Newer systems provide a directory named /dev/fd whose entries are files named 0,1, 2, and so on. Opening the file /dev/fd/n is equivalent to duplicating descriptor n (assuming that descriptor n is open).
The /dev/fd feature was developed by Tom Duff and appeared in the 8th Edition of the Research Unix System. It is supported by SVR4 and 4.3+BSD. It is not part of POSIX.1.
In the function call
fd = open("/dev/fd/0", mode);
most systems ignore the specified mode, while others require that it be a subset of the mode used when the referenced file (standard input in this case) was originally opened. Since the open above is equivalent to
fd = dup(0);
the descriptors 0 and fd share the same file table entry (Figure 3.4). For example, if descriptor 0 was opened read-only, we can only read on fd. Even if the system ignores the open mode, and the call
fd = open("/dev/fd/0", O_RDWR);
succeeds, we still can't write to fd.
We can also call creat with a /dev/fd pathname argument, as well as specifying O_CREAT in a call to open. This allows a program that calls creat to still work if the pathname argument is /dev/fd/1, for example.
Some systems provide the pathnames /dev/stdin, /dev/stdout, and /dev/stderr. These are equivalent to /dev/fd/0, /dev/fd/1, and /dev/fd/2.
The main use of the /dev/fd files is from the shell. It allows programs that use pathname arguments to handle standard input and standard output in the same manner as other pathnames. For example, the cat(1) program specifically looks for an input filename of - and uses this to mean standard input. The command
filter file2 | cat file1 - file3 | lpr
is an example. First cat reads file1, next its standard input (the output of the filter program on file2), then file3. If /dev/fd is supported, the special handling of - can be removed from cat, and we can enter
filter file2 | cat file1 /dev/fd/0 file3 | lpr
The special meaning of – as a command-line argument to refer to the standard input or standard output is a kludge that has crept into many programs. There are also problems if we specify - as the first file, since it looks like the start of another command-line option. /dev/fd is a step toward uniformity and cleanliness.