Franske ITC-2480 Lab 14 - Revision history

BenFranske: Update to mjnk version

2021-12-03T20:07:10Z

Update to mjnk version

BenFranske: /* Using Loops and Reading User Input */

2018-04-19T05:50:46Z

Using Loops and Reading User Input

← Older revision		Revision as of 05:50, 19 April 2018
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	There are a couple of differences to note when comparing this with the similar BASH script we created. First, instead of using a separate program, like the BASH '''read''' program, we are obtaining user input with the ''raw_input'' function. This program is then storing the user's input into a certain variable. By default the ''raw_input'' function saves information as a STRING which is a type of variable which can hold letters and numbers but can't be used for numeric operations like adding, subtracting, looping, etc. Because of this we use the ''int()'' and ''str()'' funtions to switch the data back and forth from the string type to the integer numeric type as needed in various parts of the script. Second, the for statement is using the built-in ''range()'' function instead of the '''seq''' command as we did in BASH to generate the number of loops needed.</li>		There are a couple of differences to note when comparing this with the similar BASH script we created. First, instead of using a separate program, like the BASH '''read''' program, we are obtaining user input with the ''raw_input'' function. This program is then storing the user's input into a certain variable. By default the ''raw_input'' function saves information as a STRING which is a type of variable which can hold letters and numbers but can't be used for numeric operations like adding, subtracting, looping, etc. Because of this we use the ''int()'' and ''str()'' funtions to switch the data back and forth from the string type to the integer numeric type as needed in various parts of the script. Second, the for statement is using the built-in ''range()'' function instead of the '''seq''' command as we did in BASH to generate the number of loops needed.</li>
	<li> Try running the script and see if you were correct about what it does and how it works.</li>		<li> Try running the script and see if you were correct about what it does and how it works.</li>
	<li> Also, it is important to note that while this script works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.</li>		<li> The script as provided does not actually produce the correct number of files requested by the user. See if you can fix the script to produce the correct number of files.</li>
			<li> Also, it is important to note that while this script mostly works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.</li>
	<li> See if you can modify the script so you can specify a starting and an ending number for the files rather than just a number of files to be created.</li>		<li> See if you can modify the script so you can specify a starting and an ending number for the files rather than just a number of files to be created.</li>
	<li> See if you can add a check to the script which will display an example of a filename that will be created and asking the user if that looks correct before actually creating all the files. If the user enters N then do not create the files. Hint, you will need to do a little research on using python IF statements to check user input.</li>		<li> See if you can add a check to the script which will display an example of a filename that will be created and asking the user if that looks correct before actually creating all the files. If the user enters N then do not create the files. Hint, you will need to do a little research on using python IF statements to check user input.</li>

BenFranske at 17:58, 12 April 2018

2018-04-12T17:58:39Z

Show changes

BenFranske: /* Using Variables and Conditionals in Python Scripts */

2014-04-24T02:04:04Z

Using Variables and Conditionals in Python Scripts

← Older revision		Revision as of 02:04, 24 April 2014
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	print "Hello " + sys.argv[1] + " from Python!"		print "Hello " + sys.argv[1] + " from Python!"
	</pre></li>		</pre></li>
	<li> What do you think will happen if you run the script like this "./hello.py yourfirstname"? Try it and find out if you were correct in your hypothesis!<li>		<li> What do you think will happen if you run the script like this "./hello.py yourfirstname"? Try it and find out if you were correct in your hypothesis!</li>
	<li> Wait, what if someone doesn't know they are supposed to include their name as an argument though? Try running "./hello.py" and see what happens. Unlike BASH Python is a little more particular about things and if you try to access a variable that doesn't exist it will give you an error. This isn't very user friendly so let's add a conditional to check and see if an argument was supplied or not. Try updating your hello.py script again and adding a conditional like this:		<li> Wait, what if someone doesn't know they are supposed to include their name as an argument though? Try running "./hello.py" and see what happens. Unlike BASH Python is a little more particular about things and if you try to access a variable that doesn't exist it will give you an error. This isn't very user friendly so let's add a conditional to check and see if an argument was supplied or not. Try updating your hello.py script again and adding a conditional like this:
	<pre>		<pre>

BenFranske: /* Functions and Experimenting */

2014-04-24T01:15:20Z

Functions and Experimenting

← Older revision		Revision as of 01:15, 24 April 2014
Line 102:		Line 102:
	== Functions and Experimenting ==		== Functions and Experimenting ==
	<ol>		<ol>
	<li> ~~One final programming structure~~ we ~~should discuss is the function. In shell scripting you~~ can think of a function like something you could have put into a separate script so that you could call it to do something multiple times from inside of your script but instead of making it a completely separate file you decided to include it in ~~a single script~~. ~~Here is a~~ simple sample:		<li> Just as in BASH we can create functions in python as well. Try entering this simple sample as hello-func.py:
	<pre>		<pre>
	#!/bin/~~bash~~		#!/usr/bin/python
	~~function~~ helloname {		def helloname():
	~~echo~~ "Enter your name, followed by enter:"		name = raw_input("Enter your name, followed by enter: ")
	~~read NAME~~		print "Hello "+name+"!"
	~~echo~~ "Hello "~~$NAME~~"!"		print "This program will welcome you twice to be extra nice!"
	}		helloname()
	~~echo~~ "This program will welcome you twice to be extra nice!"		print "At this point we could be doing lots of other stuff in our script or be in a loop, etc."
	helloname		helloname()
	~~echo~~ "At this point we could be doing lots of other stuff in our script or be in a loop, etc."		print "Thanks for playing!"
	helloname
	~~echo~~ "Thanks for playing!"
	</pre>		</pre>
	See if you can figure out what will happen when you run it, then try it out and see if you were correct. Note that functions need to be declared (listed) before you use them in your script so it would be common to see them at the beginning of the file as in this example. ~~Also~~, it~~'s possible to~~ pass ~~information~~ to a function ~~using arguments just like~~ you ~~would with~~ a ~~separate~~ script.</li>		This should work pretty much the same way that the BASH equivalent script did. See if you can figure out what will happen when you run it, then try it out and see if you were correct. Note that functions need to be declared (listed) before you use them in your script so it would be common to see them at the beginning of the file as in this example. One difference with functions in PYthon is how you pass information in and out of functions. Instead of using arguments you include the information in the function call itself and you can return information from the function which can be used immediately or can be stored into a variable. Try modifying hello-func.py like this:
	<li> Now that you have a grasp on the fundamentals of ~~BASH~~ scripting see if you can write a script which can be used to modify the name of all files in the current directory which share the same extension such as those created by file-generator.sh. The goals for your script should be:		<pre>
			#!/usr/bin/python
			def askname():
			yourname = raw_input("Enter your name, followed by enter: ")
			return yourname
			def helloname(thename):
			print "Hello "+thename+"!"
			print "There are lots of ways to make use of the two functions in this script. We can pass a pre-specified string to a function:"
			helloname("world")
			print "We can ask for your name and use it immediately as input into the hello function:"
			helloname(askname())
			print "We can take a string already stored in a variable and pass the variable to a function:"
			somevariable="Linux Lover"
			helloname(somevariable)
			print "Or we can ask you your name again and store your name in a variable once and then use it a few times:"
			userentry = askname()
			helloname(userentry)
			print "Thanks for playing "+userentry+"!"
			</pre>
			See if you can figure out what this script will do and how it works before running it, then try it out and see if you were correct.
			</li>
			<li> Now that you have a grasp on the fundamentals of python scripting for system administration see if you can write a python script which can be used to modify the name of all files in the current directory which share the same extension such as those created by file-generator.sh. The goals for your script should be:
	* The user has a choice to enter the original and the new extension for the files either using command line arguments or interactively by answering questions your script asks.		* The user has a choice to enter the original and the new extension for the files either using command line arguments or interactively by answering questions your script asks.
	* The script should display each old file name and then the new file name next to it so you can see what is going on		* The script should display each old file name and then the new file name next to it so you can see what is going on
	* You will want to use conditionals to figure out if the user entered arguments to use as the old and new extensions or should be prompted for them interactively.		* You will want to use conditionals to figure out if the user entered arguments to use as the old and new extensions or should be prompted for them interactively.
	* You should use functions and loops in order to make the most efficient script possible. Hints: The actual renaming of the files and displaying old and new names would be a good thing to put in a function because you will be calling it over and over again~~. Also, loops can be used to operate on a list of things such as a list of filenames~~.		* You should use functions and loops in order to make the most efficient script possible. Hints: The actual renaming of the files and displaying old and new names would be a good thing to put in a function because you will be calling it over and over again.
	Don't be afraid to try using search engines for help with this. The goal of this lab is not to make you into a professional programmer but to introduce you enough to shell scripting so that you can create useful scripts with resources such as search engines available to you.		Don't be afraid to try using search engines for help with this. The goal of this lab is not to make you into a professional programmer but to introduce you enough to python shell scripting so that you can create useful scripts with resources such as search engines available to you.
	</li>		</li>
	</ol>		</ol>

BenFranske: /* Using Loops and Reading User Input */

2014-04-24T00:39:50Z

Using Loops and Reading User Input

← Older revision		Revision as of 00:39, 24 April 2014
Line 93:		Line 93:
	print "Created "+str(NUM)+" file(s)."		print "Created "+str(NUM)+" file(s)."
	</pre>		</pre>
	There are a couple of differences to note when comparing this with the similar BASH script we created. First, instead of using a separate program, like the BASH read program, we are obtaining user input with the raw_input function. This program is then storing the user's input into a certain variable. By default the raw_input program saves information as a STRING which is a type of variable which can hold letters and numbers but can't be used for numeric operations like adding, subtracting, looping, etc. Because of this we use the int() and str() funtions to switch the data back and forth from the string type to the integer numeric type as needed in various parts of the script. Second, the for statement is using the built-in range() function instead of the seq command as we did in BASH.</li>		There are a couple of differences to note when comparing this with the similar BASH script we created. First, instead of using a separate program, like the BASH read program, we are obtaining user input with the raw_input function. This program is then storing the user's input into a certain variable. By default the raw_input program saves information as a STRING which is a type of variable which can hold letters and numbers but can't be used for numeric operations like adding, subtracting, looping, etc. Because of this we use the int() and str() funtions to switch the data back and forth from the string type to the integer numeric type as needed in various parts of the script. Second, the for statement is using the built-in range() function instead of the seq command as we did in BASH to generate the number of loops needed.</li>
	<li> Try running the script and see if you were correct about what it does and how it works.</li>		<li> Try running the script and see if you were correct about what it does and how it works.</li>
	<li> Also, it is important to note that while this script works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.</li>		<li> Also, it is important to note that while this script works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.</li>

BenFranske: /* Using Loops and Reading User Input */

2014-04-24T00:38:07Z

Using Loops and Reading User Input

← Older revision		Revision as of 00:38, 24 April 2014
Line 93:		Line 93:
	print "Created "+str(NUM)+" file(s)."		print "Created "+str(NUM)+" file(s)."
	</pre>		</pre>
	~~Note that~~ instead of using a separate program, like the BASH read program, we are obtaining user input with the raw_input function. This program is then storing the user's input into a certain variable. By default the raw_input program saves information as a STRING which is a type of variable which can hold letters and numbers but can't be used for numeric operations like adding, subtracting, looping, etc. Because of this we use the int() and str() funtions to switch the data back and forth from the string type to the integer numeric type as needed in various parts of the script. </li>		There are a couple of differences to note when comparing this with the similar BASH script we created. First, instead of using a separate program, like the BASH read program, we are obtaining user input with the raw_input function. This program is then storing the user's input into a certain variable. By default the raw_input program saves information as a STRING which is a type of variable which can hold letters and numbers but can't be used for numeric operations like adding, subtracting, looping, etc. Because of this we use the int() and str() funtions to switch the data back and forth from the string type to the integer numeric type as needed in various parts of the script. Second, the for statement is using the built-in range() function instead of the seq command as we did in BASH.</li>
	<li> Try running the script and see if you were correct about what it does and how it works.</li>		<li> Try running the script and see if you were correct about what it does and how it works.</li>
	<li> Also, it is important to note that while this script works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.</li>		<li> Also, it is important to note that while this script works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.</li>
Line 99:		Line 99:
	<li> See if you can add a check to the script which will display an example of a filename that will be created and asking the user if that looks correct before actually creating all the files. If the user enters N then do not create the files. Hint, you will need to do a little research on using python IF statements to check user input.</li>		<li> See if you can add a check to the script which will display an example of a filename that will be created and asking the user if that looks correct before actually creating all the files. If the user enters N then do not create the files. Hint, you will need to do a little research on using python IF statements to check user input.</li>
	</ol>		</ol>

	== Functions and Experimenting ==		== Functions and Experimenting ==
	<ol>		<ol>

BenFranske at 00:36, 24 April 2014

2014-04-24T00:36:26Z

← Older revision		Revision as of 00:36, 24 April 2014
Line 80:		Line 80:
	== Using Loops and Reading User Input ==		== Using Loops and Reading User Input ==
	<ol>		<ol>
	<li> Sometimes you want a program to do something over and over again several times. This is typically done using a loop. Let's say you wanted to create a bunch of sample files to do some further practice with in the current directory named "sample-fileX.extension" where X was a number that would keep incrementing by 1 each time and you could specify any extension you wanted. We could certainly use arguments to capture that user input but it is also possible to accept input from the user directly while the script is running. This would create what we call an "interactive" script because the user is interacting with it.</li>		<li> Create a file-generator.py script on your system like the one below and see if you can figure out how it works before testing it:
	<li> Create a file-generator.sh script on your system like the one below and see if you can figure out how it works before testing it:
	<pre>		<pre>
	#!/bin/~~bash~~		#!/usr/bin/python
	BASENAME=sample-file		import subprocess
	~~echo~~ "Welcome to the sample file generation utility!"		BASENAME="sample-file"
	~~echo~~ "Files will be created using the filename format "$BASENAME"X.extension"		print "Welcome to the sample file generation utility!"
	~~echo~~ "Enter the number of files you wish to create, then press Enter:"		print "Files will be created using the filename format "+BASENAME+"X.extension"
	~~read NUM~~		NUM = int(raw_input("Enter the number of files you wish to create, then press Enter: "))
	~~echo~~ "Enter the extension (without the leading period) to put on the files"		EXTENSION = raw_input ("Enter the extension (without the leading period) to put on the files: ")
	~~read EXTENSION~~		for CURNUM in range(1,NUM):
	for CURNUM in ~~`seq~~ 1 $NUM~~`; do~~		subprocess.call(["touch", BASENAME+str(CURNUM)+"."+EXTENSION])
	touch $BASENAME$CURNUM.$EXTENSION		print "Created "+str(NUM)+" file(s)."
	~~done~~
	~~echo~~ "Created "$NUM" file(s)."
	</pre>		</pre>
	~~There are~~ a ~~few things to note about this script. First~~, we ~~have used one type of loop, a FOR loop, but there~~ are ~~others and other ways to make this work~~. ~~Second you can see several more examples of calling another~~ program ~~inside your script~~. ~~In this case we have used~~ the ~~read, seq, and touch programs to accomplish our goal. The seq~~ program is ~~especially useful while writing scripts. It~~ can be used ~~to generate a series~~ of ~~numbers from a starting number~~ (~~1 in this case~~) ~~to a stopping number~~ (~~$NUM in this case~~). There are many other helpful programs you can use while writing scripts including some you know about already such as grep and find, and others you don't such as sed and awk. You can even use programs like wget to ~~get information from~~ the ~~Internet~~ and ~~process it for use in a script instead of using it for just downloading a file. For example, if you know of a web site that lists~~ the ~~sunrise and sunset times you can use a combination of wget and grep/awk/sed~~ to ~~pull that time out of~~ the ~~webpage and use it~~ as ~~part~~ of a script. ~~The possibilities are almost endless, time spent learning about the ins and outs of these little utilities is time well spent if you will be writing shell scripts!~~</li>		Note that instead of using a separate program, like the BASH read program, we are obtaining user input with the raw_input function. This program is then storing the user's input into a certain variable. By default the raw_input program saves information as a STRING which is a type of variable which can hold letters and numbers but can't be used for numeric operations like adding, subtracting, looping, etc. Because of this we use the int() and str() funtions to switch the data back and forth from the string type to the integer numeric type as needed in various parts of the script. </li>
	<li> Try running the script and see if you were correct about what it does and how it works.</li>		<li> Try running the script and see if you were correct about what it does and how it works.</li>
	<li> Also, it is important to note that while this script works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.</li>		<li> Also, it is important to note that while this script works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.</li>
	<li> See if you can modify the script so you can specify a starting and an ending number for the files rather than just a number of files to be created.</li>		<li> See if you can modify the script so you can specify a starting and an ending number for the files rather than just a number of files to be created.</li>
	<li> See if you can add a check to the script which will display an example of a filename that will be created and asking the user if that looks correct before actually creating all the files. If the user enters N then do not create the files.</li>		<li> See if you can add a check to the script which will display an example of a filename that will be created and asking the user if that looks correct before actually creating all the files. If the user enters N then do not create the files. Hint, you will need to do a little research on using python IF statements to check user input.</li>
	</ol>		</ol>
	== Functions and Experimenting ==		== Functions and Experimenting ==

BenFranske at 23:59, 23 April 2014

2014-04-23T23:59:20Z

← Older revision		Revision as of 23:59, 23 April 2014
Line 56:		Line 56:
	TODAY=subprocess.check_output("date +%Y%m%d", shell=True)		TODAY=subprocess.check_output("date +%Y%m%d", shell=True)
	TODAY=TODAY.rstrip()		TODAY=TODAY.rstrip()
			BACKUPFILE="/var/"+USER+"-backup-"+TODAY+".tar.gz"
			print "Beginning backup for "+USER+" on "+TODAY
			subprocess.call(["tar", "-czf", BACKUPFILE, "/home/"+USER])
			print "Backup completed for "+USER+" saved to "+BACKUPFILE
			</pre></li>
			<li> Try running the above example and see if it works as you expect. This script used the output from the date command to get date information to be used as part of the filename but if you were actually writing the script in python you would probably use some of the python built in date functions instead. The advantages of doing it with the built-in functionality are that you will no longer be depending on the date program being installed on the system and you won't have to remove the newline from the end of the date output with the .rstrip() command and you will just generally have cleaner code. After testing the above script and seeing how it works try modifying it like this to use built in date functionality:
			<pre>
			#!/usr/bin/python
			import subprocess
			import datetime
			USER="jsmith"
			TODAY=datetime.date.today().strftime("%Y%m%d")
	BACKUPFILE="/var/"+USER+"-backup-"+TODAY+".tar.gz"		BACKUPFILE="/var/"+USER+"-backup-"+TODAY+".tar.gz"
	print "Beginning backup for "+USER+" on "+TODAY		print "Beginning backup for "+USER+" on "+TODAY
Line 63:		Line 75:
	<li> See if you can predict what this script will do and then try it out to see if you are correct. Once you have figured out how this script works see if you can modify it so you can specify a username as an argument instead of as a fixed variable.</li>		<li> See if you can predict what this script will do and then try it out to see if you are correct. Once you have figured out how this script works see if you can modify it so you can specify a username as an argument instead of as a fixed variable.</li>
	<li> After successfully doing that try modifying the script so that it gives instructions about how to use the script if you forget to give a username as an argument.</li>		<li> After successfully doing that try modifying the script so that it gives instructions about how to use the script if you forget to give a username as an argument.</li>
	<li> It is important to note that while ~~this script works~~, ~~it's~~ just a simple ~~example~~ and not well written because ~~it is~~ easily broken by giving a username which doesn't actually exist on the system, or trying to backup files you don't have permission to access, etc. ~~A better script~~ could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input such as an argument.</li>		<li> It is important to note that while these scripts work, they are just simple examples and not well written because they are easily broken by giving a username which doesn't actually exist on the system, or trying to backup files you don't have permission to access, etc. Better scripts could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input such as an argument.</li>
	</ol>		</ol>

BenFranske at 22:54, 23 April 2014

2014-04-23T22:54:24Z

← Older revision		Revision as of 22:54, 23 April 2014
Line 49:		Line 49:
	</pre></li>		</pre></li>
	<li> See if you can predict what these changes will do and then try it out to see if you are correct. Because python stores all of the arguments into a special type of variable which can hold multiple pieces of data, called an array, we can use the len() function to check the length of the sys.argv array. If it's more than one (because sys.argv[0] always holds the script name so there is always at least one) we know arguments have been added. If not we know they have not been added. You'll also notice that indenting properly is important in Python scripts this is how python knows if a line is part of the if/else statement or something that comes after it.</li>		<li> See if you can predict what these changes will do and then try it out to see if you are correct. Because python stores all of the arguments into a special type of variable which can hold multiple pieces of data, called an array, we can use the len() function to check the length of the sys.argv array. If it's more than one (because sys.argv[0] always holds the script name so there is always at least one) we know arguments have been added. If not we know they have not been added. You'll also notice that indenting properly is important in Python scripts this is how python knows if a line is part of the if/else statement or something that comes after it.</li>
	<li> It's also possible to capture the standard output of a command and put that into a variable for use. For example try running the "date +%Y%m%d" command. We can use the output of this command in a variable to make our backup script a little bit nicer. Save this script as better-backup.sh with the appropriate permissions to run it as a script.		<li> It's also possible to capture the standard output of a command and put that into a variable for use just as we did with BASH. For example try running the "date +%Y%m%d" command. We can use the output of this command in a variable to make our backup script a little bit nicer. You'll also see that in order to capture the output we need to use the subprocess library instead of the os library we were using before. It's suggested that you switch to using the subprocess library anyway as the os.system function has been deprecated. Save this script as better-backup.py with the appropriate permissions to run it as a script.
	<pre>		<pre>
	#!/bin/~~bash~~		#!/usr/bin/python
			import subprocess
	USER="jsmith"		USER="jsmith"
	TODAY=$(date +%Y%m%d)		TODAY=subprocess.check_output("date +%Y%m%d", shell=True)
	BACKUPFILE="/var/"$USER"-backup-"$TODAY".tar.gz"		TODAY=TODAY.rstrip()
	~~echo~~ Beginning backup for $USER on $TODAY		BACKUPFILE="/var/"+USER+"-backup-"+TODAY+".tar.gz"
	tar -czf $BACKUPFILE /home/$USER/		print "Beginning backup for "+USER+" on "+TODAY
	~~echo~~ Backup completed for $USER saved to $BACKUPFILE		subprocess.call(["tar", "-czf", BACKUPFILE, "/home/"+USER])
			print "Backup completed for "+USER+" saved to "+BACKUPFILE
	</pre></li>		</pre></li>
	<li> See if you can predict what this script will do and then try it out to see if you are correct. Once you have figured out how this script works see if you can modify it so you can specify a username as an argument instead of as a fixed variable. ~~Can you do this with a change to only one line?~~</li>		<li> See if you can predict what this script will do and then try it out to see if you are correct. Once you have figured out how this script works see if you can modify it so you can specify a username as an argument instead of as a fixed variable.</li>
	<li> After successfully doing that try modifying the script so that it gives instructions about how to use the script if you forget to give a username as an argument.</li>		<li> After successfully doing that try modifying the script so that it gives instructions about how to use the script if you forget to give a username as an argument.</li>
	<li> It is important to note that while this script works, it's just a simple example and not well written because it is easily broken by giving a username which doesn't actually exist on the system, or trying to backup files you don't have permission to access, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input such as an argument.</li>		<li> It is important to note that while this script works, it's just a simple example and not well written because it is easily broken by giving a username which doesn't actually exist on the system, or trying to backup files you don't have permission to access, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input such as an argument.</li>