More Intro. to Shell Scripting

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Today's schedule
  • Take short quiz #6
  • Return homework 3 -- range 5-10, mean 8.6
    • 6 points for code
    • 1 point for naming the style used
    • 1 point for say why the style was chosen
    • 2 points for describing the (major) changes made
  • Continue Introduction to Shell Scripting

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 in turn

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."
  

• 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
  

Bash shell constructs

• Use of variables (using $)

  echo $variable_name
  echo $PATH
  

• 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
  
  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, until, for and select

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