Index

1. Command arguments

• typical examples include:
  • ftp somemachine.co.uk
  • cat --help
• we may want our scripts to handle arguments as well
  • fortunately we can do this

• #!/bin/sh
  
  echo $1 $2 $3

• create a file called args1.sh and make this script executable
  

2. Command arguments

• if we type: ./args1.sh apple pear orange then we see the output:

• apple pear orange

• so we see that $1 represents the first argument, $2 the second etc
• now we alter script to contain:

• #!/bin/sh
  
  shift 1
  echo $1 $2 $3

3. Shift

• the shift operator moves all argument values to the preceding argument index

4. Usage

• every useful script should have a usage description
  • under GNU/Linux the tradition is that this is printed
    • either if the user specified -h or --help
    • or if the user specified an incorrect argument or no arguments
• let us add a function usage to autoftp

5. Improving autoftp

• #!/bin/sh
  
  scriptname=$0   # $0 is the name of the program
  
  function usage () {
     cat <<EOF
  Usage: scriptname [-p] [-v] [-h] filename
     -p   probes the ftp sites and check urls in
          filename
     -v   executes and prints out verbose messages
     -h   displays basic help
  EOF
  }
  
  
  if [ "$1" = "-h" ]
  then
     usage
     exit 0
  fi

6. Better argument handling

• previous method rather ad hoc
  • we might also like to combine arguments so
    • user could type: autoftp -vp autoftprc rather than
    • autoftp -v -p autoftprc
• bash provides us with a better mechanism, getopts

7. getopts

• takes two arguments
  • the first is a string
  • second is a variable
• the string defines the various options and whether they take further arguments

8. getopts example

• #!/bin/sh
  
  progname=$0
  
  function usage () {
     cat <<EOF
  Usage: $progname [-a] [-b name] [-c] etc
  EOF
     exit 0
  }
  
  while getopts ":ab:ch" opt; do
     case $opt in
  
     a )  echo "found -a" ;;
     b )  echo "found -b and $OPTARG is after -b" ;;
     c )  echo "found -c" ;;
     h )  echo "found $opt" ; usage ;;
     \?)  usage ;;
     esac
  done
  
  shift $(($OPTIND - 1))
  
  
  echo "the remaining arguments are: $1 $2 $3"

9. getopts example run

• we can run the script args3.sh and see the effect of different arguments

• found h
  Usage: ./args3.sh [-a] [-b name] [-c] etc

• note that -h was detected and usage called
  • also note that $opt was set to h

• ./args3.sh -a -c
  found -a
  found -c
  the remaining arguments are:

10. getopts example run

• ./args3.sh -a -c foo bar mumble
  found -a
  found -c
  the remaining arguments are: foo bar mumble

• ./args3.sh -a -b file foo bar mumble
  found -a
  found -b and file is the argument to -b
  the remaining arguments are: foo bar mumble

• note that the line shift $(($OPTIND - 1)) removes all arguments that have been processed by getopts
  

11. the getopts string

• ":ab:ch"
• initial colon
  • means getopts should not produce an internal error message if the user passes an option which is not declared in the string
• a means -a is legal
• b: means -b is legal
  • furthermore -b takes an additional parameter
    • obtained via: $OPTARG
• c means -c is legal
  • h means -h is legal

12. getopts justification

• getopts may seen complex, but we can just use the above template and modify it slightly whenever we need to manipulate command arguments
• the getopts mechanism appears in other languages, C
  • learn once use many times..

13. Tutorial exercise

• improve your copy of autoftp to include command line argument handling using getopts
  • it should set probeonly=yes if the -p option was specified
  • it should set verbose=yes if the -v option was specified
  • -h should be honoured
• remember to initialise the variables at the beginning

14. autoftp with getopts

• #!/bin/sh
  
  scriptname=$0   # $0 is the name of the program
  verbose=no
  probeonly=no
  
  function usage () {
     cat <<EOF
  Usage: $scriptname [-p] [-v] [-h] filename
     -p   probes the ftp sites and check urls in
          filename
     -v   executes and prints out verbose messages
     -h   displays basic help
  EOF
     exit 0
  }
  
  while getopts ":pvh" opt; do
     case $opt in
  
     v )  verbose=yes ;;
     p )  probeonly=yes ;;
     h )  usage ;;
     \?)  usage ;;
     esac
  done
  
  shift $(($OPTIND - 1))
  
  
  echo "verbose=$verbose probeonly=$probeonly"
  echo "remaining arguments=$*"
  # etc etc

15. example execution

• ./autoftp3.sh -v
  verbose=yes probeonly=no other args=

• ./autoftp3.sh -v -p foo bar
  verbose=yes probeonly=yes other args=foo bar

• ./autoftp3.sh -h
  Usage: ./autoftp3.sh [-p] [-v] [-h] filename
     -p   probes the ftp sites and check urls in
          filename
     -v   executes and prints out verbose messages
     -h   displays basic help

16. Laboratory work

• try out the examples given in this section of the notes and extend the examples to include more arguments
• add the usage command to your assignment

Index

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