IT 244: Introduction to Linux/Unix - Shell Scripts for Homework Assignments

IT 244: Introduction to Linux/Unix
Shell Scripts for Homework Assignments

Topics

What Is a Shell Script?
Format for Homework Shell Scripts
Running a Shell Script
Example
Things Not to Do

What Is a Shell Script?

A shell script is a text file that contains a list of Unix commands ...
with one command on each line
You can then run a shell script from the command line ...
and it will perform each Unix command in the script
Shell scripts are used to automate routine Unix administrative tasks

Format for Homework Shell Scripts

Each shell script you submit for a homework assignment ...
must have a certain format
You need to create a directory for each homework assignment ...
inside your hw directory ...
inside your it244 directory
The homework directory name will begin with "hw" ...
and end with the number of the assignment
The filename for each homework shell script must
- Begin with "hw"
- Followed by the homework number
- Followed by ".sh"
So the homework shell script for homework 3 ...
should be named hw3.sh
and the script should be inside an hw3 directory
The .sh extension is not required by Unix ...
but I insist on it ...
because it makes the purpose of the file clear
I also want each step in the homework labeled ...
with an echo statement ...
giving the step number from the homework assignment
So, the line before the Unix commands that perform Step 1 ...
should read
```
    echo Step 1
```
And the line before the Unix commands that perform Step 2 ...
should read
```
    echo Step 2 
```
There should also be a blank line before each echo statement ...
to separate it from the Step that comes before
I need you to do this so I can score the assignments quickly
If you do not, you will lose points

Running a Shell Script

You run a shell script as follows
```
    bash SHELL_SCRIPT
```
So if I wanted to run the script test.sh ...
I would type
```
    bash test.sh
```
Later on in the course I'll show you an easier way
If your shell script does not work ...
bash has a feature that will help

Here are the contents of a shell script called test.sh

echo I am going to my home directory
cd
echo I will show you that I am in my home directory
pwd
echo Here are the files in my home directory
ls

If I run bash with the -x option ...

it will print out every line before it executes that line

$ bash -x test.sh 
+ echo "I am going to my home directory"
I am going to my home directory
+ cd
+ echo "I"\""ll show you that I am in my home directory"
I"ll show you that I am in my home directory
+ pwd
/home/tsoro
+ echo "Here are the files in my home directory"
Here are the files in my home directory
+ ls
assignments_submitted   html   mail      public_html
bin                     it244  memo.bak  scans
code                    it341  memo.txt  tests_taken
course_files            it441  nsmail    tmp

When used with the -x option, bash
- Prints each command before it executes that command ...
- placing a + at the beginning of the line
- Executes the command on that line

Example

Let"s look at an example
Let"s say that homework 99 asks you to do the following:
1. Go to your home directory
  Run a command that shows that you are in your home directory
2. Display the contents of your directory
3. Display the contents of /home/tsoro/course_files/it244_files
For this assignment you would have to create an hw99 directory ...
inside your hw directory ...
inside the hw99 directory
Inside this directory you would create the script file hw99.sh

Here is the text of the file

echo Step 1
cd
pwd

echo Step 2
ls

echo Step 3
ls /home/tsoro/course_files/it244_files

Running this script file would give me the following results

$ bash hw99.sh 
Step 1
/home/tsoro
Step 2
assignments_submitted  it114  News
bin              it244  nsmail
course_files     it341  public_html
examples         it441  test
html             mail   test_taken
hw99.sh          Mail   vp
Step 3
bother.sh        numbers1.txt
comment_test.sh  numbers2.txt
foo1.txt         numbers.txt
foo2.txt         print_arg_numbers.sh
foo.sh           print_foo.sh
foo.txt          print_positionals.sh
for_1.sh         red_sox.txt
for_2.sh         repeat.py
for_3.sh         shell_test_1.sh
fruit.txt        shell_test_2.sh
hello.sh         test.sh
hi_bye.sh

Things Not to Do

Never use one of the following commands inside your homework script
- more or less
- Editors like nano, vi, vim or emacs
- man or info
Calling these programs inside your script introduces Control characters ...
which make if very hard to grade
Do not include the text of the step in the echo command
The following is wrong
```
echo Step 5 go to your home directory
```
Do not use a lowercase letter when spelling "Step" in the echo command
The following is also wrong
```
echo step 4
```
Since all Unix command use lowercase letters, capitalizing "Step" makes it stand out
Do not combine steps
I have to score each step individually and if you combine steps ...
it will make things harder for me
The Step numbers should be the steps from the assignment ...
not the number of each of your commands
So if Step 4 asked you to
- Go to your home directory
- List the contents of your home directory

this would be wrong

    echo Step 4
    cd
    echo Step 5
    ls

Instead, you should write
```
    echo Step 4
    cd
    ls
```
I will deduct 10 points for each type of mistake
If you create your shell script by copying and pasting from the command line ..
don"t paste the entire line from your shell session into the script file
The script file should contain Linux commands only ...
not your prompt followed by the command ...
or the results of that command

The following is wrong

    it244gh@vm75:~$ echo Step 1
    it244gh@vm75:~$ cd ~
    it244gh@vm75:~$ pwd
    /home/it244gh