Positional Parameters in Shell Script

Positional parameters are a set of special variables in shell scripting (like $1, $2, $#) that hold the arguments passed to your script from the command line.

• Using them is the key to creating powerful, flexible tools.
• Instead of a script that can only back up one specific file, you can create a script that can back up any file you "pass" to it.

You will use positional parameters to:

• Make your scripts reusable and non-static.
• Get required information, like a filename, a host to ping, or a directory.
• Make your scripts behave like real Linux commands (e.g., cp [source] [destination]).

A Simple Greeting Script

#!/bin/bash

# $1 will be the first argument
# $2 will be the second argument

echo "Hello, $1!"
echo "Welcome to $2."

Output:

The Special Variables

When your script runs, the shell automatically populates these special variables.

• $1, $2, $3...$9: The first nine positional parameters.
• ${10}, ${11}...: To access arguments beyond the 9th, you must use curly braces.
• $0: The name of the script itself. This is great for error messages.
• $#: The total number (count) of arguments provided. This is your most important tool for error checking.
• $*: All arguments as a single string.
• $@: All arguments as a list of separate strings.

The Most Important Concept: "$@" Vs "$*"

This is the most critical and confusing part of positional parameters. The difference is how they behave when quoted.

Let's say you run ./myscript.sh "My File.txt" "Other File.txt".

Important:

• To loop through arguments and preserve spaces in filenames, always use for arg in "$@".
• To print all arguments in one go, you might use echo "$*.

Looping Through All Arguments ($@)

This is the main use for "$@". You can create a script that processes an unlimited number of arguments.

Here is a ping_hosts.sh script that pings every host you give it.

#!/bin/bash

# If no hosts are given, show an error
if [ "$#" -eq 0 ]; then
    echo "Error: No hosts provided."
    echo "Usage: $0 [host1] [host2] ..."
    exit 1
fi

echo "Pinging all hosts..."

# This loop will run once for each argument
# Using "$@" ensures that filenames with spaces are treated as one item
for host in "$@"
do
    echo "--- Pinging $host ---"
    ping -c 3 "$host"
    echo "---------------------"
done

echo "All pings complete."

Running it:

./ping_hosts.sh google.com 8.8.8.8

Processing Arguments with shift

The shift command is an advanced tool. It "shifts" all arguments one to the left:

• $2 becomes $1
• $3 becomes $2
• The original $1 is discarded.
• $# is decreased by one.

This is useful for processing arguments one by one in a while loop.

#!/bin/bash

# Loop as long as the number of arguments (-gt) is "greater than" 0
while [ "$#" -gt 0 ]
do
    echo "Processing argument: $1"
    echo "Arguments left: $#"
    
    # Discard $1 and move $2 into its place
    shift
    
    echo "---"
done

echo "All arguments processed."

Running it :

./shift_test.sh apple banana cherry

Output:

Positional Parameters Options: