← All topics

43 questions

Shell

Work through every question currently mapped to this canonical topic.

• What does this line in shell scripts means?: #!/bin/bash
  Answer

  #!/bin/bash is She-bang

  /bin/bash is the most common shell used as default shell for user login of the linux system. The shell’s name is an acronym for Bourne-again shell. Bash can execute the vast majority of scripts and thus is widely used because it has more features, is well developed and better syntax.

• True or False? When a certain command/line fails in a shell script, the shell script, by default, will exit and stop running
  Answer

  Depends on the language and settings used. If the script is a bash script then this statement is true. When a script written in Bash fails to run a certain command it will keep running and will execute all other commands mentioned after the command which failed.

  Most of the time we might actually want the opposite to happen. In order to make Bash exist when a specific command fails, use 'set -e' in your script.

• What do you tend to include in every script you write?
  Answer

  Few example:

  • Comments on how to run it and/or what it does
  • If a shell script, adding "set -e" since I want the script to exit if a certain command failed

  You can have an entirely different answer. It's based only on your experience and preferences.

• Today we have tools and technologies like Ansible, Puppet, Chef, ... Why would someone still use shell scripting?
  Answer

  • Speed
    • Flexibility
    • The module we need doesn't exist (perhaps a weak point because most CM technologies allow to use what is known as "shell" module)
    • We are delivering the scripts to customers who don't have access to the public network and don't necessarily have Ansible installed on their systems.
• How to define a variable with the value "Hello World"?
  Answer

  HW="Hello World

• How to define a variable with the value of the current date?
  Answer

  DATE=$(date)

• How to print the first argument passed to a script?
  Answer

  echo $1

• Write a script to print "yay" unless an argument was passed and then print that argument
  Answer

  echo "${1:-yay}"
  

• What would be the output of the following script?

  #!/usr/bin/env bash
  NINJA_TURTLE=Donatello
  function the_best_ninja_turtle {
          local NINJA_TURTLE=Michelangelo
          echo $NINJA_TURTLE
  }
  NINJA_TURTLE=Raphael
  the_best_ninja_turtle
  

  Answer

  Michelangelo

• Explain what would be the result of each command:
  • echo $0
    • echo $?
    • echo $$
    • echo $#

  🚧 Answer not written yet.

• What is $@?

  🚧 Answer not written yet.

• What is difference between $@ and $*?
  Answer

  $@ is an array of all the arguments passed to the script $* is a single string of all the arguments passed to the script

• How do you get input from the user in shell scripts?
  Answer

  Using the keyword read so for example read x will wait for user input and will store it in the variable x.

• How to compare variables length?
  Answer

  if [ ${#1} -ne ${#2} ]; then
      ...
  

• Explain conditionals and demonstrate how to use them

  🚧 Answer not written yet.

• In shell scripting, how to negate a conditional?

  🚧 Answer not written yet.

• In shell scripting, how to check if a given argument is a number?
  Answer

  regex='^[0-9]+$'
  if [[ ${var//*.} =~ $regex ]]; then
  ...
  

• How to perform arithmetic operations on numbers?
  Answer

  One way: $(( 1 + 2 )) Another way: expr 1 + 2

• How to perform arithmetic operations on numbers?

  🚧 Answer not written yet.

• How to check if a given number has 4 as a factor?
  Answer

  if [ $(($1 % 4)) -eq 0 ]; then

• What is a loop? What types of loops are you familiar with?

  🚧 Answer not written yet.

• Demonstrate how to use loops

  🚧 Answer not written yet.

• How do you debug shell scripts?
  Answer

  Answer depends on the language you are using for writing your scripts. If Bash is used for example then:

  • Adding -x to the script I'm running in Bash
  • Old good way of adding echo statements

  If Python, then using pdb is very useful.

• Running the following bash script, we don't get 2 as a result, why?

  x = 2
  echo $x
  

  Answer

  Should be x=2

• How to extract everything after the last dot in a string?
  Answer

  ${var//*.}

• How to extract everything before the last dot in a string?
  Answer

  ${var%.*}

• Generate 8 digit random number
  Answer

  shuf -i 9999999-99999999 -n 1

• Can you give an example to some Bash best practices?

  🚧 Answer not written yet.

• What is the ternary operator? How do you use it in bash?
  Answer

  A short way of using if/else. An example:

  [[ $a = 1 ]] && b="yes, equal" || b="nope"

• What does the following code do and when would you use it?

  diff

  Answer

  It is called 'process substitution'. It provides a way to pass the output of a command to another command when using a pipe | is not possible. It can be used when a command does not support STDIN or you need the output of multiple commands. https://superuser.com/a/1060002/167769

• What are you using for testing shell scripts?
  Answer

  bats

• Argument Check

  Objectives

  Note: assume the script is executed with an argument

  1. Write a script that will check if a given argument is the string "pizza"
  2. If it's the string "pizza" print "with pineapple?"
  3. If it's not the string "pizza" print "I want pizza!"
  Answer

  Objectives

  Note: assume the script is executed with an argument

  1. Write a script that will check if a given argument is the string "pizza"
  2. If it's the string "pizza" print "with pineapple?"
  3. If it's not the string "pizza" print "I want pizza!"

  Solution

  #!/usr/bin/env bash
  
  [[ ${1} == "pizza" ]] && echo "with pineapple?" || echo "I want pizza!"
  

• Basic Date

  Objectives

  1. Write a script that will put the current date in a file called "the_date.txt"
  Answer

  Objectives

  1. Write a script that will put the current date in a file called "the_date.txt"

  Solution

  #!/usr/bin/env bash
  
  echo $(date) > the_date.txt
  

• Count Chars

  Objectives

  1. Read input from the user until you get empty string
  2. For each of the lines you read, count the number of characters and print it

  Constraints

  1. You must use a while loop
  2. Assume at least three lines of input
  Answer

  Objectives

  1. Read input from the user until you get empty string
  2. For each of the lines you read, count the number of characters and print it

  Constraints

  1. You must use a while loop
  2. Assume at least three lines of input

  Solution

  #!/usr/bin/env bash
  
  echo -n "Please insert your input: "
  
  while read line; do
      echo -n "$line" | wc -c
      echo -n "Please insert your input: "
  done
  

• Directories Comparison

  Objectives

  1. You are given two directories as arguments and the output should be any difference between the two directories
  Answer

  Objectives

  1. You are given two directories as arguments and the output should be any difference between the two directories

  Solution 1

  Suppose the name of the bash script is dirdiff.sh

  #!/bin/bash
  
  if test $# -ne 2
  then
  	echo -e "USAGE: ./dirdiff.sh directory1 directory2"
  	exit 1
  fi
  
  # check for the checksums.
  # If both the checksums same, then both directories are same
  if test `ls -1 $1 | sort | md5sum | awk -F "  " '{print $1}'` == `ls -1 $2 | sort | md5sum | awk -F "  " '{print $1}'`
  then
  	echo -e "No difference between the 2 directories"
  	exit 0
  fi
  
  diff -q $1 $2
  

  Solution 2

  With gnu find, you can use diff to compare directories recursively.

  diff --recursive directory1 directory2
  

• Empty Files

  Objectives

  1. Write a script to remove all the empty files in a given directory (including nested directories)
  Answer

  Objectives

  1. Write a script to remove all the empty files in a given directory (including nested directories)

  Solution

  #! /bin/bash
  for x in *
  do
      if [ -s $x ]
      then
          continue
      else
          rm -rf $x
      fi
  done
  

• Shell Scripting - Factors

  Objectives

  Write a script that when given a number, will:

  • Check if the number has 2 as factor, if yes it will print "one factor"
  • Check if the number has 3 as factor, if yes it will print "one factor...actually two!"
  • If none of them (2 and 3) is a factor, print the number itself
  Answer

  Objectives

  Write a script that when given a number, will:

  • Check if the number has 2 as factor, if yes it will print "one factor"
  • Check if the number has 3 as factor, if yes it will print "one factor...actually two!"
  • If none of them (2 and 3) is a factor, print the number itself

  Solution

  #!/usr/bin/env bash
  
  (( $1 % 2 )) || res="one factor"
  (( $1 % 3 )) || res+="...actually two!"
  
  echo ${res:-$1}
  

• Files Size

  Objectives

  1. Print the name and size of every file and directory in current path

  Note: use at least one for loop!

  Answer

  Objectives

  1. Print the name and size of every file and directory in current path

  Note: use at least one for loop!

  Solution

  #!/usr/bin/env bash
  
  for i in $(ls -S1); do
      echo $i: $(du -sh "$i" | cut -f1)
  done
  

• Great Day

  Objectives

  1. Write a script that will print "Today is a great day!" unless it's given a day name and then it should print "Today is "

  Note: no need to check whether the given argument is actually a valid day

  Answer

  Objectives

  1. Write a script that will print "Today is a great day!" unless it's given a day name and then it should print "Today is "

  Note: no need to check whether the given argument is actually a valid day

  Solution

  #!/usr/bin/env bash
  
  echo "Today is ${1:-a great day!}"
  

• Shell Scripting - Hello World

  Objectives

  1. Define a variable with the string 'Hello World'
  2. Print the value of the variable you've defined and redirect the output to the file "amazing_output.txt"
  Answer

  Objectives

  1. Define a variable with the string 'Hello World'
  2. Print the value of the variable you've defined and redirect the output to the file "amazing_output.txt"

  Solution

  #!/usr/bin/env bash
  
  HW_STR="Hello World"
  echo $HW_STR > amazing_output.txt
  

• It's Alive!

  Objectives

  1. Write a script to determine whether a given host is down or up
  Answer

  Objectives

  1. Write a script to determine whether a given host is down or up

  Solution

  #!/usr/bin/env bash
  SERVERIP=<IP Address>
  NOTIFYEMAIL=test@example.com
  
  ping -c 3 $SERVERIP > /dev/null 2>&1
  if [ $? -ne 0 ]
  then
     # Use mailer here:
     mailx -s "Server $SERVERIP is down" -t "$NOTIFYEMAIL" < /dev/null
  fi
  

• Number of Arguments

  Objectives

  • Write a script that will print "Got it: " in case of one argument
  • In case no arguments were provided, it will print "Usage: ./ "
  • In case of more than one argument, print "hey hey...too many!"
  Answer

  Objectives

  • Write a script that will print "Got it: " in case of one argument
  • In case no arguments were provided, it will print "Usage: ./ "
  • In case of more than one argument, print "hey hey...too many!"

  Solution

  #!/usr/bin/env bash
  
  set -eu
  
  main() {
    case $# in
      0) printf "%s" "Usage: ./<program name> <argument>"; return 1 ;;
      1) printf "%s" "Got it: $1"; return 0 ;;
      *) return 1 ;;
    esac
  }
  
  main "$@"
  

• Sum

  Objectives

  1. Write a script that gets two numbers and prints their sum
  2. Make sure the input is valid (= you got two numbers from the user)
  3. Test the script by running and passing it two numbers as arguments

  Constraints

  1. Use functions
  Answer

  Objectives

  1. Write a script that gets two numbers and prints their sum
  2. Make sure the input is valid (= you got two numbers from the user)
  3. Test the script by running and passing it two numbers as arguments

  Constraints

  1. Use functions

  Solution

  #!/usr/bin/env bash
  
  re='^[0-9]+$'
  
  if ! [[ $1 =~ $re && $2 =~ $re ]]; then
      echo "Oh no...I need two numbers"
      exit 2
  fi
  
  function sum {
      echo $(( $1 + $2 ))
  }
  
  sum $1 $2