proc_pid_fd(5)                File Formats Manual               proc_pid_fd(5)

NAME
       /proc/pid/fd/ - file descriptors

DESCRIPTION
       /proc/pid/fd/
              This is a subdirectory containing one entry for each file which
              the process has open, named by its file descriptor, and which is
              a symbolic link to the actual file.  Thus, 0 is standard input,
              1 standard output, 2 standard error, and so on.

              For file descriptors for pipes and sockets, the entries will be
              symbolic links whose content is the file type with the inode.  A
              readlink(2) call on this file returns a string in the format:

                  type:[inode]

              For example, socket:[2248868] will be a socket and its inode is
              2248868.  For sockets, that inode can be used to find more
              information in one of the files under /proc/net/.

              For file descriptors that have no corresponding inode (e.g.,
              file descriptors produced by bpf(2), epoll_create(2),
              eventfd(2), inotify_init(2), perf_event_open(2), signalfd(2),
              timerfd_create(2), and userfaultfd(2)), the entry will be a
              symbolic link with contents of the form

                  anon_inode:file-type

              In many cases (but not all), the file-type is surrounded by
              square brackets.

              For example, an epoll file descriptor will have a symbolic link
              whose content is the string anon_inode:[eventpoll].

              In a multithreaded process, the contents of this directory are
              not available if the main thread has already terminated
              (typically by calling pthread_exit(3)).

              Programs that take a filename as a command-line argument, but
              don't take input from standard input if no argument is supplied,
              and programs that write to a file named as a command-line
              argument, but don't send their output to standard output if no
              argument is supplied, can nevertheless be made to use standard
              input or standard output by using /proc/pid/fd files as command-
              line arguments.  For example, assuming that -i is the flag
              designating an input file and -o is the flag designating an
              output file:

                  $ foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...

              and you have a working filter.

              /proc/self/fd/N is approximately the same as /dev/fd/N in some
              UNIX and UNIX-like systems.  Most Linux MAKEDEV scripts
              symbolically link /dev/fd to /proc/self/fd, in fact.

              Most systems provide symbolic links /dev/stdin, /dev/stdout, and
              /dev/stderr, which respectively link to the files 0, 1, and 2 in
              /proc/self/fd.  Thus the example command above could be written
              as:

                  $ foobar -i /dev/stdin -o /dev/stdout ...

              Permission to dereference or read (readlink(2)) the symbolic
              links in this directory is governed by a ptrace access mode
              PTRACE_MODE_READ_FSCREDS check; see ptrace(2).

              Note that for file descriptors referring to inodes (pipes and
              sockets, see above), those inodes still have permission bits and
              ownership information distinct from those of the /proc/pid/fd
              entry, and that the owner may differ from the user and group IDs
              of the process.  An unprivileged process may lack permissions to
              open them, as in this example:

                  $ echo test | sudo -u nobody cat
                  test
                  $ echo test | sudo -u nobody cat /proc/self/fd/0
                  cat: /proc/self/fd/0: Permission denied

              File descriptor 0 refers to the pipe created by the shell and
              owned by that shell's user, which is not nobody, so cat does not
              have permission to create a new file descriptor to read from
              that inode, even though it can still read from its existing file
              descriptor 0.

SEE ALSO
       proc(5)

Linux man-pages 6.9.1             2024-05-02                    proc_pid_fd(5)