Bash Programming Exercises

Math

Exercise 9

1. Look at the man pages for bc.

2. Try doing some math in bc interactively.

3. Try writing some equations in a file and then provide that file as an argument to bc.

Solution to

1. bc looks like a programmable calculator with its own language. Nowadays one would do the calculations using a programming language that they are cognizant of. Still bc could be easier to integrate in shell tasks, because it comes with most Unix-based operating systems (without installation).

   You probably read the word POSIX many times. POSIX is an official standard which defines which shell tools a operating system should have, what the tools should do, and even how the tool interfaces look like. The goal is interoperability between POSIX-compliant operating systems. In other words you should be able to move your shell scripts from a POSIX-compliant system to another, e.g., from macOS to Ubuntu.

   Did you also notice that the bc has the option --standard? If not, look what it does. Even most commands try to adhere to the POSIX standard, they also add extensions for better usability. If you use these extensions, then your program may not be easily transferable to another operating system, in other words not portable as in the abbreviation POSIX - portable operating system interface. This is for users who do not change their operating systems not a problem at all.

2. We see that bc has its own command prompt like Matlab. Like in other decent command prompts you can access command history by using arrow keys.

3. Listing 1 pythagorean_triple_13_12_5.bc

   y=sqrt(13^2 - x^2)
   x=12
   y
   

Variables

Exercise 10

1. Write a Bash program where you assign two numbers to different variables, and then the program prints the sum of those variables.

2. Write another Bash program where you assign two strings to different variables, and then the program prints both of those strings. Write a version where the strings are printed on the same line, and a version where the strings are printed on different lines.

3. Write a Bash program that prints the number of arguments provided to that program multiplied by the first argument provided to the program.

Solution to

1. Listing 2 sum_a_b.sh

   a=2 b=1; echo $((a+b))
   

   Execute:

   sh sum_a_b.sh
   

2. a=Bunsen
   b=burner
   echo $a $b      # on the same line
   echo -e $a\\n$b  # on different lines (use \\n because the arguments are expanded before running the actual command)
   echo $a
   echo $b         # also on different lines, more readable
   

   -e option for echo enables the interpretation of backslash characters. A more readable way would be to use echo twice.

3. Listing 3 number_of_arg_multiplied_by_first_arg.sh

   echo $(($# * $1))
   

   sh number_of_arg_multiplied_by_first_arg.sh 2 3 4
   

User Input

Exercise 11

1. Write a script that asks the user for an adjective, a noun, and a verb, and then use those words in a sentence (like Mad Libs).

Solution to

Listing 4 mad_libs.sh

echo Let us play Mad Libs!
echo Give me an adjective!
read adjective
echo Give me a noun!
read noun
echo Give me a verb!
read verb
echo The $adjective $noun used to $verb often in the nineties, but nowadays people use less $adjective $noun.

Logic and If/Else

Exercise 12

1. Write a Bash script that takes a string as an argument and prints “how proper” if the string starts with a capital letter.

2. Write a Bash script that takes one argument and prints “even” if the first argument is an even number or “odd” if the first argument is an odd number.

3. Write a Bash script that takes two arguments. If both arguments are numbers, print their sum, otherwise just print both arguments.

4. Write a Bash script that prints “Thank Moses it’s Friday” if today is Friday. (Hint: take a look at the date program).

Solution to

1. Listing 5 evaluate_punctuation.sh

   [[ $# != 1 ]] && echo Usage $0 STRING && exit 1
   if [[ $1 =~ ^[[:upper:]] ]]
   then
   	echo how proper!
   fi
   

   sh evaluate_punctuation.sh Hello!
   sh evaluate_punctuation.sh how?
   

   Note that we do not have to specify the whole word ^[[:upper:]].*. The =~ expression returns true if only part of the string matches. This is the same reason why the regular expression [[:upper:]] would fail, because this regex would return true if there is any capital letter in the word, e.g., cApital.

2. Listing 6 even_or_odd.sh

   [[ $# != 1 ]] && echo Usage $0 NUMBER && exit 1
   if (($1 % 2 == 0))
   then
   	echo even
   else
   	echo odd
   fi
   

   sh even_or_odd.sh 4
   sh even_or_odd.sh 3
   

3. Listing 7 sum_of_two_or_print.sh

   [[ $# != 2 ]] && echo Usage $0 NUMBER_OR_STR1 NUMBER_OR_STR2 && exit 1
   if [[ $1 =~ ^[[:digit:]]+$ && $2 =~ ^[[:digit:]]+$ ]]
   then
   	echo $(( $1 + $2 ))
   else
   	echo $@
   fi
   

   sh sum_of_two_or_print.sh 1 5
   sh sum_of_two_or_print.sh High 5
   

   Note that ^ and $ needed to match the whole word but not part of it.

4. Listing 8 is_it_friday.sh

   [[ $(date +%a) == Fri ]] && echo Thank Moses it\'s Friday!
   exit 0  # always exit with success to not annoy jupytext
   

   sh is_it_friday.sh
   

Arrays

Exercise 13

1. Write a bash script where you define an array inside of the script, and the first argument for the script indicates the index of the array element that is printed to the console when the script is run.

2. Write a bash script where you define two arrays inside of the script, and the sum of the lengths of the arrays are printed to the console when the script is run.

Solution to

1. Listing 9 select_from_array_by_index.sh

   [[ $# != 1 ]] && echo Usage $0 INDEX_THAT_SHOULD_BE_PRINTED && exit 1
   INDEX=$1
   array=({1..100})
   echo ${array[$INDEX]}
   

   sh select_from_array_by_index.sh 5
   sh select_from_array_by_index.sh 94
   

2. Listing 10 sum_of_length_of_two_arrays.sh

   arr1=({1..37})
   arr2=({1..13})
   echo $(( ${#arr1[*]} + ${#arr2[*]} ))
   

   sh sum_of_length_of_two_arrays.sh
   

Braces

Exercise 14

1. Create 100 text files using brace expansion.

Solution to

touch f{01..100}.txt

Loops

Exercise 15

• Write several programs with three levels of nesting and include FOR loops, WHILE loops, and IF statements. Before you run your program try to predict what your program is going to print. If the result is different from your prediction try to figure out why.

• Enter the yes command into the console, then stop the program from running. Take a look at the man page for yes to learn more about the program.

Solution to

1. for f in $(ls)
   do
   	if [[ $(head $f) =~ ^[[:digit:]]+$ ]]
   	then
   		echo processing $f
   		max_number_of_files_with_this_directory_prefix=$(head $f)
   		echo $f wants to have at least $max_number_of_files_with_this_directory_prefix files
   		# the file $f is inclusive
   
   		while [[ $(ls $f* | wc -l) -lt $max_number_of_files_with_this_directory_prefix ]]
   		do
   			touch $f.$RANDOM
   		done
   	else
   		echo skipping $f
   	fi
   done
   

   This script searches for files containing only numbers in the first line. Then for every such a file the script stores the number as the max_number_of_files_with_this_directory_prefix ensures that there are as many files prefixed with the filename $f as indicated by this number.

2. yes command can be used for overriding shell [yes/no] prompts which want to continue or break dependent on user input. When we automatize some tasks, we may want that our program is self-sufficient and does not need any user input. If there is a command which needs user input we can feed this program with yes. For example in Ubuntu or Debian apt install vim will stop at Do you want to continue? [Y/n] but yes | apt install vim will provide the needed input by providing y in the standard input.

Functions

Below this list of exercises you can find examples of how these programs should work when used on the command line.

Exercise 16

1. Write a function called plier which multiplies together a sequence of numbers.

2. Write a function called isiteven that prints 1 if a number is even or 0 a number is not even.

3. Write a function called nevens which prints the number of even numbers when provided with a sequence of numbers. Use isiteven when writing this function.

4. Write a function called howodd which prints the percentage of odd numbers in a sequence of numbers. Use nevens when writing this function.

5. Write a function called fib which prints the number of fibonacci numbers specified.

plier 7 2 3

## 42

isiteven 42

## 1

nevens 42 6 7 9 33

## 2

howodd 42 6 7 9 33

## .60

fib 4

## 0 1 1 2

fib 10

## 0 1 1 2 3 5 8 13 21 34

Solution to

1. function plier {
   	local product=1
   	
   	for multiplicand in $@
   	do
   		((product*=$multiplicand))
   	done
   	echo $product
   }
   plier 7 2 3
   

2. function isiteven {
   	input=$1
   	echo $(($input % 2 == 0))
   }
   isiteven 42
   

3. function isiteven {
   	input=$1
   	echo $(($input % 2 == 0))
   }
   
   function nevens {
   	local evens_count=0
   	
   	for n in $@
   	do
   		(( evens_count += $(isiteven $n) ))
   	done
   
   	echo $evens_count
   }
   nevens 42 6 7 9 33
   

4. function isiteven {
   	input=$1
   	echo $(($input % 2 == 0))
   }
   
   function nevens {
   	local evens_count=0
   	
   	for n in $@
   	do
   		(( evens_count += $(isiteven $n) ))
   	done
   
   	echo $evens_count
   }
   
   function howodd {
   	echo $(( 100 * ($# - $(nevens $@)) / $# )) 
   }
   howodd 42 6 7 9 33
   

5. function fib {
   	fibonacci_number_count=$1
   	sequence=()
   	for i in $(seq $fibonacci_number_count)
   	do
   		if (($i == 1))
   		then
   			sequence+=(0)
   		elif (($i == 2))
   		then
   			sequence+=(1)
   		else
   			# note that i begins at 1, but the array is zero-indexed.
   			# to get the previous number we have to -1, and then -1 again to
   			# get the actual index
   			prev_number=${sequence[$i-2]}
   			prev_prev_number=${sequence[$i-3]}
   			sequence+=( $(( $prev_number + $prev_prev_number )) )
   		fi
   	done
   	echo ${sequence[*]}
   }
   fib 10
   

Writing Programs

Below this list of exercises you can find examples of how the programs described here should work when used on the command line.

Exercise 17

1. Make a script executable.

2. Put that script in a directory that you create and make that directory part of your PATH.

3. Write a program called range that takes one number as an argument and prints all of the numbers between that number and 0.

4. Write a program called extremes which prints the maximum and minimum values of a sequence of numbers.

range 6

## 0 1 2 3 4 5 6

range -3

## -3 -2 -1 0

extremes 8 2 9 4 0 3

## 0 9

Solution to

1. chmod +x ~/scripts/SCRIPT

2. echo 'PATH=~/scripts:$PATH' >> ~/.bash_profile

3. function range {
   	if (($1 > 0))
   	then
   		echo $(eval echo {0..$1})
   	else
   		echo $(eval echo {$1..0})
   	fi
   }
   range 6
   range -3
   

   To reuse this program again, we can paste this function into .bashrc. Instead of a function we could also use a script.

4. function extremes {
   	local minimum=$1
   	local maximum=$1
   	shift  # remove first element from parameters
   
   	for n in $@
   	do
   		if (($n > maximum))
   		then
   			maximum=$n
   		fi
   
   		if (($n < minimum))
   		then
   			minimum=$n
   		fi
   	done
   	echo $minimum $maximum
   }
   extremes 8 2 9 4 0 3
   

Cleaning up created files

rm evaluate_punctuation.sh
rm even_or_odd.sh
rm f*.txt
rm is_it_friday.sh
rm number_of_arg_multiplied_by_first_arg.sh
rm select_from_array_by_index.sh
rm sum_a_b.sh
rm sum_of_length_of_two_arrays.sh
rm sum_of_two_or_print.sh