Special Caveats - The GNU Awk User's Guide

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4.7.4 Special File Name Caveats

Here is a list of things to bear in mind when using the special file names that gawk provides:

Recognition of these special file names is disabled if gawk is in compatibility mode (see Options).
The As mentioned earlier, the special files that provide process-related information are now considered obsolete and will disappear entirely in the next release of gawk. gawk prints a warning message every time you use one of these files. To obtain process-related information, use the PROCINFO array. See Built-in Variables.
Starting with version 3.1, gawk always interprets these special file names.¹ For example, using `/dev/fd/4' for output actually writes on file descriptor 4, and not on a new file descriptor that is 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. Doing so results in unpredictable behavior.

Footnotes

[1] Older versions of gawk would interpret these names internally only if the system did not actually have a /dev/fd directory or any of the other special files listed earlier. Usually this didn't make a difference, but sometimes it did; thus, it was decided to make gawk's behavior consistent on all systems and to have it always interpret the special file names itself.