Python Cookbook, 3rd Edition

Chapter 13. Utility Scripting and System Administration

A lot of people use Python as a replacement for shell scripts, using it to automate common system tasks, such as manipulating files, configuring systems, and so forth. The main goal of this chapter is to describe features related to common tasks encountered when writing scripts. For example, parsing command-line options, manipulating files on the filesystem, getting useful system configuration data, and so forth. Chapter 5 also contains general information related to files and directories.

13.1. Accepting Script Input via Redirection, Pipes, or Input Files

Problem

You want a script you’ve written to be able to accept input using whatever mechanism is easiest for the user. This should include piping output from a command to the script, redirecting a file into the script, or just passing a filename, or list of filenames, to the script on the command line.

Solution

Python’s built-in fileinput module makes this very simple and concise. If you have a script that looks like this:

#!/usr/bin/env python3
import fileinput

with fileinput.input() as f_input:
    for line in f_input:
        print(line, end='')

Then you can already accept input to the script in all of the previously mentioned ways. If you save this script as filein.py and make it executable, you can do all of the following and get the expected output:

$ ls | ./filein.py          # Prints a directory listing to stdout.
$ ./filein.py /etc/passwd   # Reads /etc/passwd to stdout.
$ ./filein.py < /etc/passwd ...