There are various ways to pass data into a Bash shell script.

• Pass in arguments (also known as positional arguments)
• Pass in options (also known as key value pairs)
• Pass in flags

Passing in arguments

In this example, "foo" and "bar" are passed into the example.sh script.

example.sh foo bar

In the example.sh script, foo will be accessed using $1 and bar will be accessed using $2, like this.

#!/bin/bash
echo $1
echo $2

Invoking this script will return the following output.

foo
bar

Or, like this (array)

#!/bin/bash
echo "$@"

Invoking this script will return the following output.

foo bar

And here is a very simple case statement that will echo "got foo" or "got bar" if $1 is foo or bar, else "opps" will be returned.

case "$1" in
  foo) echo "got foo";;
  bar) echo "got bar";;
    *) echo "opps";;
esac

Pass in options

A case statement can be used to create both command line options and flags. An option must be followed by a value. In this example, the --name option is followed by a value of Jeremy.

~]# example.sh --name Jeremy

A flag is not followed by a value on the command line. In this example, the --help flag is used. A flag will be associated with a value in the case statement.

~]# example.sh --help

One of the most practical uses of a case statement is to be able to create command line options. For example, lets say you have a bash script and you want options such as -n or --name and -o or --occupation.

example.sh -n Jeremy -o Engineer

First, you will create a variable that will contain the short and long verision of the command line options. In this example n:o: creates the -n and -o options, and name:,occupation: creates the --name and --occupation options.

ARGS=$(getopt -a --options n:o: --long "name:,occupation:" -- "$@")

Next you evaluate the $ARGS variable.

eval set -- "$ARGS"

Then a while loop is used to associate each command line option with it's value. For example, if -n Jeremy or --name Jeremy is used on the command line, then a variable called name will contain a value of Jeremy.

while true; do
  case "$1" in
    -n|--name)
      name="$2"
      shift 2;;
    -o|--occupation)
      occupation="$2"
      shift 2;;
    --)
      break;;
     *)
      printf "Unknown option %s\n" "$1"
      exit 1;;
  esac
done

You can then do something based on the  value associated with the command line options.

echo "Your name is $name"

Pass in flags

You can also create flags. For example, let's say you want to have flags for -v or --verbose and -h or --help.

~]# example.sh -v -h

First, you will create a variable that will contain the short and long verision of the command line flags. In this example, vh creates the -v and -h options, and verbose,help creates the --verbose and --help options. The difference between an option and a flag is that an option has the : character, and a flag does not.

ARGS=$(getopt -a --options vh --long "verbose,help" -- "$@")

Next you evaluate the $ARGS variable.

eval set -- "$ARGS"

You will also want to set default values for each flag. In this example, the default value is "false".

verbose="false"
help="false"

Then a while loop is used to update the verbose and help value to "true" if the flags are used.

while true; do
  case "$1" in
    -v|--verbose)
      verbose="true"
      shift;;
    -h|--help)
      help="true"
      shift;;
    --)
      break;;
     *)
      printf "Unknown option %s\n" "$1"
      exit 1;;
  esac
done

You can then use an if statement to do something with the verbose and help flags.

if [ $help == true ]
then
  echo "Here is some helpful information"
fi