> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Bash Unary Minus
The `-` (hyphen or minus) character in Bash is a heavily overloaded lexical token. Its behavior is strictly determined by its syntactic context, functioning variously as an arithmetic operator, a parameter expansion modifier, a special parameter, a file descriptor closure token, a pattern matching range indicator, a conditional test prefix, or a directory navigation shortcut.
## Arithmetic Operators
Within an arithmetic evaluation context (`$(( ))`, `(( ))`, `let`, or variables typed with `declare -i`), `-` functions as a binary subtraction operator, a unary negation operator, and forms the unary pre-decrement and post-decrement operators (`--`).
```bash theme={"dark"}
# Binary subtraction
(( result = operand1 - operand2 ))
# Unary negation
(( result = -operand ))
# Pre-decrement and post-decrement
(( --operand ))
(( operand-- ))
```
## File Descriptor Closure
In redirection operations, appending `-` to the file descriptor duplication operators (`>&` or `<&`) instructs the shell to close the specified file descriptor rather than duplicating it.
```bash theme={"dark"}
# Close standard output (fd 1)
>&-
# Close standard input (fd 0)
<&-
# Close a specific file descriptor (e.g., fd 3)
3>&-
```
## Special Parameter (Shell Options)
The hyphen acts as a core special parameter (`$-`). When expanded, it yields a string containing the current set of single-letter shell option flags enabled in the current Bash environment (e.g., `himBH`).
```bash theme={"dark"}
# Expands to current shell options
echo $-
```
## Parameter Expansion (Default Value Substitution)
In brace parameter expansion, `-` acts as a fallback operator. It instructs the shell to evaluate to a specified default string if the target parameter is unset. When combined with a colon (`:-`), it triggers the fallback if the parameter is either unset or null (empty).
```bash theme={"dark"}
# Substitutes 'default_value' only if 'var' is strictly unset
${var-default_value}
# Substitutes 'default_value' if 'var' is unset OR null
${var:-default_value}
```
## Pattern Matching Range Operator
Within bracket expressions `[ ]` used in pathname expansion (globbing) and regular expression matching (`[[ =~ ]]`), `-` defines a contiguous character range based on the current locale's collating sequence.
```bash theme={"dark"}
# Globbing character range
ls *[a-z]*
# Regular expression character range
[[ $string =~ [0-9] ]]
```
## Conditional Test Prefix
Within the `test` builtin, POSIX brackets `[ ]`, and Bash extended test keywords `[[ ]]`, `-` is the required prefix for unary file/string operators and binary algebraic comparison operators.
```bash theme={"dark"}
# Unary operator prefix (e.g., -z for zero-length string, -f for regular file)
[[ -z $variable ]]
# Binary operator prefix (e.g., -eq for arithmetic equality)
[[ $val1 -eq $val2 ]]
```
## Here-Document Tab Stripping
When appended to the here-document redirection operator (`<<`), forming `<<-`, the hyphen modifies the parsing of the here-document. It instructs the shell to strip all leading tab characters (but not spaces) from the subsequent lines of the document body and the terminating delimiter.
```bash theme={"dark"}
command <<-DELIMITER
text_with_leading_tabs
DELIMITER
```
## Previous Working Directory Alias
When passed as the sole argument to the `cd` builtin, `-` is parsed as a reserved alias for the `$OLDPWD` environment variable. It instructs the shell to change the current working directory to the previous working directory and prints the new path to standard output.
```bash theme={"dark"}
cd -
```
## Option Prefix and End-of-Options Marker
The `-` character is the standard prefix for short (single-character) options passed to Bash builtins and external commands. Furthermore, a standalone double hyphen (`--`) is parsed as the end-of-options indicator, forcing all subsequent arguments to be treated as positional parameters (operands), even if they begin with a `-`.
```bash theme={"dark"}
# Short option prefix
command -x -y
# End-of-options marker
command -- -filename_starting_with_hyphen
```
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