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# Bash Pre-decrement
The `--` (double dash) token is a POSIX-standard end-of-options indicator (or delimiter). When a command parser encounters `--`, it ceases option processing and treats all subsequent arguments strictly as literal operands, regardless of whether those arguments begin with a hyphen (`-`).
```bash theme={"dark"}
command [options] -- [operands]
```
## Parsing Mechanics
In Bash, argument parsing is typically handled by the `getopts` builtin for shell scripts, or by external utilities utilizing C library functions like `getopt()` or `getopt_long()`. These parsers recognize `--` as a hardcoded termination token defined by POSIX Utility Syntax Guideline 10.
When the parser reads the argument vector (`argv`):
1. It evaluates arguments starting with `-` as flags or options.
2. Upon encountering the exact string `--`, the parser halts its scanning loop.
3. The parser increments its internal index (such as `OPTIND` in Bash) past the `--` token. It does *not* automatically consume or discard the `--` token from the shell's positional parameters.
4. All remaining elements in the argument vector are treated as non-option operands. For external commands, these are processed internally via their C `argv` array. For Bash builtins or shell scripts, they are processed as literal strings or assigned to the shell's positional parameters (e.g., `$1`, `$2`), depending on the specific command.
## Interaction with `set`
While `--` is a universal POSIX guideline implemented by most standard utilities (e.g., `rm`, `grep`, `ls`), it has a specific, notable behavior when used with the Bash `set` builtin. The `set` command uses `--` to safely assign values to the shell's positional parameters without them being interpreted as shell options.
```bash theme={"dark"}
# The shell interprets '-x' as an option, enabling xtrace.
set -x
# The shell stops option processing at '--'.
# '-x' is assigned to the positional parameter $1.
set -- -x
# Passing '--' with no subsequent arguments unsets all positional parameters.
set --
```
## Interaction with `getopts`
When writing custom Bash scripts, the `--` delimiter directly affects the `OPTIND` (Option Index) variable used by the `getopts` builtin.
```bash theme={"dark"}
#!/bin/bash
# Standard option parsing loop.
# The leading colon in ":x:y" enables silent error reporting.
# This suppresses default shell errors, allows $OPTARG to capture the invalid
# option character for the \? case, and redirects missing required arguments
# to the : case (instead of \?).
while getopts ":x:y" opt; do
case ${opt} in
x ) echo "Option x: $OPTARG" ;;
y ) echo "Option y triggered" ;;
: ) echo "Missing argument for option: -$OPTARG" >&2; exit 1 ;;
\? ) echo "Invalid option: -$OPTARG" >&2; exit 1 ;;
esac
done
# Shift the parsed options and the '--' token out of the positional parameters
shift $((OPTIND - 1))
# Use "$*" to safely join positional parameters into a single string
# without unintended word splitting or pathname expansion.
echo "Remaining operands: $*"
```
If this script is invoked as `./script.sh -x foo -- -y bar`:
1. `getopts` parses `-x` and its argument `foo`.
2. `getopts` encounters `--`, stops parsing, and increments `OPTIND` past the `--` token.
3. The `shift` command removes `-x`, `foo`, and `--` from the parameter list.
4. The string `-y` is never evaluated as an option; it is retained as the literal string `$1`, and `bar` becomes `$2`.
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