Ubuntu Manpage: /proc/pid/fd/

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.

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