> ## Documentation Index
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> Use this file to discover all available pages before exploring further.
# Bash Test Expression
A Bash test expression evaluates a conditional statement and returns an exit status indicating success (`0` for true) or failure (`1` for false). It is the foundational mechanism for boolean logic in Bash, relying on shell builtins or keywords to parse arguments and operators.
## Syntax Constructs
Bash provides three primary constructs for evaluating test expressions:
**1. The `test` Builtin**
The traditional POSIX-compliant command.
```bash theme={"dark"}
test expression
```
**2. The Single Bracket `[ ]` Builtin**
A syntactic alias for the `test` command. It requires a closing `]` as its final argument. Because it is parsed as a standard command, unquoted variables within the brackets are subject to word splitting and pathname expansion (globbing).
```bash theme={"dark"}
[ expression ]
```
**3. The Double Bracket `[[ ]]` Keyword**
A Bash-specific extended test construct. Because it is a shell keyword rather than a command, it suppresses word splitting and pathname expansion for its internal arguments. It also supports advanced pattern matching and regular expressions.
```bash theme={"dark"}
[[ expression ]]
```
## Operator Categories
Test expressions utilize specific unary and binary operators to evaluate data types, string literals, and file metadata.
### File Test Operators
Evaluate file system attributes and metadata.
* `-e file` : True if the file exists.
* `-f file` : True if the file is a regular file.
* `-d file` : True if the file is a directory.
* `-s file` : True if the file exists and has a size greater than zero.
* `-r file` : True if the file has read permission.
* `-w file` : True if the file has write permission.
* `-x file` : True if the file has execute permission.
### String Operators
Evaluate string length and lexicographical equivalence.
* `-z string` : True if the string length is exactly zero.
* `-n string` : True if the string length is non-zero.
* `str1 = str2` : True if strings are identical (POSIX standard). Within `[[ ]]`, the right operand is treated as a glob pattern rather than a literal string unless quoted.
* `str1 == str2`: Synonym for `=`. True if strings are identical. Within `[[ ]]`, the right operand is treated as a glob pattern rather than a literal string unless quoted.
* `str1 != str2`: True if strings are not identical.
* `str1 < str2` : True if `str1` sorts before `str2` lexicographically (requires escaping `\<` in `[ ]`).
* `str1 > str2` : True if `str1` sorts after `str2` lexicographically (requires escaping `\>` in `[ ]`).
* `str1 =~ regex`: True if `str1` matches the extended regular expression `regex` (Valid only within `[[ ]]`).
### Integer Operators
Evaluate algebraic relationships between integers. Within `[ ]` and `test`, these operators strictly require numeric literals or expanded variables. Within `[[ ]]`, these operators force an arithmetic evaluation context, meaning they can evaluate variables without the `$` prefix and unquoted arithmetic expressions (e.g., `[[ x+2 -eq 5 ]]`).
* `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
Combine or invert multiple expressions. The syntax depends on whether the operator is used internally within a construct or externally as a shell control operator.
* **Negation (NOT):**
* `! expression` : Inverts the exit status. Valid internally within all constructs.
* **Logical AND:**
* `expr1 -a expr2` : Internal AND operator. Valid only within `[ ]` or `test` (POSIX obsolescent).
* `expr1 && expr2` : Internal AND operator within `[[ ]]`. Outside of test constructs, `&&` is a standard shell control operator used to chain multiple `[ ]` or `test` commands (e.g., `[ expr1 ] && [ expr2 ]`).
* **Logical OR:**
* `expr1 -o expr2` : Internal OR operator. Valid only within `[ ]` or `test` (POSIX obsolescent).
* `expr1 || expr2` : Internal OR operator within `[[ ]]`. Outside of test constructs, `||` is a standard shell control operator used to chain multiple `[ ]` or `test` commands (e.g., `[ expr1 ] || [ expr2 ]`).
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