TELNET AND SSH in Font

Generate QR Code 2d barcode in Font TELNET AND SSH

CHAPTER 16 TELNET AND SSH
Painting Denso QR Bar Code In None
Using Barcode drawer for Font Control to generate, create QR Code 2d barcode image in Font applications.
www.OnBarcode.com
DataMatrix Creator In None
Using Barcode drawer for Font Control to generate, create Data Matrix 2d barcode image in Font applications.
www.OnBarcode.com
Listing 16 2. Shell Supporting Whitespace-Separated Arguments #!/usr/bin/env python # Foundations of Python Network Programming - 16 - shell.py # A simple shell, so you can try running commands in the absence of # any special characters (except for whitespace, used for splitting). import subprocess while True: args = raw_input('] ').split() if not args: pass elif args == ['exit']: break elif args[0] == 'show': print "Arguments:", args[1:] else: subprocess.call(args) Of course, this means that you cannot use this shell to talk about files with spaces in their names, since without at least one other special character an escape or quoting character you cannot make the spaces mean anything but the argument separator! But you can quickly bring up this shell to try out all sorts of special characters of which you have always been afraid, and see that they mean absolutely nothing if passed directly to the common commands you use (the shell in Listing 16 2 uses a ] prompt, to make it easy to tell apart from your own shell): $ python shell.py ] echo Hi there! Hi there! ] echo An asterisk * is not special. An asterisk * is not special. ] echo The string $HOST is not special, nor are "double quotes". The string $HOST is not special, nor are "double quotes". ] echo What No *<>!$ special characters What No *<>!$ special characters ] show "The 'show' built-in lists its arguments." Arguments: ['"The', "'show'", 'built-in', 'lists', 'its', 'arguments."'] ] exit You can see here absolute evidence that Unix commands in this case, the /bin/echo command that we are calling over and over again do not generally attempt to interpret their arguments as anything other than strings. The echo command happily accepts double-quotes, dollar signs, and asterisks, and treats them all as literal characters. As the foregoing show command illustrates, Python is simply reducing our arguments to a list of strings for the operating system to use in creating a new process. What if we fail to split our command into separate arguments >>> import subprocess >>> subprocess.call(['echo hello']) Traceback (most recent call last): ... OSError: [Errno 2] No such file or directory
Barcode Maker In None
Using Barcode generator for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
UPC-A Supplement 2 Drawer In None
Using Barcode printer for Font Control to generate, create UPC A image in Font applications.
www.OnBarcode.com
CHAPTER 16 TELNET AND SSH
Encode Code 3 Of 9 In None
Using Barcode creator for Font Control to generate, create Code 3/9 image in Font applications.
www.OnBarcode.com
QR Printer In None
Using Barcode drawer for Font Control to generate, create Quick Response Code image in Font applications.
www.OnBarcode.com
Do you see what has happened The operating system does not know that spaces should be special; that is a quirk of shell programs, not of Unix-like operating systems themselves! So the system thinks that it is being asked to run a command literally named echo [space] hello, and, unless you have created such a file in the current directory, it fails to find it and raises an exception. Oh I said at the beginning of this whole section that its whole premise was a lie, and you probably want to know what character is, in fact, special to the system! It turns out that it is the null character the character having the Unicode and ASCII code zero. This character is used in Unix-like systems to mark the end of each command-line argument in memory. So if you try using a null character in an argument, Unix will think the argument has ended and will ignore the rest of its text. To prevent you from making this mistake, Python stops you in your tracks if you include a null character in a commandline argument: >>> import subprocess >>> subprocess.call(['echo', 'Sentences can end\0 abruptly.']) Traceback (most recent call last): ... TypeError: execv() arg 2 must contain only strings Happily, since every command on the system is designed to live within this limitation, you will generally find there is never any reason to put null characters into command-line arguments anyway! (Specifically, they cannot appear in file names for exactly the same reason as they cannot appear in arguments: file names are null-terminated in the operating system implementation.)
Paint UCC - 12 In None
Using Barcode encoder for Font Control to generate, create GS1 128 image in Font applications.
www.OnBarcode.com
Encoding USPS Confirm Service Barcode In None
Using Barcode drawer for Font Control to generate, create USPS Confirm Service Barcode image in Font applications.
www.OnBarcode.com
Creating QR In None
Using Barcode drawer for Online Control to generate, create QR Code JIS X 0510 image in Online applications.
www.OnBarcode.com
Creating QR Code 2d Barcode In None
Using Barcode generation for Software Control to generate, create QR Code image in Software applications.
www.OnBarcode.com
Universal Product Code Version A Printer In None
Using Barcode printer for Office Word Control to generate, create Universal Product Code version A image in Microsoft Word applications.
www.OnBarcode.com
Code 128 Code Set A Creator In Java
Using Barcode generator for BIRT reports Control to generate, create USS Code 128 image in BIRT applications.
www.OnBarcode.com
Data Matrix Scanner In C#
Using Barcode recognizer for .NET framework Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
Barcode Encoder In Visual Studio .NET
Using Barcode encoder for Reporting Service Control to generate, create Barcode image in Reporting Service applications.
www.OnBarcode.com
QR Code 2d Barcode Generator In Java
Using Barcode creator for Eclipse BIRT Control to generate, create QR image in BIRT applications.
www.OnBarcode.com
Data Matrix Recognizer In VB.NET
Using Barcode decoder for .NET Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
Make QR Code JIS X 0510 In .NET
Using Barcode printer for Reporting Service Control to generate, create QR Code image in Reporting Service applications.
www.OnBarcode.com
UPC-A Supplement 5 Encoder In .NET Framework
Using Barcode maker for .NET Control to generate, create UPC-A image in VS .NET applications.
www.OnBarcode.com
Barcode Printer In C#
Using Barcode maker for VS .NET Control to generate, create Barcode image in VS .NET applications.
www.OnBarcode.com
Draw QR Code 2d Barcode In Java
Using Barcode drawer for Android Control to generate, create QR image in Android applications.
www.OnBarcode.com
Copyright © OnBarcode.com . All rights reserved.