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# Bash Conditional Expression
A Bash conditional expression evaluates a specific condition and yields an exit status of `0` (true) or `1` (false). These expressions are evaluated using test constructs and are the primary mechanism for resolving boolean logic in shell control flow structures.
## Evaluation Constructs
Bash provides three primary constructs for evaluating conditional expressions:
```bash theme={"dark"}
# 1. The test builtin command
test expression
# 2. Single brackets (POSIX standard, alias for test)
[ expression ]
# 3. Double brackets (Bash extended shell keyword)
[[ expression ]]
```
### `[ ]` vs `[[ ]]`
* **`[ ]` (POSIX `test`)**: Evaluated as a standard command. Variables inside single brackets are subject to word splitting and pathname expansion (globbing). Variables must typically be quoted (e.g., `[ "$VAR" = "string" ]`) to prevent syntax errors if the variable is empty or contains spaces.
* **`[[ ]]` (Bash Keyword)**: Evaluated as a shell keyword. It suppresses word splitting and pathname expansion for its internal arguments. It introduces advanced features like unquoted variable safety, pattern matching (globbing) on the right-hand side of `==`, and regular expression matching via `=~`.
## Operator Categories
Conditional expressions rely on unary and binary operators to perform evaluations.
### File Test Operators
Unary operators used to evaluate file attributes and existence.
```bash theme={"dark"}
[ -e FILE ] # True if file exists (any type)
[ -f FILE ] # True if file exists and is a regular file
[ -d FILE ] # True if file exists and is a directory
[ -s FILE ] # True if file exists and has a size greater than zero
[ -r FILE ] # True if file exists and is readable
[ -w FILE ] # True if file exists and is writable
[ -x FILE ] # True if file exists and is executable
[ -L FILE ] # True if file exists and is a symbolic link
```
### String Operators
Operators used to evaluate string length and perform lexicographical comparisons.
```bash theme={"dark"}
[ -z STRING ] # True if the length of the string is zero
[ -n STRING ] # True if the length of the string is non-zero
[ STRING1 = STRING2 ] # True if strings are identical (== is also valid)
[ STRING1 != STRING2 ] # True if strings are not identical
[[ STRING1 < STRING2 ]] # True if STRING1 sorts before STRING2 lexicographically
[[ STRING =~ REGEX ]] # True if STRING matches the extended regular expression REGEX
```
*Note: The `<` and `>` operators must be escaped (`\<`, `\>`) when used inside single brackets `[ ]` to prevent them from being interpreted as redirection operators.*
### Integer Operators
Binary operators strictly used for algebraic comparison of integers.
```bash theme={"dark"}
[ INT1 -eq INT2 ] # Equal to
[ INT1 -ne INT2 ] # Not equal to
[ INT1 -lt INT2 ] # Less than
[ INT1 -le INT2 ] # Less than or equal to
[ INT1 -gt INT2 ] # Greater than
[ INT1 -ge INT2 ] # Greater than or equal to
```
### Logical Operators
Operators used to combine or negate expressions. The syntax differs strictly between the POSIX `[ ]` and Bash `[[ ]]` constructs.
```bash theme={"dark"}
# Negation (Unary)
[ ! expression ]
[[ ! expression ]]
# Logical AND (Binary)
[ expression1 -a expression2 ]
[[ expression1 && expression2 ]]
# Logical OR (Binary)
[ expression1 -o expression2 ]
[[ expression1 || expression2 ]]
```
## Grouping and Precedence
In complex expressions, precedence can be explicitly defined using parentheses.
```bash theme={"dark"}
# Inside single brackets, parentheses must be escaped
[ \( expression1 -a expression2 \) -o expression3 ]
# Inside double brackets, parentheses do not require escaping
[[ ( expression1 && expression2 ) || expression3 ]]
```
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