Python scripting in Linux

9. Python scripting in Linux#

Python libraries for interaction with Linux OS
Multiple account creation example
Python scripts in command line

9.1. Interaction with operating system#

There are various python standard libraries utilized for interaction with the Linux environment, file systems, and processes.

We review only a few most commonly used:

sys — System-specific parameters and functions
os — Miscellaneous operating system interfaces
shutil - File operations
subprocess — Process management
paramiko - Remote commands via SSH and file transfer via SFTP
re - Regular expressions

9.2. Run Jupyter notebook in server mode (Exercise)#

Check if jupyter is already running:

ps -ef | grep jupyter

If it shows up running without using port 8888, terminate it:

killall -9 jupyter-notebook

Start jupyter notebook in the server mode with listening on port tcp/8888:

cd Python
jupyter-notebook --no-browser --port=8888 --ip=* &

Find out the IP address and the port number on the Ravada server for your jupyter notebook by login to the vdi portal and starting the desktop.

Navigate the browser on your laptop or desktop to the IP address and port, for example

http://172.16.26.112:60000

Login with the password you assigned to the jupyter notebook in the previous exercises.

Start a new python3 project in the jupyter notebook.

This is a Jupyter Notebook.

This computational tool allows for Python programming in a web browser without having to use a multi-window environment. You can write and run your code all in one sitting.

9.3. sys — System-specific parameters and functions - path, argv, … (Exercise)#

Identify the OS platform within Python:

import sys
sys.platform

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Example how it can be used:

if sys.platform.startswith('linux'):
    # Linux-specific code here...
    print('we are working in Linux environment')
else:
    print('different OS')

Get the current PYTHONPATH:

print(sys.path)

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To see what Python we are using:

sys.executable

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Second input parameter for the python script:

sys.argv[2]

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Check what methods are in sys:

dir(sys)

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9.4. os — Miscellaneous operating system interfaces - chdir, rmdir, system, … (Exercise)#

In your Jupyter notebook, copy and paste the python code from the cell below:

#Current working directory
import os
os.getcwd()

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Try the following in the new cell:

#Change the directory
os.chdir("/etc")
os.getcwd()

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Try listing the files and directories in /etc:

# see the list of files and directories
os.listdir()

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Check if file hosts exists:

os.path.isfile("hosts")

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os.path.isfile('no_hosts')

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Get the file system statistics on a file

os.stat('/etc/hosts')

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Get the size of a file in bytes:

os.path.getsize("/etc/hosts")

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Get the directory path separator:

os.sep

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Generate the file names in a directory tree by walking the tree either top-down or bottom-up.

top='/etc'
for dirpath, dirnames, filenames in os.walk(top):
    path = dirpath.split(os.sep)
    print(path)

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Derive the directory path from a full path:

full_path="/etc/libvirt/qemu/kvm1.xml"
os.path.dirname(full_path)

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Create, rename, and remove directories:

os.chdir('/home/hostadm')
os.mkdir('DIR1')
os.rename('DIR1','DIR2')
os.rmdir('DIR2')

Execute a system command without controlling the stdout and stderr:

os.system('cd /etc; pwd')

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The output number is the last command error code.

Check what methods are included in os

dir(os)

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9.5. shutil - file operations - copy, chown, … (Exercise)#

import shutil

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Copy file /etc/hosts into list2.txt:

shutil.copy('/etc/hosts','list2.txt')

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See what files we have in the current working directory:

os.listdir()

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Copy directory tree /etc/network into directory NEW_DIR1

shutil.copytree('/etc/network','NEW_DIR1')

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9.6. Calling Linux commands from Python (Exercise)#

The subprocess module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. Example of class call

# Create directory DIR1 and check the disk usage in the directory
import subprocess
s=subprocess.run("mkdir DIR1; du -sh DIR1", shell=True)

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If you need to get the output of the system command, use method getoutput”

  user = "user1"
  stdoutdata = subprocess.getoutput(f'id {user}')
  if 'no such user' in stdoutdata:
     print(stdoutdata)

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For full control of the standard input and the standart output to the command, we use class Popen:

import subprocess

s=subprocess.Popen("du -sh /etc", shell=True, 
                        stdout=subprocess.PIPE,
                        stderr=subprocess.PIPE,)
    
stdout_value,stderr_value = s.communicate()

print("std output=", stdout_value) 
print("std error=", stderr_value) 

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9.7. Scripts runnable from command line (Exercise)#

Choose what version of Python you want the script to use:

either the system python, /usr/bin/python3,

or Anaconda, /home/hostadm/miniconda3/bin/python3,

or select the one that is the default in the user environment:

#!/usr/bin/env python3

Say, we want to use Anaconda python. Create, for example, file call.py with the first line containing:

#!/home/hostadm/miniconda3/bin/python3

The snippets from the jupyter cells can be placed in file call.py, for example:

#!/home/hostadm/miniconda3/bin/python3

# see the list of files and directories
import os
listf = os.listdir()
print(listf)

Make the file executable:

chmod a+x call.py

Run the script:

./call.py

9.8. Creating multiple user accounts with python (Exercise)#

In directory Python, create a new file, users.txt, with editor nano:

users.txt:

mike 2000
jerry 2001
sam 2003
mary 2004

In jupyter notebook, run the following script:

import os

def user_cmd(user, uid, group, hdir):
    s = f'useradd -m -d {hdir}/{user} -s /bin/bash -u {uid} -g {group} {user}'
    return s
    
os.chdir('/home/hostadm/Python')
HDIR = '/NFS/home'

f = open('users.txt','r')

account = {}
for line in f:
    key, value = line.rstrip().split()
    account[key] = value

f.close() 

for keys in account:
    grp_cmd = f'groupadd -g {account[keys]} {keys}'
    print(grp_cmd)
    comm = user_cmd(keys, account[keys], account[keys], HDIR)
    print(comm)

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Function print() above is a placeholder for function os.system() that would create accounts.

9.9. Transferring files via SFTP with paramiko (Exercise)#

On your desktop install paramiko from conda-forge python module collection:

conda install -c conda-forge paramiko

Start run01 VM and find out its IP address. If you no longer have run01, pickup any recent VM, for example, master:

virsh start run01
virsh domifaddr run01

For example, IP is 192.168.122.134

In Jupyter notebook, import module paramiko:

import paramiko

Initialize SSH client constructor ssh:

ssh = paramiko.SSHClient()

Automatically trust unknown hosts (not recommended for production). Otherwise, you can SSH to the client from the command line.

ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())

Establish SSH connection to the client:

ssh.connect(hostname='192.168.122.134', username='...', password='...')

Initialize sftp constructor:

sftp = ssh.open_sftp()

Transfer file users.txt from local directory Python into the user home directory on the SSH server.

sftp.put('users.txt', 'users.txt' )

Verify that the file is there via running ls command via SSH to the server:

stdin, stdout, stderr = ssh.exec_command('ls')
print(stdout.read().decode())

You can also fetch a file via SFTP, for example:

sftp.get('vm_id_reset.sh','/tmp/vm_id_reset.sh')

Verify that file ‘vm_id_reset.sh’ is already located in /tmp directory:

os.path.isfile('/tmp/vm_id_reset.sh')

Close connections:

sftp.close()
ssh.close()

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9.10. Regular expressions (RE) for pattern search/replacement (Exercise)#

Files can be read line by line into python lists. The lists can be searched for patterns.

Here we use re library with its methods.

Copy file list.txt from the shell scripting exercises into directory Python:

cd
cp bash_scripts/list.txt Python

Run the python script below to extract the lines containing “Reboot”, “reboot” or “support” strings (aka grep -E '[Rr]eboot|support' list.txt command):

import re

f=open('list.txt','r')
    
for line in f:
    match = re.search(r'[Rr]eboot|support', line)
    if match:
        print(line)
            
f.close()

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The combination of two commands for opening and closing a file, f=open(...) and f.close() can be replaced with one line: with open('...','r') as f:. To eliminate extra blank lines in the printed output, one can add method rstrip('\n') to variable line. Below is a concise form of the above script:

import re

with open('list.txt','r') as f:
  for line in f:
    match = re.search(r'[Rr]eboot|support', line)
    if match:
        print(line.rstrip('\n'))

Substitute underscore for any number of spaces wherever they occur on the line (aka sed 's/ */_/g' list.txt):

import re

f=open('list.txt','r')
    
for line in f:
    newline = re.sub(r' *', r'_', line)
    print(newline)
        
f.close()

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A concise form of the script:

import re

with open('list.txt','r') as f:    
  for line in f:
    newline = re.sub(r' *', r'_', line)
    print(newline.rstrip('\n'))

9.11. Splitting lines into fileds (Exercise)#

Display user names, home directories, and login shell (fields 1, 6 and 7) in /etc/passwd file.

Aka awk -F: '{ print $1, $6, $7 }' /etc/passwd command:

f=open('/etc/passwd','r')

for line in f:
    f1,f2,f3,f4,f5,f6,f7 = line.split(':')
    print(f1, f6, f7)
    
f.close()

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A concise form of the script:

with open('/etc/passwd','r') as f:
  for line in f:
    f1,f2,f3,f4,f5,f6,f7 = line.rstrip('\n').split(':')
    print(f1, f6, f7)

9.12. References#

Python standard library

Python scripting in Linux

Contents

9. Python scripting in Linux#

9.1. Interaction with operating system#

9.2. Run Jupyter notebook in server mode (Exercise)#

9.3. sys — System-specific parameters and functions - path, argv, … (Exercise)#

9.4. os — Miscellaneous operating system interfaces - chdir, rmdir, system, … (Exercise)#

9.5. shutil - file operations - copy, chown, … (Exercise)#

9.6. Calling Linux commands from Python (Exercise)#

9.7. Scripts runnable from command line (Exercise)#

9.8. Creating multiple user accounts with python (Exercise)#

9.9. Transferring files via SFTP with paramiko (Exercise)#

9.10. Regular expressions (RE) for pattern search/replacement (Exercise)#

9.11. Splitting lines into fileds (Exercise)#

9.12. References#