Bash, quiz 3

Question 1

what is a shell?

Answer 1

A shell is a command interpreter, an interface between a
human (or another program) and the OS

Question 2

what is the terminal?

Answer 2

The terminal is a program that opens a graphical window and
lets you interact with the shell.

Question 3

what are the shell’s responsibilities?

Answer 3

Program execution
Variable and file name substitution
I/O Redirection
Pipeline Hookup
Environment Control
Interpreted Programming Languages

Question 4

shell I/O redirection metacharacters

Answer 4

< > > > |

Question 5

shell wildcard metacharacters

Answer 5

Asterix ? [ ]

Question 6

shell other metacharacters

Answer 6

& ; $ ! \ ( ) space tab newline

Question 7

shell substitutions/globbing

Answer 7

• Matches 0 or more of any character – ls *.c
  ? Matches any single character – ls file0?.c
  […] Matches any single character if it is in list provided
  – ls file[0-9].c

Question 8

what is a shell script?

Answer 8

A shell script is a file that contains shell commands

Question 9

How do you specify the shell to execute the program?

Answer 9

The script must begin with #! (pronounced “shebang”) to identify shell to be executed
– #! /bin/bash

Question 10

What are the steps to run a shell script?

Answer 10

To run:
– Make executable: chmod +x hello.sh
– Invoke via: ./hello.sh

Question 11

What are the two types of variables that Bash supports?

Answer 11

Bash supports two types of variables:
– Local variables
– Environment variables,

Question 12

How are environment variables set?

Answer 12

created using export[variable]

Environment variables are set by the system and can usually
be found using the env command
env [options] [varname=value] [command]
– Displays the current environment or modifies the specified variables
– Specified commands are executed in the new environment

Question 13

what does the $PATH variable do?

Answer 13

Initially set when the shell is created (login process)
– Provides a list of available directories where an executable command may be found

Question 14

List some shell scripting features

Answer 14

Full scripting language
– Conditional statements (if-then-else, case, …)
– Arithmetic, string, file, environment variables
– Input (prompt user, command-line arguments, …)
– Loop statements (for, while, do-while, repeat-until, …)
– Lists (and, or)
– Functions
– Traps

Question 15

What is the syntax for declaring a local variable in bash?

Answer 15

Variable structure format
varname=value
– Variable name must begin with alphabetic or underscore character, followed by zero or more alphanumeric or underscore characters
– Note there should not be any spaces around the ‘=’ sign
– Variables are case-sensitive
– Do not use globbing metacharacters, such as ? and *, in your variable names
– Once defined, use by prefixing $ symbol to variable name

Question 16

How do you export a Bash shell variable?

Answer 16

• Variables can be exported
  – The value of the variable can be passed to other subshell
• Export command format:
  export [varname]

– Changing an exported variable in a subshell does not change the value in the parent shell

Question 17

how do you destroy an exported variable? [bash scripting]

Answer 17

by using ‘unset’

Question 18

How does ‘export’ with no arguments behave in a bash script

Answer 18

export with no arguments lists the variables that are exported
to the user’s shell

Question 19

How do you read in user input, bash scripting

Answer 19

The read command allows you to prompt for input and store it in a
variable
* Syntax:
read varname [varname1] [varname2] … [varnameN]
– or
read –p “prompt” varname1 [varname2] … [varnameN]
* Input entered by user are assigned to varname1, varname2,
etc.
* If more input is entered than there are variables, the
remaining input will be assigned to the last variable

Question 20

quoting mechanisms, bash scripting

Answer 20

Quote characters and the backslash character have special
meaning in shell scripts
` perform command substitution ` uname `
“ allow some variable expansion, but prevent wildcard
replacement
‘ prevent wildcard replacement as well as variable and command
substitution
\ preserve the literal value of the next character

Question 21

how do you perform arithmetic evaluation in bash scripting?

Answer 21

• let statement can be used to perform arithmetic operations
• Available operators are: +, –, multiplication, /, %
  let X=10+2 times 7
  echo $X
  let Y=X+2 times 4
  echo $Y
• An arithmetic expression can be evaluated by:
  $[expression] or $((expression))
  echo “$((123+20))”
  VALORE=$[123+20]
  echo “$[123*$VALORE]”

Question 22

what is the difference between single and double brackets, bash scripting?

Answer 22

Single brackets [ … ] are built-in in bash as an alias for the test command
Double brackets [[ … ]] is a bash keyword and is much more capable than single brackets

Question 23

test file operator commands, bash scripting
-d, -f, -r, -s, -w, -x, -G, -O, -u, -g

Answer 23

• The test file operator returns true if:
  –d file: file is a directory
  –f file: file is an ordinary file, and exists
  –r file: file is readable by the process
  –s file: file is not empty
  –w file: file is writable by the process
  –x file: file is executable
  –G file: file is owned by the group user belongs to
  –O file: file is owned by the user
  –u file: set_user_id bit is set
  –g file: set_group_id bit is set

Question 24

bash numeric relational operators

Answer 24

greater than: -gt
greater than or equal: -ge
less than: -lt
less than or equal: -le
equal: -eq
not equal: -ne

Question 25

bash string relational operators

Answer 25

equal: = or ==
not equal: !=
str1 is less than str2: str1 < str2
str1 is greater than str2: str1 > str2
string length is greater than zero: -n str
string length is zero: -z str

Question 26

bash logical operators

Answer 26

! expression logical negation
expression1 –a expression2 logical and
expression –o expression logical or

The && (and) and || (or) logical operators may also
be used, but must be enclosed in [[ … ]]

Question 27

bash script example if… statement

Answer 27

if [[ condition ]]; then

fi

Question 28

bash script case statement

Answer 28

Syntax:
case value in
pattern1) statement-list1
;;
pattern2) statement-list2
;;
…
patternN) statement-listN
;;
esac

Question 29

bash script while loop

Answer 29

while [ expression ]
do
statement1
statement2
…
statementN
done

Question 30

bash script until loop

Answer 30

until [ expression ]
do
statement1
statement2
…
statementN
done

Question 31

bash script for loop

Answer 31

for varname in arg1 arg2 … argN
do
statement1
statement2
…
statementN
done

Question 32

bash script C style for loop

Answer 32

for (( expression1 ; expression2 ; expression3 ))
do
statement1
statement2
…
statementN
done

Question 33

bash script select command

Answer 33

select command constructs simple menu from a list
– Allows user to enter a number instead of a string value
– User enters sequence number corresponding to the argument in the
list
select value in LIST
do
statement(s)
done

Question 34

usage of break and continue in bash scripting

Answer 34

• Interrupts the for, while, or until loop
• The break statement
  – Transfers control to the statement after the done statement
  – Terminates execution of the loop
• The continue statement
  – Transfers control to the done statement
  – Skips the test statements for the current iteration
  – Continues execution of the loop

Question 35

bash script shell function format

Answer 35

A shell function is similar to a shell script
– Stores a series of commands for execution later
– Shell stores functions in memory
– Shell executes a shell function in the same shell that called it (not a
subshell)
* Must be defined before they can be referenced
– Usually placed at the beginning of the script
function-name()
{
statements
}

Question 36

What does the following line in a Bash script do?

chmod +x script.sh

Answer 36

Makes script.sh executable

Question 37

Which command is used to set an environment variable in Bash?

Answer 37

export

Question 38

Which symbol is used to comment a line in a Bash script?

Answer 38

#

Question 39

!/bin/bash

What does the following Bash script do?

if [ -e myfile.txt ]; then
echo “File exists.”
else
echo “File does not exist.”
fi

Answer 39

Checks if a file exists

Question 40

Which of the following is a correct way to compare two numeric values using [ ] in Bash?

Answer 40

[ 3 -lt 8 ]

Question 41

What does the expression (( 3 * 4 )) evaluate to in Bash?

Answer 41

12

Question 42

Which command is used in Bash to evaluate and compute the result of an arithmetic expression?

Answer 42

let

Question 43

Which construct in Bash is used to create a menu for selecting options from a list?

Answer 43

select

Question 44

What is the correct syntax for evaluating an arithmetic expression in Bash?

Answer 44

(( expression ))

Question 45

In bash, if
a=someletters_12345_moreletters.txt
b=${a:12:5}

What would be your next statement to print the value of b

Answer 45

echo $b

Question 46

! /bin/bash

What will be the output of this program?

global=”pretty good variable”

checkvar()

{

local inside=”not so good variable”

echo $inside

global=”better variable”

echo $global

}

echo $global

checkvar

echo $global

checkvar

Answer 46

pretty good variable
not so good variable
better variable
better variable
not so good variable
better variable

Question 47

In Bash, which command is used to send the “interrupt” signal to a running process?

Answer 47

kill -2

Question 48

The output of this program
#!/bin/bash
string1=”Hello”
string2=”World”
echo “$string1.$string2”

Answer 48

Hello.World