal, terminated with a newline.

`/dev/user'

Reading this file returns a single record terminated with a newline. The fields are separated with spaces. The fields represent the following information:

$1

The return value of the getuid system call (the real user ID number).

$2

The return value of the geteuid system call (the effective user ID number).

$3

The return value of the getgid system call (the real group ID number).

$4

The return value of the getegid system call (the effective group ID number).

If there are any additional fields, they are the group IDs returned by getgroups system call. (Multiple groups may not be supported on all systems.)

These special file names may be used on the command line as data files, as well as for I/O redirections within an awk program. They may not be used as source files with the `-f' option.

Recognition of these special file names is disabled if gawk is in compatibility mode (see section Command Line Options).

Caution: Unless your system actually has a `/dev/fd' directory (or any of the other above listed special files), the interpretation of these file names is done by gawk itself. For example, using `/dev/fd/4' for output will actually write on file descriptor 4, and not on a new file descriptor that was dup'ed from file descriptor 4. Most of the time this does not matter; however, it is important to not close any of the files related to file descriptors 0, 1, and 2. If you do close one of these files, unpredictable behavior will result.

The special files that provide process-related information may disappear in a future version of gawk. See section Probable Future Extensions.

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