What's "set -- "$progname" "$@"" means in shell script?

Understanding 'set -- "$progname" "$@"' in Shell Scripts

Demystify a common shell idiom used for robust argument handling and script self-referencing. Learn how it protects against unexpected behavior and ensures correct parameter passing.

When working with shell scripts, you might encounter the seemingly cryptic line set -- "$progname" "$@". This idiom is a powerful, yet often misunderstood, technique used to manipulate a script's positional parameters. It's particularly useful for ensuring that a script can reliably refer to itself and correctly process its arguments, even when those arguments contain spaces or special characters. This article will break down each component of this command, explain its purpose, and illustrate why it's a best practice in many shell scripting scenarios.

The 'set' Command and Positional Parameters

In shell scripting, positional parameters ($1, $2, $3, etc.) represent the arguments passed to a script or function. The special parameter $@ expands to all positional parameters, individually quoted, which is crucial for preserving arguments containing spaces. The set command is a built-in shell command used to modify shell options and set positional parameters. When set is used without any options, followed by --, it clears the existing positional parameters and replaces them with the arguments that follow --.

# Original arguments
echo "Original arguments: $@"

# Change positional parameters
set -- "new_arg1" "new arg2 with spaces"
echo "New arguments: $@"

# Demonstrate effect on $1, $2
echo "First new argument: $1"
echo "Second new argument: $2"

Basic usage of set -- to redefine positional parameters.

Understanding Each Component

Let's dissect set -- "$progname" "$@" piece by piece:

1. set: As discussed, this command is used to set shell options or positional parameters.
2. --: This double-dash is a standard convention that signals the end of options for a command. Any arguments following -- are treated as positional arguments, even if they start with a hyphen (e.g., -f or --help). This prevents potential misinterpretation of arguments as options to the set command itself.
3. "$progname": This is the first new positional parameter. $progname is typically a variable that holds the name of the script itself (often derived from $0). By including it here, the script effectively re-adds its own name as the first argument. This is useful if the script needs to call itself or refer to its own name later, especially after its original $0 might have been altered or if the script is sourced.
4. "$@": This is the second part of the new positional parameters. As mentioned, $@ expands to all current positional parameters, each quoted individually. This is critical for preserving arguments that contain spaces or special characters. For example, if an argument was "hello world", $@ ensures it remains a single argument "hello world" rather than splitting into "hello" and "world".

flowchart TD
    A[Script Execution Start] --> B{Initial Positional Parameters: $0, $1, $2, ...}
    B --> C["set -- \"$progname\" \"$@\""]
    C --> D{`set` command invoked}
    D --> E{`--` signals end of options}
    D --> F{`"$progname"` becomes new $1}
    D --> G{`"$@"` expands original $1, $2, ... to new $2, $3, ...}
    F & G --> H[New Positional Parameters: $progname, original $1, original $2, ...]
    H --> I[Script Continues with Modified Parameters]

Flowchart illustrating the effect of set -- "$progname" "$@" on script arguments.

Why Use This Idiom?

This construct is primarily used for two main reasons:

1. Robust Argument Handling: It provides a clean way to re-process or re-align arguments. If a script needs to perform some initial parsing (e.g., shift off some options) and then pass the remaining arguments to another command or function, this ensures that the arguments are correctly quoted and passed.
2. Self-Referencing and exec: In scenarios where a script might exec itself with modified arguments, or if it needs to ensure that $0 (the script's name) is always available as the first argument for internal functions, this idiom guarantees consistency. For instance, if a script needs to restart itself with exec "$0" "$@", ensuring $0 is correctly set is vital.

Practical Example: Argument Preprocessing

Consider a script that needs to handle some internal options before passing the rest of the arguments to an external command. This idiom allows you to 'reset' the arguments after processing the internal ones.

#!/bin/bash

progname="$(basename "$0")"

# Simulate some internal options
internal_option_a=false
internal_option_b="default"

# Process internal options
while [[ $# -gt 0 ]]; do
    case "$1" in
        -a|--option-a)
            internal_option_a=true
            shift # Remove -a from arguments
            ;;
        -b|--option-b)
            internal_option_b="$2"
            shift 2 # Remove -b and its value
            ;;
        --)
            shift # Remove --
            break # Stop processing options
            ;;
        -*)
            echo "Unknown option: $1" >&2
            exit 1
            ;;
        *)
            break # Stop processing options, remaining are positional
            ;;
    esac
done

# Now, reset positional parameters for external command
# This ensures that the script's name is the first argument, 
# followed by any remaining arguments for the external command.
set -- "$progname" "$@"

echo "--- Script Internal State ---"
echo "Internal Option A: $internal_option_a"
echo "Internal Option B: $internal_option_b"
echo "-----------------------------"

echo "Arguments for external command (as seen by this script): $@"
echo "First argument: $1"
echo "Second argument: $2"
echo "All arguments: $@"

# Imagine calling an external command here, e.g.,
# external_command "$@"

A script demonstrating argument preprocessing and resetting positional parameters.

In this example, after the script processes its own internal options, set -- "$progname" "$@" re-establishes the positional parameters. $1 will now be the script's name, and $2, $3, etc., will be the arguments originally intended for the external command, correctly quoted and ordered. This makes the script more robust and predictable when dealing with complex argument structures.