Week 4 - Bourne Again Shell (Bash)

Question 1

What is Bash?

Answer 1

A shell (command interpreter + programming language)

Question 2

What standard does Bash and most Linux/Unix systems adhere to?

Answer 2

POSIX standard

Question 3

Bash executes several files before you are in control what are some directories that these files reside in?

Why do we need to execute these files?

Answer 3

Login shell:
/etc/profile (global)
~/.bash_login

Non-login shell:
~/.bashrc

Non-login, non interactive:
BASH_ENV

We execute these files to setup the environment settings.

Question 4

What command do we use to redirect standard error to a file?

Answer 4

2>
cat x y > hold 2 > errors

(2 = standard error,
1 = standard output,
0 = standard input,
& differentiates from file name)

Question 5

What command do we use to redirect standard error to standard output?

Answer 5

2>&1
cat x y > hold 2>&1

(2 = standard error,
1 = standard output,
0 = standard input,
& differentiates from file name)

Question 6

chmod u+x filename

What does u+x do?

Answer 6

Gives execution (x) privileges to the owner of the file (u)

Question 7

The first token is always interpreted as a(n) _______

Answer 7

Command

Question 8

What does # represent?

Answer 8

A shell comment

Question 9

What does #! represent?

Answer 9

!/usr/bin/python3

“shebang” defines the interpreter used to run the script

print(“This is a Python script.”)

Question 10

___ separates sequential commands on a line

Answer 10

;

Question 11

What are the two uses of &

Answer 11

a& runs task in background

> &2 denotes standard error file descriptor instead of filename.

Question 12

Why is a job a command pipeline?

Answer 12

Under the hood, a job is multiple commands with pipes:

command1 | command2 | command3

Question 13

fg %jobnumber
kill %jobnumber

what is the % for?

Answer 13

% is used to refer to a job number

Question 14

What does CTRL-Z do?

Answer 14

Suspends foreground job

Question 15

What is a directory stack?
What command displays the stack?

Answer 15

Stores history of visited directories like a navigation stack.

dirs

Question 16

Give an example of a variable

Answer 16

name=tommy_j2525

NOTE: No whitespaces and cannot start with a digit

Question 17

What special character do we use to evaluate variables?

Answer 17

$

echo $myvar

Question 18

What are some special parameters?

Answer 18

$0: The name of the script or command.
$#: The number of arguments passed to the script.
$@: All arguments passed to the script.
$?: The exit status of the last command.
$$ contains the process identification (PID)
$! contains the PID of the last process run in
background

Question 19

What does the shift built-in do?

Answer 19

Shifts arguments to the left when you are using positional parameters.

Question 20

What is the difference between a command and a built-in?

Answer 20

Commands are external scripts, built-ins are integrated into the shell.

Question 21

What does the set builtin do?

Answer 21

assigns arguments to positional parameters

NOTE: set -v (verbose) is useful for debugging

Question 22

What is the output for each? (person=alex)

echo person
echo $person
echo “$person”
echo ‘$person’
echo $person

Answer 22

person
alex
alex
person
person

Question 23

What is the output for each?
(person=”alex and jenny”)

echo $person
echo “$person”

Answer 23

alex and jenny

alex and jenny

$ directly plops tokens into the command with positional arguments which results in only one space between the words
(echo alex and jenny)

$”” will directly output the original string from the variable
(echo “alex and jenny”)

Question 24

memo=alex*

is
echo “$memo”
the same as echo $memo?

Why?

Answer 24

No

echo “$memo” will output alex* (the original string)

echo $memo will replace $memo with alex*
(echo alex*)
which performs string matching
output alex.report alex.summary

Question 25

What does the read builtin do?

Answer 25

Reads a line from the terminal and assigns it to a variable read my_var

Question 26

PATH=/sbin:/usr/sbin:$PATH what does this command do?

Answer 26

An environment variable that specifies where to look for executable files when you run a command. It is a comma-separated list of directories INCLUDING the old path directory (itself)

Question 27

What does the declare built in do?

Answer 27

– -a declares a variable as an array (later) – -f makes the variable a function name – -i marks a variable as an integer – -r marks a variable as readonly – -x marks a variable for export

Question 28

what does pstree -p do?

Answer 28

Shows process tree with PIDs

Question 29

What does ps -Al do?

Answer 29

Shows processes -A shows all processes including those belonging to other users -l long format (more details)

Question 30

What does ps aux do?

Answer 30

shows table or processes using human readable format (Task manager)

Question 31

what does history store?

Answer 31

A history of previously entered commands !! executes last command !n executes command with number n !string executes command starting with string !?string executes command containing string

Question 32

alias ls='ls -color' alias rm='rm -i' What's the difference between single and double quotes? Why do we use single quotes here?

Answer 32

Single quotes ('): Everything inside is taken literally (no variable expansion). Double quotes ("): Variables and escape sequences are evaluated and expanded. We use single quotes because we do not want the command to be expanded and modified. NOTE: Similar to $ which expands variables, '' is similar to $""

Question 33

What is the syntax for a function?

Answer 33

function-name () { commands } NOTE: functions cannot have arguments, and they can overwrite commands with their names!

Question 34

what does the export built in do?

Answer 34

Exports a variable so that it available to child processes or sub-shells.

Question 35

Brace expansion: output of echo page_{one, two, three} ?

Answer 35

page_one page_two page_three NOTE: uses expansion to run the command multiple times with each token.

Question 36

Syntax for evaluating integer expression?

Answer 36

$[expression]

Question 37

How do you perform command substitution?

Answer 37

$(command) or `command` echo "The files in this directory are: $(ls)" NOTE: uses the output of a command as part of another command.