Difference between revisions of "CSC231 Bash Tutorial 5"
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for i in a b c d e f ; do | for i in a b c d e f ; do | ||
for j in 0 1 2 3 4 5 ; do | for j in 0 1 2 3 4 5 ; do | ||
− | echo -n "$i$j " | + | echo -n "$i$j " |
done | done | ||
echo "" | echo "" | ||
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<br /> | <br /> | ||
+ | * 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. | ||
<br /> | <br /> | ||
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<br /> | <br /> | ||
<br /> | <br /> | ||
+ | =Solutions= | ||
+ | <showafterdate after="20170303 11:45" before="20170601 00:00"> | ||
+ | ==Challenge 2== | ||
+ | |||
+ | for i in `seq 10` ; do | ||
+ | for j in `seq $i` ; do | ||
+ | echo -n "*" | ||
+ | done | ||
+ | echo "" | ||
+ | done | ||
+ | |||
+ | </showafterdate> | ||
[[Category:Nasm]][[Category:CSC231]][[Category:Assembly]] | [[Category:Nasm]][[Category:CSC231]][[Category:Assembly]] |
Revision as of 08:55, 3 March 2017
--D. Thiebaut (talk) 08:04, 3 March 2017 (EST)
Contents
Testing 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.
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
- 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.
Solutions
<showafterdate after="20170303 11:45" before="20170601 00:00">
Challenge 2
for i in `seq 10` ; do for j in `seq $i` ; do echo -n "*" done echo "" done
</showafterdate>