Checking Input Arguments in Bash Shell Scripts

When writing Bash shell scripts, it’s essential to check if input arguments have been provided. This ensures that your script behaves as expected and avoids errors. In this tutorial, we’ll explore how to check the existence of input arguments in a Bash shell script.

Understanding Input Arguments

In Bash, input arguments are stored in variables such as $1, $2, $3, and so on. The number of input arguments can be accessed using the $# variable. To check if an argument exists, you need to verify if the corresponding variable is set or not.

Checking for Empty Strings

One way to check if an argument exists is by verifying if the corresponding variable is an empty string. You can use the -z test operator in a conditional statement to achieve this:

if [ -z "$1" ]; then
  echo "No argument supplied"
fi

In this example, the script checks if the first argument $1 is an empty string. If it is, the script prints a message indicating that no argument was supplied.

Checking the Number of Arguments

Another approach is to check the number of input arguments using the $# variable:

if [ $# -eq 0 ]; then
  echo "No arguments supplied"
fi

Here, the script checks if the number of input arguments is equal to zero. If it is, the script prints a message indicating that no arguments were supplied.

Using Expansion Operators

Bash provides several expansion operators that can be used to check for the existence of variables and provide default values if they are not set. For example:

scale=${2:-1}

In this case, the scale variable is assigned the value of $2 (the second argument) if it exists. If $2 does not exist or is an empty string, the scale variable defaults to 1.

Some other useful expansion operators include:

• ${varname:=word}: sets the undefined varname to word
• ${varname:?message}: returns varname if it’s defined and not null; otherwise, prints message and aborts the script
• ${varname:+word}: returns word only if varname is defined and not null

Best Practices

When checking for input arguments in a Bash shell script:

• Always verify that the required number of arguments has been provided.
• Use expansion operators to provide default values when possible.
• Consider exiting the script with an error message if insufficient or invalid arguments are provided.

By following these guidelines, you can write robust and reliable Bash shell scripts that handle input arguments correctly.