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Python | os.dup() method
  • Last Updated : 18 Jun, 2019

OS module in Python provides functions for interacting with the operating system. OS comes under Python’s standard utility modules. This module provides a portable way of using operating system dependent functionality.

A file descriptor is small integer value that corresponds to a file or other input/output resource, such as a pipe or network socket. A File descriptor is an abstract indicator of a resource and act as handle to perform various lower level I/O operations like read, write, send etc.

For Example: Standard input is usually file descriptor with value 0, standard output is usually file descriptor with value 1 and standard error is usually file descriptor with value 2.
Further files opened by the current process will get the value 3, 4, 5 an so on.

os.dup() method in Python is used to duplicate the given file descriptor. The duplicated file descriptor is non-inheritable, But on Windows platform, file descriptor associated with standard stream (standard input : 0, standard output : 1, standard error : 2) which can be inherited by child processes.

Inheritable file descriptor means if the parent process has a file descriptor 4 in use for a particular file and parent creates a child process then the child process will also have file descriptor 4 in use for that same file.

Syntax: os.dup(fd)

fd: A file descriptor, which is to be duplicated.

Return Type: This method returns the duplicated file descriptor, which is an integer value.

Code: Use of os.dup() method to duplicate a file descriptor

# Python program to explain os.dup() method 
# importing os module 
import os
# File path 
path = "/home/ihritik/Desktop/file.txt"
# open the file and get
# the file descriptor associated
# with it using method
fd =, os.O_WRONLY)
# Print the value of
# file descriptor
print("Original file descriptor:", fd)
# Duplicate the file descriptor
dup_fd = os.dup(fd)
# The duplicated file will have 
# different value but it 
# will correspond to the same 
# file to which original file
# descriptor was refering 
# Print the value of
# duplicated file descriptor
print("Duplicated file descriptor:", dup_fd)
# Get the list of all 
# file Descriptors Used
# by the current Process
# (below code works on UNIX systems)
pid = os.getpid()
os.system("ls -l/proc/%s/fd" %pid)
# Close file descriptors
print("File descriptor duplicated successfully")
Original file descriptor: 3
Duplicated file descriptor: 4
total 0
lrwx------ 1 ihritik ihritik 64 Jun 14 06:45 0 -> /dev/pts/0
lrwx------ 1 ihritik ihritik 64 Jun 14 06:45 1 -> /dev/pts/0
lrwx------ 1 ihritik ihritik 64 Jun 14 06:45 2 -> /dev/pts/0
l-wx------ 1 ihritik ihritik 64 Jun 14 06:45 3 -> /home/ihritik/Desktop/file.txt
l-wx------ 1 ihritik ihritik 64 Jun 14 06:45 4 -> /home/ihritik/Desktop/file.txt
File descriptor duplicated successfully

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