0110 | Scripting

Scripting

Bash Scripting | Basics

• shebang | #!
• specify the shell after the shebang | #!/bin/bash
• example | hello-world

  #!/bin/bash
  
  # This is a comment
  echo "Hello world"
• running scripts
  • bash hello-world.sh
    • here the bash program is the executable program
  • or by making the script executable
    • chmod +x hello-world.sh --> ./hello-word.sh
• example | executing commands example

  #!/bin/bash
  
  user=$(whoami)
  hostname=$(hostname)
  directory=$(pwd)
  # display info
  echo "User=[$user] Host=[$hostname] Working dir=[$directory]"
  echo "Contents:"
  ls

Bash Scripting | Control Structures

• if | -d (dirs) | -e (files)
• example | if-examples.sh

  #!/bin/bash
  
  if [[ -d /etc/ ]]; then
      echo /etc/ is indeed a directory
  fi      
  
  # check to see if a file exists
  if [[ -e sample.txt ]]; then
      echo the file sample.txt exists
  else    
      echo the file sample.txt does NOT exist
  fi      
  
  TEST_VAR="test"
  if [[ $TEST_VAR == "test" ]]; then
      echo TEST_VAR has a value of "test"
  elif [[ $TEST_VAR == "again" ]]; then
      echo TEST_VAR has a value of "again"
  else    
      echo TEST_VAR has an unknown value
  fi
• numerical values | -ne | -eq | -gt | -ge | -lt | -le
• string values | == | != | < | >
• example | comparison-examples.sh

  #!/bin/bash
  
  # create variables
  x=1
  echo x=["$x"]
  y=2
  echo y=["$y"]
  z=2
  echo z=["$z"]
  
  # numeric -- not equals
  if [[ "$x" -ne "$y" ]]; then
      echo ["$x"] ne ["$y"]
  fi
  
  # numeric -- equals
  if [[ "$y" -eq "$z" ]]; then
      echo ["$y"] eq ["$z"]
  fi
  
  # numeric -- greater than
  if [[ "$y" -gt "$x" ]]; then
      echo ["$y"] gt ["$x"]
  fi
  
  # numeric -- greater than or equal to
  if [[ "$y" -ge "$z" ]]; then
      echo ["$y"] ge ["$z"]
  fi
  
  # numeric -- less than
  if [[ "$x" -lt "$y" ]]; then
      echo ["$x"] lt ["$y"]
  fi
  
  # numeric -- less than or equal to
  if [[ "$y" -le "$z" ]]; then
      echo ["$y"] le ["$z"]
  fi
  
  # ***************** string comparison ********************
  a="A"
  echo a=["$a"]
  b="B"
  echo b=["$b"]
  anotherA="A"
  echo anotherA=["$anotherA"]
  
  # string -- equals
  if [[ "$a" == "$anotherA" ]]; then
      echo ["$a"] "==" ["$anotherA"]
  fi
  # string -- not equals
  if [[ "$a" != "$b" ]]; then
      echo ["$a"] "!=" ["$b"]
  fi
  # string -- less than
  if [[ "$a" < "$b" ]]; then
      echo ["$a"] "<" ["$b"]
  fi
  # string -- greater than
  if [[ "$b" > "$a" ]]; then
      echo ["$b"] ">" ["$a"]
  fi
• example | case-example.sh

  #!/bin/bash
  
  # switch off of the first command line argument
  case $1 in
  [1-3])
      message="Argument is between 1 and 3 inclusive"
      ;;
  [4-6])
      message="between 4 and 6 incl"
  
      ;;
  [7-9])
      message="between 7 and 9 inc"
      ;;
  1[0-9])
      message="between 10 and 19 inc"
      ;;
  *)
      message="I don't understand the argument or it is missing"
      ;;
  esac
  
  # print msg
  echo $message

Bash Scripting | Loops

• Definite loops
  • know the number of loops before it starts
  • most common: for loop
• Indefinite Loops
  • do NOT know the number of loops until the end
  • most common: while loop
• Indefinite loop != Infinite loop
• example | loop-examples.sh

  #!/bin/bash
  
  echo ---------------------------------------
  echo For loops
  
  echo Print out a hard-coded sequence
  for i in 1 2 3 4 5; do
      echo Index=[$i]
  done
  
  echo Print out a generated sequence
  for i in {1..5}; do
      echo Index=[$i]
  done
  
  
  echo Print out a generated sequence using a 3-expression format
  # Note: Double parenthesis are used since we are 
  # doing arithmetic
  for(( i=1; i<=5; i++))
  do
      echo Index=[$i]
  done
  
  echo Print out the last line of each shell script
  for FILE in *.sh
  do 
      echo ===================================
      echo File=[$FILE]
      tail -n 1 $FILE
  done
  echo ''
  
  #******* while loop ********************
  echo ----------------------------
  echo While loop
  
  echo Executing a while loop to countdown to blastoff
  counter=5
  while [[  $counter -gt 0 ]]; do
      echo Countdown [$counter]
      counter=$(($counter - 1))
  done
  echo Blastoff

Bash Scripting | Examples

• command line arguments

  • $0 | the name of the script
  • $# | argument count
  • $@ | argument list
    • iterate-able
    • $0 is not included
  • $* | argument string
    • all of the args in 1 string
  • arg 12 is ${12}
    • NOT $12 (=$1 and "2")

• example | command-line-arguments-example.sh

  #!/bin/bash
  
  # Processing command line arguments
  
  echo Name of script [$0]
  echo Command line argument count [$#]
  
  # iterate through each argument
  for arg in $@; do
      echo Argument [$arg]
  done
  
  echo All arguments [$*]
  
  # use parenthesis for arguments with numbers 10 or larger
  if [ "${12}" != "" ]; then
      echo Argument 12 is [${12}]
      echo Argument 12 is NOT [$12]
  fi

• example | generate-password.sh | !not working!

  #!/bin/bash
  
  # Grab the command line arguments
  passwd_word_count=$1
  separator=$2
  
  # Start with a blank password
  password=''
  
  # Get the total number of words in the word list
  total_word_count=`wc -l ../wordlist.txt | awk '{print $1;}'`
  
  # Build the password using the specified number of words
  for (( i=1; i<=$passwd_word_count; i++ ))
  do
      # Generate a random number using OpenSSL to by cryptographically secure
      rand_num_hex=`openssl rand -hex 4`
      rand_num_dec=$((16#$rand_num_hex))
  
      # Use the random number as an index into the word list
      word_index=$(($rand_num_dec % $total_word_count))
      random_word=`awk -v idx="$word_index" '{if (NR==idx) print $1}'`
  
      # Capitalize the word
      random_word_upper=`echo ${random_word^}`
  
      # Insert a separator if this isn't the first word in the passwod
      if [[ ${password} -gt 0 ]]; then
          password=$password$separator$random_word_upper
      else
          password=$random_word_upper
      fi
  done
  
  echo $password
  • running it | ./generate-password.sh 4 '-'