Command-Line Arguments

To make scripts dynamic, you can pass data to them when executing from the terminal.

Positional Parameters

Arguments passed to a script are stored in positional parameters:

• $0 : The script name itself.
• $1, $2, $3, ... : The first, second, third arguments, and so on.
• $# : The total number of arguments.
• $@ : All arguments as separate words.
• $* : All arguments as a single word.

#!/bin/bash

echo "Script name: $0"
echo "Total arguments: $#"
echo "First argument: $1"
echo "All arguments: $@"

Run this script as: ./script.sh arg1 arg2

The shift Command

The shift command shifts all positional parameters to the left by one. $2 becomes $1, $3 becomes $2, and so on. $1 is discarded. This is useful for processing arguments in a loop.

#!/bin/bash

echo "Processing arguments:"
while [ "$#" -gt 0 ]; do
    echo "Current argument: $1"
    shift
done

Parsing Flags with getopts

For more complex scripts, you usually want to support flags (e.g., -f filename or -v). getopts is the standard Bash built-in for parsing short options.

#!/bin/bash

# Default values
VERBOSE=false
FILENAME=""

# A colon after a letter means it requires an argument
while getopts "vf:" OPTION; do
    case "$OPTION" in
        v)
            VERBOSE=true
            ;;
        f)
            FILENAME="$OPTARG"
            ;;
        ?)
            echo "Usage: $(basename $0) [-v] [-f filename]"
            exit 1
            ;;
    esac
done

echo "Verbose mode: $VERBOSE"
if [ -n "$FILENAME" ]; then
    echo "File specified: $FILENAME"
fi

Run as: ./script.sh -v -f data.txt

Best Practices

• Always validate arguments. If an argument is required but missing, print a usage message and exit with an error.
• Use getopts over manual shift parsing when you have multiple flags or optional arguments.
• Remember to quote "$@" when iterating to preserve arguments that contain spaces.