Topic 8 - Unix Shell Programming

Question 1

What is a shell script?

Answer 1

1) A computer program designed to be run by the Unix shell

2) a text file that can be read and understood by humans and by machines

Question 2

What does a shell script contain?

Answer 2

1) Unix commands

2) Flow Control command

Question 3

How are commands executed by the shell?

Answer 3

1) In order as they appear in the file or

2) in an order determined by the flow control statements

Question 4

Different shells have different ______________

Answer 4

Different shells have different control structure syntaxes

Question 5

How is a shell script created?

Answer 5

Like all high-level language programs, programmers create shell scripts with a text editor.

Question 6

What is the main difference between shell scripts and high-level language programs?

Answer 6

Shell scripts do not have to be converted to machine language by a compiler - unix shell acts as an interpreter when reading script files.

Question 7

What are the main uses of shell scripts?

Answer 7

1) To avoid repetition

2) To automate difficult tasks

Question 8

There are two methods one can use to inform Unix which shell will be used to run the script. Describe the first method:

Answer 8

Calling the shell followed by the script file name - no need to set the permission of the script to executable.

Question 9

There are two methods one can use to inform Unix which shell will be used to run the script. Describe the second method:

Answer 9

Invoking the shell as the first working line in the script file (using #!) - permission of the script must be set to executable.

Question 10

With the second method of running a script, what happens when #1 is not present at beginning of first line?

Answer 10

a default shell will be used to run the script.

Question 11

What happens if both methods are presented?

Answer 11

The first method has precedence

Question 12

How can you assign the output of a command to a variable?

Answer 12

Using backquotes - example:
#!/bin/sh
files=`ls`
echo $files
-
#!/bin/sh
value=`expo 23456 + 6789`
echo $value

Question 13

What is the syntax for for loops?

Answer 13

for variable in value1 value 2... ;
do
----command_set
done;
example:
for i in 1 2 3;
do
----exectueline
done;

Question 14

How do you debug your program?

Answer 14

use the -x when invoking the shell

or -xv

Question 15

how are conditional statements done in bourne shell script?

Answer 15

Instead of evaluating a boolean variable to true or false, in born shell script, the only thing ou can test is whether a command is “successful” or “not successful”

Question 16

What are the exit status codes for commands?

Answer 16

0 = successful
Non-zero = unsuccessful

Question 17

What is another way to check the value of the exit status of a program immediately after running the program?

Answer 17

echo $?

Question 18

Syntax for if-statement?

Answer 18

if decision_command;
then
—-command_set;
fi

Question 19

Syntax for if-else statement?

Answer 19

if decision_command;
then
----command_set;
else
----command_set;
fi

Question 20

[NOT A QUESTION] read this example of Nested If:

Answer 20

if decision_command_1;
then
----command_set;
else
----if decision_command_2;
----then
-------command_set2;
----else
--------command_set_3;
----fi
fi

Question 21

Syntax for else-if-statement?

Answer 21

if decision_command_1;
then
----command_set;
elif decision_command_2;
---then
-------command_set_2
fi

Question 22

What is the command to do nothing? You can place this command inside an if-then statement.

Answer 22

”:” aka the colon.

Question 23

What command can check file type/size, compare strings and compare integer values?

Answer 23

“test”

Question 24

As a result of check/comparison done by “test”, what is returned?

Answer 24

An exit status code where:
0 = successful
non-zero = unsuccessful

Question 25

What do each of these test commands with each of their options do?
test -f file
test -L file
test -d file
test -x file
test -s file
test -z string
test siring1 = string2
test string1 != string2

Answer 25

test -f file –> does file exist and is not a directory or a link?
test -L file –> does file exist and is a link?
test -d file –> does file exist and is a directory?
test -x file –> does file exist and the executable flag is on?
test -s file –> does file exist and its size is bigger than 0 bytes?
test -z string –> does the length of string equal 0?
test string1 = string2 –> does str1 equal str2?
test string1 != –> does string1 not equal string2?

Question 26

What do each of these options mean?

• eq,
• ne,
• lt,
• le,
• gt,
• ge

Answer 26

• eq, = true if n1 and n2 are equal
• ne, = true if n1 and n2 are not equal
• lt, = true if n1 is less than n2
• le, = true if n1 is less than or equal to n2
• gt, = true if n1 is greater than n2
• ge = true if n1 is greater than or equal to n2

Question 27

What is the shorthand notation for the “test” command?

Answer 27

[ ] where the brackets are surrounded by spaces

Question 28

While-do-done syntax:

Answer 28

While [ ];
do
—command-execute;
done

Question 29

What is the case-Statement used for?

Answer 29

It supports multi-way branding, based on the value of a single string

Question 30

What does $0 mean?

Answer 30

name of the running command

Question 31

What does $* mean?

Answer 31

All the arguments starting from 1$

Question 32

What does $# mean?

Answer 32

Number of arguments, $0 is not included in the count

Question 33

What does the shift command do?

Answer 33

Shift all arguments to the left. Results in losing the first argument ($1)