BASH

Question 1

BASH

Answer 1

= Bourne Again Shell

A command processor

Question 2

How to grant a file the execute permission?

Answer 2

chmod +x filename

Question 3

!

Answer 3

!/bin/bash

= sha-bang

The first line of a shell script file must begin with it.

It is followed by the full path to the shell interpreter:

or: #!/usr/bin/env bash

Question 4

sh / print to console

Answer 4

echo ‘Hello’

Question 5

BASH script

Answer 5

A list of shell commands written in a file.

Usually ends with the .sh extension

Question 6

man ${command}

Answer 6

Get command’s manual

Question 7

Data types in Bash

Answer 7

Only strings and numbers

Question 8

Types of variables in Bash

Answer 8

1. Local variables
2. Environment variables
3. Variables as positional arguments

Question 9

Local variables in Bash

Answer 9

Exist only within a single script. Inaccessible to other programmes and scripts.

• declare variable:
  foo=”value”
• display value
  echo $foo
• delete varible
  unset foo

Question 10

Using Bash variables in strings

Answer 10

Only possible within double quotes.

Question 11

Environment variables in Bash

Answer 11

Variables accessible outside of the current shell session, to other programmes, scripts, etc.

Defined like local variables, just preceded by ‘export’

export foo=’bar’

Question 12

Most commonly used global variables in Bash

Answer 12

$HOME
$PWD
$USER
$RANDOM - random integer between 0 and 32767

Question 13

Positional parameters in Bash

Answer 13

Variables allocated when a function is evaluated and they are given positionally.

Those are:
$0 - script’s name
$1..$9 - the parameter list elements from 1 to 9
${10-N} the parameter list elements from 10 to N
$* or $@ all positional parameters except $0
$# number of parameters, not counting $0
$FUNCNAME - the function name, has value only inside a function.

Question 14

Default variables in Bash

Answer 14

Useful when you’re processing positional parameters which can be omitted

FOO=${FOO:-‘default’} or

echo “1: ${1:-‘one’}”

Question 15

Assigning a variable the value of a command line output in Bash

Answer 15

It can be done by encapsulating the command with `` or $().

FILE_LIST=ls or
FILE_LIST=$(ls)

Question 16

Array declaration in Bash

Answer 16

You can assign a value to an index in the array variable:

fruits[0]=Apple
fruits[1]=Pear
fruits[2]=Plum

or use compound assignment:

fruits=(Apple Pear Plum)

Question 17

List all values in Bash array

Answer 17

${fruits[*]} or

${fruits[@]}

Question 18

Bash array slice

Answer 18

Extracts the slice of the length 2 that starts at index 0.

${fruits[*]:0:2} #Apple Pear

${fruits[@]:0:2}

Question 19

Bash slice of positional arguments

Answer 19

${@:1:2} or

${*:1:2}

Question 20

Add elements to Bash array

Answer 20

You use new compound array assignment to substitute the existing array in it, and then assign the new value to the original variable.

fruits=(Orange ${fruits[*]} Banana)

or you can assign a value to the next available index

fruits[3]=Banana

Question 21

Delete elements from an array

Answer 21

unset fruits[0]

Question 22

Get the total number of elements in an array

Answer 22

${#fruits[*]}

Question 23

Shell expansion

Answer 23

A mechanism to calculate arithmetical operations, to save results of command’s executions, …

Question 24

Brace expansions

Answer 24

Allow us to generate:

1. Arbitrary strings
   echo beg{i,a,u}n # begin began begun
2. Ranges
   echo {0..5} # 0 1 2 3 4 5
   echo {00..8..2} # 00 02 04 06 08

Question 25

Command substitution

Answer 25

Allows us to evaluate a command and substitute its value into another command or variable assignment.

It is performed when a command is enclosed by `` or $():

now=date +%T or
now=$(date +%T)

echo now # 15:45:03

Question 26

Arithmetic expansion

Answer 26

The expression that performs an arithmetic operation must be enclosed by $(( ))

result=$(( ((10 + 5*3) - 7) / 2 ))
echo result # 9

Question 27

Create a folder structure where src, dest, and test folders each have index.js and util.js files

Answer 27

mkdir -p project/{dest,src,test}

touch project/{dest,src,test}/{index,util}.js

Question 28

What does mkdir’s -p flag do?

Answer 28

Makes parent directories, as needed.

mkdir -p project/{dist,src,test}

project would be a parent directory

Question 29

Streams in Bash

Answer 29

Sequences of characters Bash can receive input, and send output as.

Question 30

Redirection in Bash

Answer 30

Redirection controls where the output of a command can go, and where the input of a command can come from.

These operators are used for redirecting streams:

> redirect output
&> redirect output and error output
&>> append redirected output and error output
< redirect input
<< here documents
<<< here strings

# output of ls will be written to list.txt
ls -l > list.txt

# append output to list.txt
ls -a >> list.txt

# all errors will be written to errors.txt
grep da * 2> errors.txt

# read from errors.txt
less < errors.txt

Question 31

Pipes in Bash

Answer 31

Let us use output of a program as the input of another.

command1 | command2 | command3

command1 sends its output to command2, which then passes it on to the input of command3.

Constructions like this are called pipelines.

This can be used to process data through several programs.

ls -l | grep .md$ | less

The output of ls is sent to the grep programme, which prints only files with a .md extension, and this output is finally passed to the less programme.

Question 32

List of commands in Bash

Answer 32

A sequence of one or more pipelines separated by one of the following operators:

; execute sequentially, one after another. the shell waits for the finish of each command

& if a command is terminated by &, it is executed asynchronously in a subshell (background)

&& right command will be executed only if its preceding command finishes successfully

|| right command will be executed only if its preceding command finishes unsuccessfully.

Question 33

Conditionals in Bash

Answer 33

if [[]]; then elif[[]]; then else fi

     if [[ `uname` == "Adam" ]]; then  
       echo "Do not eat an apple!"  
     elif [[ `uname` == "Eva" ]]; then  
       echo "Do not take an apple!"  
     else  
       echo "Apples are delicious!"  
     fi

Question 34

Conditionals for working with file system in Bash

Answer 34

[[ -e FILE ]] - if file exists

[[ -f FILE ]] - if file exists and is a file

[[ -d FILE ]] - if file exists and is a direcorry

[[ -s FILE ]] - if file exists and is not empty

[[ -r FILE ]] - if file exists and is readable

[[ -w FILE ]] - if file exists and is writable

[[ -x FILE ]] - if file exists and is executable

[[ -L FILE ]] - if file exists and is symbol

[[ FILE1 -nt FILE2 ]] - if FILE1 is newer than FILE2

[[ FILE1 -ot FILE2]] - if FILE1 is older than FILE2

Question 35

Conditionals for working with strings

Answer 35

[[ -z STR ]] - string is empty

[[ -n STR ]] - string is not empty

[[ STR1 == STR2 ]] - are equal

[[ STR1 != STR2 ]] - are not equal

Question 36

Arithmetic operators in Bash

Answer 36

= -eq

!= -ne

< -lt

> -gt

<= -le

> = -ge

Question 37

Combining expressions in Bash

Answer 37

Conditions may be combined using combining expressions

[[ ! EXPR ]] if EXPR is fals

[[ (EXPR) ]] returns the value of EXPR

[[ EXPR1 -a EXPR2 ]] - logical end

[[ EXPR1 -o EXPR2 ]] - logical or

Question 38

Case statements in Bash

Answer 38

case "$FRUIT" in
  (apple)
    echo 'I like them'
      ;;
  (banana)
    echo 'It is ok"
      ;;
  (orange|tangerine)
    echo 'i do not like' && exit 1
      ;;
  (*)
    echo 'unknown fruit'
      ;;
esac

Question 39

Kinds of loops in Bash

Answer 39

1. for
2. while
3. until
4. select

Question 40

For loop in Bash

Answer 40

for arg in elem1 elem2 ... elemN
do
  # statements
done

as a one liner (there needs to me a ; before do):

for i in {1..5}; do echo $i; done

C-like style:
for (( i = 0; i < 10; i++ )); do
echo $i
done

Question 41

When are for loops useful in Bash

Answer 41

!/bin/bash

When we want to perform the same operation over each file in a directory. E.g. to move all .sh files into the script folder and give them executable permissions.

for FILE in $HOME/*.sh; do
mv “$FILE” “${HOME}/scripts”
chmod +x “${HOME}/scripts/${FILE}”
done

Question 42

While loop in Bash (squares of nums from 0-9)

Answer 42

!/bin/bash

x = 0
while [[ $x -lt 10 ]]; do
  echo $(( $x * $x ))
  x = `expr $x + 1`
done

Question 43

Until loop

Answer 43

until [[ condition ]]; do
  #statements
done

Question 44

Loop control

Answer 44

prints all odd numbers from 0 - 9

There are the break and continue statements for situations where we need to:

• stop a loop before its normal ending or
• step over an iteration

for (( i = 0; i < 10; i++ )); do
if [[ $(($i % 2)) == 0 ]]; then continue; fi
echo $i
done

Question 45

Function in Bash

Answer 45

A sequence of commands grouped under a single name.

Calling a function is like calling any program, you just write the name and the function will be invoked.

Declaring a function:
my_func () {
  # statements
}

my_func # call my_func

Functions can take arguments and return a result - exit code

greeting () {
  if [[ -n $1 ]]; then
    echo "Hello, $1!"
  else
    echo "Hello, unknown!"
  fi
  return 0
}

greeting World # Hello, World!
greeting # Hello, unknown

Question 46

Exit status codes in Bash

Answer 46

0 - success

1 - failure

Question 47

Local variables in Bash functions

Answer 47

Declared using the local keyword, accessible only within a single function’s scope.

local local_var=”I’m a local variable”

Question 48

How can you make your own commands in terminal

Answer 48

Define functions in your ~/.bashrc file

Question 49

Define command aliases

Answer 49

alias ll=’ls -alF’

in ~/.bashrc

Question 50

Change a variable’s value in Bash

Answer 50

x=0

x = expr $x + 1

Question 51

Run scripts in debug mode

Answer 51

!/bin/bash -x

1. Use an option in a script’s shebang

2. Use the set command to debug just a part of a script. Options are turned on/off using -/+

echo "xtrace is turned off"
set -x
echo "xtrace is enabled"
set +x
echo xtrace is turned off again

Question 52

Debugging options in Bash

Answer 52

• f (noglob) disable filename expansion (globbing)
• i (interactive) script runs in interactive mode
• n (noexec) read commands, but don’t execute them (syntax check)
• t exit after first command
• v (verbose) print each command to stdout before executing it
• x (xtrace) print each command before executing it and expands commands

Question 53

Move files in Shell

Answer 53

mv - takes two arguments, the source and the destination

mv old new # renames old to new

mv file subdir/file # moves file to subdir file

mv file* subdir # moves all matched to ‘name*’ files to the ‘subdir’