Conditional Logic

In this lesson, we explore how conditional logic works in shell scripts by demonstrating a real-world example: checking a rocket's launch status and executing the appropriate commands based on the result.

When executing the script, a command checks the rocket's status. The possible statuses returned are:

• "launching": when the launch is in progress,
• "success": when the launch is successful, or
• "failed": when the rocket crashes and the mission fails.

If the launch fails, the script triggers the rocket-debug command to investigate the cause of the failure. This command is executed solely when the launch fails, ensuring that no unnecessary operations occur when the launch is successful. The mechanism behind this behavior is implemented via conditional logic in the shell script.

Basic Structure of an If Statement

The fundamental structure of a conditional if statement in a shell script includes:

1. The if statement, followed by the condition enclosed in square brackets.
2. The then keyword, which indicates the start of commands that run if the condition is true.
3. Optionally, the elif or else blocks for handling alternative cases.
4. The fi statement, which marks the end of the if block.

Consider the following example, where we check if the rocket status is "failed":

mission_name=$1

mkdir $mission_name

rocket-add $mission_name

rocket-start-power $mission_name
rocket-internal-power $mission_name
rocket-start-sequence $mission_name
rocket-start-engine $mission_name
rocket-lift-off $mission_name

rocket_status=$(rocket-status $mission_name)

if [ $rocket_status = "failed" ]
then
    rocket-debug $mission_name
elif [ $rocket_status = "success" ]
then
    echo "This is successful"
fi

In this script:

• The rocket_status variable stores the result from the rocket-status command.
• The condition [ $rocket_status = "failed" ] checks if the status equals "failed". If true, it runs the rocket-debug command.
• The elif clause allows for an alternate procedure when the rocket launch is successful.

Important Syntax Tips:

• Use the comparison operator = for string equality checks.
• Ensure there is a space between the square brackets, the operator, and the values.
• Use the not equals operator (!=) for checking inequality.

Comparison and Test Operators

Below is a summary of common operators used in shell scripting for both string and numeric comparisons:

• String Comparisons:

  • Equality: =
  • Inequality: != (or use pattern matching with wildcards in the enhanced syntax)

• Numeric Comparisons:

  • Equal: -eq
  • Not equal: -ne
  • Greater than: -gt
  • Less than: -lt

Enhanced Syntax with Double Square Brackets

For more advanced conditional expression, you can utilize double square brackets [[ ]]. This syntax offers the following benefits:

• Supports advanced pattern matching.
• Allows the combination of multiple conditions without needing to separate into multiple expressions.

For example, to check if a substring exists within a string:

[[ "abcd" = *bc* ]]

In this expression, the asterisks (*) indicate that there can be any collection of characters before or after "bc", making the expression evaluate to true. Remember to avoid enclosing patterns in quotes when using this syntax.

Additional examples of enhanced syntax include:

[[ "abc" = ab[cd] ]]   # true, because the third character can be either 'c' or 'd'
[[ "abd" = ab[cd] ]]   # true
[[ "abe" = "ab[cd]" ]] # false, the pattern is taken as a literal string
[[ "abc" > "bcd" ]]    # compares alphabetical order
[[ "abc" < "bcd" ]]    # compares alphabetical order

The operators > and < are used for comparing strings in lexicographical (alphabetical) order.

Combining Multiple Conditions

Sometimes, it's necessary to test multiple conditions in a script. There are two basic logical operators:

• Logical AND (&&): Ensures that both conditions are true.

  [ COND1 ] && [ COND2 ]
  

• Logical OR (||): Executes the command if at least one condition is true.

  [ COND1 ] || [ COND2 ]
  

When using the enhanced double square bracket syntax, you can combine conditions within a single statement:

if [[ $value -gt 4 && $value -lt 10 ]]
then
    echo "Value is between 5 and 9."
fi

Even without the detailed table, the above examples demonstrate how logical operators can be employed to craft effective conditional evaluations.

File Test Operators

Shell scripts also offer file test operators that allow you to check various file properties:

• -e: Checks if a file exists.
• -d: Determines if the given path is a directory.
• -s: Verifies that the file size is greater than zero.
• -x: Confirms if the file is executable.
• -w: Checks if the file is writable.

These operators are essential for creating robust scripts that adapt based on file or directory attributes.