Introduction to Shell Scripting

Last time: Debugging and Profiling Tools

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

Announcements

• Project #5 (debugging) due Wednesday
• Should examine bash tutorials and references (see schedule and related resources for links)
• P3 scores: Low 61, Mean 87, High 100.
• SQ3 scores: Low 2, Mean 6.6, High 10.
• Today's schedule
  • Short Quiz #4
  • Return Exam #1: Low 65, High 100, Mean 84.
  • Introduction to Shell Scripting
    • See also UNIX in a Nutshell (via safari), chapters 3&4, focus on bash

Automating UNIX Operations

• You know many UNIX commands
• We will cover more...
• You know how to program in C and other languages
• You have also seen how to connect UNIX commands with pipes
• UNIX commands can also be placed into shell scripts
  • What is a shell?
  • What is a script?
• Try writing one

What does the shell do?

• After you type a command, the shell
• Performs history substitution (if necessary)
• Breaks up the command line into tokens
• Updates the shell history
• Processes quotes
• Performs alias substitution
• Sets up redirection, background processing, and pipes
• Performs variable substitution (e.g., $user)
• Performs command substitution (ls `cat filelist.txt`)
• Performs filename substitution (handling wildcards, etc.)
• Executes the program

Types of commands

• A command that you type can be
• An alias
• A function
• A built-in command
• An executable program
• The shell will attempt each of these possibilities in turn until it finds a match

Bash shell constructs

• First line contains the path to the shell to execute the script

  #!/bin/bash or #!/usr/bin/bash
  

• Comment lines

  # anything following a hash symbol
  

• Display output

  echo "Hello world."
  

Bash shell constructs

• Local variables (case sensitive)

  variable_name=value
  declare variable_name=value
  name="John Smith"
  x=5
  

• Global variables (environment variables)

  export VARIABLE_NAME=value
  export PATH=/bin:/usr/bin:.
  declare -x VARIABLE_NAME=value
  

• Use of variables (using $)

  echo $variable_name
  echo $PATH
  

Bash shell constructs

• Reading user input (read a line at a time)

  echo "Enter your name (first and last):"
  read wholename
  read firstname lastname
  

• Working with script arguments (e.g., scriptname arg1 arg2 arg3)

  echo $1 $2 $3       # individual arguments
  echo $*             # all arguments
  echo $#             # number of arguments
  

• Arrays in Bash

  declare -a array_name=(word1 word2 word3 ...)
  declare -a fruit=(apples pears plums)
  echo "I like to eat" ${fruit[0]}
  

Bash shell constructs

• Command substitution

  somevar=`command`;
  somevar=$(command)
  echo $somevar    # show result of command
  
  echo "Today is `date`"
  echo "Today is $(date)"
  

• Arithmetic

  declare -i variable name    # integer variable
  declare -i ii=44+44
  (( n= 5 + 5 ))
  echo $ii $n
  

• Operators for Expressions - similar to C language operators
  • Includes ==, !=, &&, ||, !, >, >=, <, <=

Bash shell constructs

• Conditional statement -- if

  if command
  then
     block of statements
  fi
  
  if [[ expression ]]
  then
     block1
  else
     block2
  fi
  
  if (( numeric expression ))
  then 
    block1
  else if (( another expression)
  then
    block2
  elif [[ boolean expression ]]
  then
    block3
  fi
  
  if (( 3+3 )); then echo yes; fi
  

Bash shell constructs

• Conditional statement -- case

  case variable_name in
    pattern1)
     statements
      ;;
    pattern2)
     statements
      ;;
    pattern3)
      ;;
  esac
  
  case "$color" in
    blue)
      echo $color is blue
       ;;
    red|orange)
      echo $color is red or orange
       ;;
    *)
      echo not a match
       ;;
  esac
  

Bash shell constructs

• Loops: while and until

  while [[ string expression ]]
  do
     block of statements
  done
  
  until command
  do
     block of statements
  done
  

Bash shell constructs

• Loops: for and select

  for variable in word_list
  do
     block of statements
  done
  
  for color in red green blue
  do
     echo $color
  done
  
  select variable in word_list
  do
     block
  done
  
  select fname in red green blue;
  do
          echo you picked $fname \($REPLY\)
          break;
  done
  

Bash shell constructs

• Functions in bash

  function_name() {
    block of code
  }
  
  function function_name {
    block of code
  }
  
  ...
  function_name