Much of the material in this document is taken from Appendix H.3 in the book A Primer on Scientific Programming with Python, 4th edition, by the same author, published by Springer, 2014.

Make a folder

Python applies the term directory instead of folder. The equivalent of the Unix mkdir mydir is

import os
os.mkdir('mydir')

Ordinary files are created by the open and close functions in Python.

Make intermediate folders

Suppose you want to make a subfolder under your home folder:

$HOME/python/project1/temp

but the intermediate folders python and project1 do not exist. This requires each new folder to be made separately by os.mkdir, or you can make all folders at once with os.makedirs:

foldername = os.path.join(os.environ['HOME'], 'python',
                          'project1', 'temp')
os.makedirs(foldername)

With os.environ[var] we can get the value of any environment variable var as a string. The os.path.join function joins folder names and a filename in a platform-independent way.

Move to a folder

The cd command reads os.chdir and cwd is os.getcwd:

origfolder = os.getcwd()  # get name of current folder
os.chdir(foldername)      # move ("change directory")
...
os.chdir(origfolder)      # move back

Rename a file or folder

The cross-platform mv command is

os.rename(oldname, newname)

List files

Unix wildcard notation can be used to list files. The equivalent of ls *.py and ls plot*[1-4]*.dat reads

import glob
filelist1 = glob.glob('*.py')
filelist2 = glob.glob('plot*[1-4]*.dat')

List all files and folders in a folder

The counterparts to ls -a mydir and just ls -a are

filelist1 = os.listdir('mydir')
filelist1 = os.listdir(os.curdir)  # current folder (directory)
filelist1.sort()                   # sort alphabetically

Check if a file or folder exists

The widely used constructions in Unix scripts for testing if a file or folder exist are if [ -f $filename ]; then and if [ -d $dirname ]; then. These have very readable counterparts in Python:

if os.path.isfile(filename):
    inputfile = open(filename, 'r')
    ...

if os.path.isdir(dirnamename):
    filelist = os.listdir(dirname)
    ...

Remove files

Removing a single file is done with os.rename, and a loop is required for doing rm tmp_*.df:

import glob
filelist = glob.glob('tmp_*.pdf')
for filename in filelist:
    os.remove(filename)

Remove a folder and all its subfolders

The rm -rf mytree command removes an entire folder tree. In Python, the cross-platform valid command becomes

import shutil
shutil.rmtree(foldername)

It goes without saying that this command must be used with great care!

Copy a file to another file or folder

The cp fromfile tofile construction applies shutil.copy in Python:

shutil.copy('fromfile', 'tofile')

Copy a folder and all its subfolders

The recursive copy command cp -r for folder trees is in Python expressed by shell.copytree:

shutil.copytree(sourcefolder, destination)

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