Scripting Basics

Question 1

Write out the basic structure of a bash script

Answer 1

#!/bin/sh  #this points to the intrpreter to use
echo "Hello World!"
exit 0 #The return value for the script

Question 2

How do we make a script an executable

Answer 2

chmod +x name-of-script.sh

Question 3

How do run a script

Answer 3

./name-of-script.sh

Question 4

What type of file extension does a script need to be saved as

Answer 4

.sh

Question 5

What is the shell used to run scripts

Answer 5

Bourne

If you want to use a different shell (e.g. zsh, bash), you need to update the script to know to use that environment.

Question 6

Can you run a script that doesn’t have the opening #!/bin/sh

Answer 6

Yes. You can run it using the sh command.

Question 7

What’s the comment character for Bourne scripting

Answer 7

#

Question 8

Declare a shell variable

Answer 8

v1=”Hello World”

Question 9

What type of value are shell variables

Answer 9

Strings.

Shell scripting does not support other type of variables

Question 10

How do you use a variable once it’s declared

Answer 10

$variableName e.g. $v1

Question 11

How do you determine the length of a string

Answer 11

${variableName} e.g. ${#v1}

Question 12

What does a null value mean in shell scripting

Answer 12

An empty string

Question 13

Explain the following variable modifier:

v2=${v1:-foo}

Answer 13

If v1 is not empty, use v1; else use the text ‘foo’

Question 14

Explain the following variable modifier:

v2=${v1:=bar}

Answer 14

If v1 is not empty, use v1; else assign ‘bar’ to v1 and use it

Question 15

Explain the following variable modifier:

v2=${v1:?}

Answer 15

If v1 is not empty, use v1; else exit with an error message

Question 16

How would you store the output of a command into a variable?

Answer 16

You would create a vriable and then surround the command in backticks

num=echo $v1 | wc -w

Question 17

How can you perform arithmatic since shell variables are stored as strings

Answer 17

Use the expr command

v2=3 #3 is stored as a string valueexp
expr $v2 + 1 #returns 4

Or you can use the following shorthand:

v3=$((v3 + 1)) #

Question 18

What operators do we have to chain commands together

Answer 18

&& #If the first command runs, run the next one also
command1 && command2 # if command1 succeeds, run command2 also

|| #If the first command fails, run the next one
command1 || command3 # if command1 fails, run command3

Question 19

What’s the syntax for grouping commands together

Answer 19

Wrap in curly braces. Seperate commands with a newline or semicolon. If a semi colon is used, ensure the is a space afterwards.

[ ‘id -u’ -eq 0 ] || { echo “root only!”; exit 1; }

Question 20

Wildcard characters

Answer 20

? A single character

* Multiple Characters

Question 21

Escape character

Answer 21

\

Question 22

Home path characyer

Answer 22

~

Question 23

Is there a difference between single quoutes and double quotes

Answer 23

Yes. Single quotes indicate literal assignment, Double quotes allows for enhanced features (e.g. escaped whitespaces, variable name exansion).

The backslash escape character works with double quotes, not single quotes.

When in doubt, go with double quotes.

Question 24

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

Answer 24

Use backticks.

e.g. dirlist=ls -alt

Question 25

Write a shell script function named say

Answer 25

``` #!/bin/sh say() { v1="Veni Vidi Vici" echo $v1 } say exit 0 ```

Question 26

What are positional parameters

Answer 26

Used with functions to pass in additonal information. The position they are assinged indicates what parameter is assigned. Positional parameters are read only.

Question 27

How can you break out of a function

Answer 27

Use the return keyword and exit code. e.g. return 0 If another code apart from 0 is used, the function has failed

Question 28

Special variable for process id of current shell process

Answer 28

$$

Question 29

Special varaiable for return value of last command

Answer 29

$?

Question 30

Special varaiable for name used to invoke script

Answer 30

$0

Question 31

Special variable for number of arguments

Answer 31

$#

Question 32

Special variable for consolidaed list of argumetns

Answer 32

$ *

Question 33

Special variable for iterable list of all arguments

Answer 33

$@

Question 34

Basic structure for a conditional statement containing an if, else if and else condition

Answer 34

if condition1; then elif condition2; then else fi

Question 35

Basic structure for an if then statement

Answer 35

if [ -f /etc/passwd ]; then echo "/etc/passwd exists" fi

Question 36

Basic structure for a case statement

Answer 36

case "$var" in [aA][bB][cC]*) ;; 123|xyz) ;; *) ;; esac

Question 37

When reading variables, is there a differnece if you quote the values e.g. "$v1" vs $v1

Answer 37

Yes. If the unquoted variable contains embedded white space between the values, it will not print out the value as a string.

Question 38

Write a basic while loop that counts from 0 to 9

Answer 38

``` n=0 while [ $n -lt 10 ]; do echo $n n=`expr $n + 1` done ```

Question 39

Write a basic for loop to check if files exit

Answer 39

for f in /etc/fstab /etc/passwd; do [ -f $f ] && "$f exists" || "$f does not exist" done

Question 40

How can you exit a loop

Answer 40

Use the break command

Question 41

What does the shift command do

Answer 41

It decrements the number of positional parameters in a function/loop

Question 42

How would you set up a script to use a different interptreer

Answer 42

Modify the opening instruction to the interpreter you want ``` #!/usr/bin/env # Example: #!/usr/bin/env zsh ```

Question 43

What is an alias

Answer 43

A set of commands assigned to a variable e.g. alias dir="ls -l"

Question 44

How do you exercise commands located in another script within the current shell

Answer 44

use the . command ``` . otherscript # Execute statements in otherscript within the current shell ```