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The :- operator is a shell parameter expansion modifier used to provide a default value during variable substitution. It evaluates the state of a parameter and returns a specified fallback string if the parameter is either unset or null (an empty string).

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Syntax

${parameter:-word}
  • parameter: The name of the variable being evaluated.
  • word: The fallback expression. This is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion before being returned.

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Evaluation Mechanics

The operator evaluates the parameter against three distinct memory states:
  1. Set and Not Null: If parameter contains a value of length > 0, the expansion yields the value of parameter. The word is ignored.
  2. Unset: If parameter has not been declared in the current shell execution environment, the expansion yields the evaluation of word.
  3. Null: If parameter is declared but contains an empty string (""), the expansion yields the evaluation of word.
Note: The :- operator is strictly a substitution mechanism. It does not assign the evaluated word back to the parameter. To perform substitution and assignment simultaneously, the := operator is used.

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State Visualization


# State 1: Set and not null
VAR="data"
echo "${VAR:-fallback}"  

# Output: data


# State 2: Unset
unset VAR
echo "${VAR:-fallback}"  

# Output: fallback


# State 3: Null (empty string)
VAR=""
echo "${VAR:-fallback}"  

# Output: fallback

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Technical Distinction: :- vs -

The inclusion of the colon (:) dictates how the shell handles null values.
  • ${parameter:-word}: Tests for both unset and null.
  • ${parameter-word}: Tests strictly for unset. If the parameter is null, the shell considers it a valid state and returns the empty string.
VAR=""


# Omitting the colon treats the null variable as valid
echo "${VAR-fallback}"   

# Output: (empty string)


# Including the colon treats the null variable as invalid
echo "${VAR:-fallback}"  

# Output: fallback
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