shell basics

Question 1

How do you start a script?

Answer 1

Shabang
#!

Question 2

how do you tell your interpreter which scripting language you wanna use?

Answer 2

After the Shabang (#!) you type it in.

i.e. you can be like /bin/bash to tell it you are using bash

Question 3

how to make a comment?

Answer 3

then write after the hash symbol

Question 4

what does echo do?

Answer 4

echo writes arguments to standard output (1).

Question 5

how do you execute a script via the terminal?

Answer 5

./file_name

so period then the backslash followed by file_name

Question 6

how do you make a file to an executable if it is not already?

Answer 6

chmod 755 file_name
defines who can do what with the file. 
7 means read/write/execute. 
6 means read/write. 
5 means read/execute. 
4 means read only.
3 means write/execute.
2 means write only.
1 means execute only.
0 means cant do anything.

Question 7

what do variables start with?

Answer 7

letter or underscore and by convention their lowercase.

Except if they are environment variables then all caps.

Question 8

how would you create a variable for a name.

Answer 8

variable_name=”Saidname”

no spaces

Question 9

how do you declare a constant?

Answer 9

declare -r NUM1=5

Question 10

how do you display the output of an algorithm? lets say 2*10 as example

Answer 10

echo $(( 2*10))
or
num2=2
num10=10
echo $(( num2*num10))

works with numbers or variables

Question 11

how can you use floating point numbers in your shell script?

Answer 11

you can call python in your code and assign the output to a variable to be displayed or used later

Question 12

how do you call python in a bash script?

Answer 12

python -c “here use python”

i.e. num9= $(python -c “print 3.1+4.5”)
echo $num9

Question 13

what does cat do?

Answer 13

prints a file or any string you pass to it.

Question 14

how to create a function in a script?

Answer 14

function_name(){
   //stuff inside the function
  return
}

Question 15

how to call a function previously declared?

Answer 15

simply call the function_name

Question 16

how do you use the attributes (arguments) passed to a function?

Answer 16

use $1 or $2 or $3 ect.

it depends on how many attributes are sent to the function. but the numbers correlate with the

Question 17

how do you “return” a value after function is used.

Answer 17

you pass back whatever value you wish to return by saying echo $variable_name

Question 18

how do you assign a variable the returning value of a function call?

Answer 18

variable=$(function arg1 arg2 ect)

then you can echo the variable to display it.

Question 19

how to prompt a user for input?

Answer 19

read -p “question or prompt” store_variable

the -p says we want the input of whatever is said AFTER the prompt

Question 20

how to say greater then or equal to in the script?

Answer 20

-ge

Question 21

how to say equal to in the script?

Answer 21

-eq

Question 22

how to say not equal to in the script?

Answer 22

-ne

Question 23

how to say greater than in the script

Answer 23

-gt

Question 24

how to say less than or equal to in the script

Answer 24

-le

Question 25

how to say less than in the script

Answer 25

-lt

Question 26

how to say less than or greater than using < or >?

Answer 26

you can use < or > if they are within double parentheses (())

Question 27

how to make a file in a directory

Answer 27

-d sample_dir

Question 28

how to delete a file

Answer 28

rm sample_file

Question 29

how to delete a directory

Answer 29

rmdir sample_dir

Question 30

how to check if a file exists?

Answer 30

if [ -e “$file” ]; then
echo “$file exists”
fi

Question 31

check if a valid expression

Answer 31

lets say we want to validate the date entered by user.
then you can say:
check=”^[0-9]{8}$”
variable check means that user should enter 8 numbers from 0-9. the ^ means its the beginning of string and the $ means its the end of string

Question 32

what is the IFS?

Answer 32

input field separator. you can decide what you want the special symbol that separates your arguments to be (for example the ‘,’ or ‘ ‘ or ‘.’)

Question 33

how would you get rid of all white spaces in a string?

Answer 33

variable = ${variable//[[:blank:]]//}

this replaces all blanks with nothing.

Question 34

in a string how would you replace ‘the’ with ‘a’?

Answer 34

variable=”the string to test”

echo “${variable}//the/a}”

Question 35

how to get the length of a string?

Answer 35

str=”string of something”

length=${#str}

Question 36

how do you grab everything after, lets say the letter ‘a’ in as string?

Answer 36

str=”a good question to ask”

mod=${str#*a}

Question 37

how to end an if statement?

Answer 37

fi

Question 38

how to end a while loop?

Answer 38

done

Question 39

how to end a case statement?

Answer 39

esac

Question 40

how to end an until loop?

Answer 40

done

Question 41

how to output things inside a file?

Answer 41

while read arg1 arg2 arg3; do
printf “Arg1: ${arg1}\nArg2: ${arg2}\nArg3: ${arg3}\n”
done < sample.txt

** < pipes the text file to this line