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The :? operator is a parameter expansion modifier in Bash used to assert that a variable is set and not null. If the specified parameter is unset or null, Bash evaluates a provided string, writes it to standard error, and immediately aborts the execution of a non-interactive shell with a non-zero exit status. If the parameter is valid, the expression expands to the parameter’s value. 
${parameter:?word}

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Evaluation Mechanics

The behavior of the expansion depends strictly on the state of parameter:
  • Set and Not Null: The expression evaluates to the value of parameter. The word is ignored.
  • Unset or Null: Bash expands word and writes the result to standard error.
  • Omitted Word: If word is not provided (e.g., ${parameter:?}), Bash supplies the default diagnostic message strictly as: parameter null or not set.

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Error Output Format

When the error is triggered, Bash prefixes the output message depending on the execution environment:
  • Non-interactive shells (Scripts): The prefix includes the script’s name ($0) and the line number where the error occurred. Format: $0: line N: parameter: word (e.g., ./script.sh: line 3: parameter: word).
  • Interactive shells (Terminal): The prefix strictly uses the shell name. Format: bash: parameter: word.

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Execution Context Behavior

The operator interacts directly with the shell’s execution state:
  • Non-interactive shells (Scripts): Triggering the error causes the shell to immediately terminate execution with an exit status of 1. Subsequent commands are not executed.
  • Interactive shells (Terminal): Triggering the error prints the message to standard error but does not exit the shell session.

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Safe Evaluation via the Null Command

Because the expression expands to the variable’s value when valid, using it as a standalone command will cause Bash to attempt to execute the expanded value, typically resulting in a command not found error. To evaluate the expansion strictly for its assertion side-effect without executing the resulting string, it must be passed as an argument to the shell’s null command (:). The null command evaluates its arguments and performs expansions, but takes no action and always returns a zero exit status. 
: "${parameter:?word}"

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Strict Distinction: :? vs ?

Bash differentiates between the presence and absence of the colon (:) in the operator, which alters the evaluation of null (empty) strings: 

# Fails if 'var' is UNSET or NULL (empty string)
: "${var:?Error Message}"


# Fails ONLY if 'var' is UNSET. An empty string ("") is considered valid and will not trigger the error.
: "${var?Error Message}"
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