CSC231 Bash Tutorial 5

--D. Thiebaut (talk) 08:04, 3 March 2017 (EST)

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This lab will introduce you to using Nasm as a tool for better understanding instructions. You will also look at a different way of redirecting standard out (stdout), and to the powerful for-loop in bash.

Using Nasm to Test Instructions

Writing a short assembly program to answer quick questions is simple and can be quick. We'll do a couple examples.

Testing how DIV works

• Get a copy of test_skel.asm

getcopy test_skel.asm

• Its code is shown below for completeness:

;;; Testing sandbox
	        extern  _printDec
	        extern  _printString
	        extern  _println
	        extern  _getInput
		extern	_printRegs
	
		section	.data
a		dd	100
b		dd	33
c		dd	0
ans		dd	0
	
		section	.text
		global	_start
_start: ;-------------------------------------------------

	
;;; exit -------------------------------------------------
		mov	ebx, 0
		mov	eax, 1
		int     0x80

• It has everything needed to test how short pieces of code work.
• Put the following code between the dashed lines:

 		mov	edx, 0
		mov	eax, dword[a]
 		div	dword[b]
 
 		call	_printRegs

• (If the lines are already there, but commented out, just remove the semi-colons.)
• See what the program does. What quantity will be divided by what other quantity.
• Assembly, link and run:

 nasm -f elf test_skel.asm
 ld -melf_i386 -o test_skel test_skel.o 231Lib.o
 ./test_skel 

• Do you see the quotient and remainder in the registers? Does it make sense?

You should practice doing this simple exercise every time you are not sure about a particular set of instructions. If you start with a skeleton program, such as test_skel.asm, all you have to do is fill in the middle part, assemble, link and run, and you will have your answer.

Hex, Binary, and 2's Complement

• You can use the same method for testing number representations.
• Go to this URL http://www.nasm.us/doc/nasmdoc3.html, and locate Section 3.4.1. Look at the instructions and the different ways Nasm accepts literals.
• Nasm accepts numbers in decimal, hex, binary, octal, and 2's complements.
• Let's try a few:
• Edit your test_skel.asm program and add the following lines between the dashed lines:

		mov	eax, 0x0000FFFF
		mov	ebx, 00000000111111110000000011111111b
		mov	ecx, 1111_1111_1111_1111_1111_1111_1111_1111b
		call	_printRegs

• Assemble, Link and run. You should see eax, ebx, and ecx, as hex, unsigned, and signed (2's complement) integers.

A Different Way to Redirect stdout

• If you use > at the end of a command, and add the name of a file at the end, the output of your command will be stored in the file. If the file already existed, it will be erased first, and overwritten.

Example

 du /etc | sort -n |  tail -10  > largest.txt
 cat largest.txt

Notice that you get the largest files in etc in the largest.txt

Try this, now:

du /usr/games/* | sort -n | tail -10  > largest.txt
cat largest.txt

See that the file largest.txt has been overwritten.

Break time: you will notice that Linux has a game directory! This is typical of Linux systems. There are usually a few games hidden somewhere. Ours has only a few. Some linux distributions come with a larger number of them. Let's try one with this set of commands (you will learn these commands today):

for i in hello there 231b Student ; do cowsay $i ; done

• Break over.
• What if instead of erasing the contents of largest.txt every time we wanted to keep adding to its end, i.e. appending text to it?
• The redirection symbol for that is >>

du /etc | sort -n |  tail -10  > largest.txt
du /usr/games/* | sort -n | tail -10  >> largest.txt
cat largest.txt

The first du command will create a new largest.txt and the second du command will append to it.

• In general you want to use > once, for the first command, and >> several times for all the information you need to append to the output file.

Bash Loops

Bash supports loops! It is a powerful way of repeating commands.

• Try this at the Bash prompt:

 for i in 1 2 3 4 5 6 7 8 9 10 ; do
      echo "i = $i"
 done

• Explanations:

• i is used as the index of the loop. It is declared by putting it right after the for keyword.
• i will take all the values in whatever list is given after the in keyword.
• the semi-colon ends the declaration, and is followed by do
• the next lines will be repeated for every value i will take. In our case, just the echo command, which is Bash's print command.
• we close the body of the for-loop with done

Note that when you want to use the loop variable inside the body of the loop, you write $i. Variables are declared without $ but are prefixed with $ when used.

Several Different Kinds of Loops

• Try these different variations of loops. They contain new features and constructs, and you will figure out how they work by playing with them.

Loop 2

for i in `seq 1 10` ; do
    echo "i is $i"
done

• Note: you need to use backquotes around the seq 1 10 command. With Bash, putting backquotes around a string means that it is a command that should be executed, and the output of this command becomes a list of lines through which the for command will iterate.

Loop 3

for i in `seq 1 10` ; do
    echo "i is $i"
done

Loop 4

for i in `seq 1 10` ; do echo "i is $i" ;  done

• Loops can be written on one line only. The semi-colon marks the end of commands. Make sure you put spaces around the semi-colons!

Challenge #1:

Use the man page for seq and figure out a way to make your loop print all the numbers from 20 down to 1.

Loop 5 -- Nested for-loops!

for i in a b c d e f ; do  
    for j in 0 1 2 3 4 5 ; do   
       echo -n "$i$j "  
    done  
    echo ""  
done

• The "-n" switch for the first echo indicates that the cursor should not go to the next line after printing the string.
• The echo "" is a way to go to the next line.

Challenge #2:

Use a nested for-loop to print this pattern of 10 lines with 1 to 10 stars:

*
**
***
****
*****
******
*******
********
*********
**********

Challenge #3:

Use a nested for-loop to print all the 231b-xx accounts:

231b-aa
231b-ab
231b-ac
...
231b-az
231b-ba
231b-bb
231b-bc
...
231b-bz

Note: this is the kind of loop that is used to create the 231b accounts in the fall. Instead of printing the string "231b-xx", our Linux admin, Suzanne, will use the adduser 231b-xx command which will add a new user to the system.

Loop 6 -- Looping through files

You can loop through files by using "*", or "*.asm", or some combination of string and "*" character to generate a list of files and iterate through them. Try the following examples:

for i in *.asm ; do 
    echo "file = $i"  
done

for i in *hello* ; do 
    echo "file = $i"  
done

for i in *.java ; do 
    echo "file = $i"  
done

More Complex Looping

Assume that you want to create a text file that contains all your assembly program, with the name of the program between two dashed lines, followed by the contents of the program. This is how you could do this:

• First let's make the output of the command go to the display, so that we can see the output:

for i in *.asm ; do
         echo "-------------------------------------------"
         echo "$i"
         echo "-------------------------------------------"
         cat $i
         echo ""
done

• Try it!
• Now let's redirect the output of all the commands to a file we'll call allAsmFiles.txt

for i in *.asm ; do
         echo "-------------------------------------------" >> allAsmFiles.txt
         echo "$i" >> allAsmFiles.txt
         echo "-------------------------------------------" >> allAsmFiles.txt
         cat $i >> allAsmFiles.txt
         echo "" >> allAsmFiles.txt
done

• Try this as well, and look at allAsmFiles.txt with emacs or less. See how quickly you created this? This can be used to generate quick Jupyter or RStudio notebooks, with the correct header information, for those of you using these packages.

Moodle Quizzes

• You are ready to take the Moodle quiz for today's lab!

Additional Information

In case you want to learn more about loops, there are many interesting tutorials on the Web. I'm listing here a few of them:

• ryanstutorials.net
• cyberciti.net

Solutions

(The solutions will be available at 11:45 a.m., on 3/3/17)
<showafterdate after="20170303 11:45" before="20170601 00:00">

Challenge 1

for i in `seq 10 -1 1` ; do echo "i is $i"; done

Challenge 2

for i in `seq 10` ; do
for j in `seq $i` ; do
echo -n "*"
done
echo ""
done

Challenge 3

for i in a b ; do 
for j in a b c d e f g h i j k l m n o p q r s t u v w x y z ; do 
echo "231b-$i$j"
done
done

</showafterdate>