Command Line Arguments

Command-line arguments are essential in shell scripting for creating flexible, reusable scripts. Instead of hard-coding values, you can accept inputs at runtime—much like using a remote control to switch channels on a TV. This guide covers everything from basic positional parameters to advanced iteration techniques.

Table of Contents

• Understanding Positional Parameters
• Practical Example: Cloning and Counting Files
• Parameterizing Your Script
• Common Special Variables
• Handling Maximum Argument Size (ARG_MAX)
• Iterating with shift
• References

Understanding Positional Parameters

When you invoke a script with arguments:

$ ./myscript.sh foo bar baz

Inside myscript.sh, the inputs map to:

#!/bin/bash
echo "First argument: $1"
echo "Second argument: $2"
echo "Third argument: $3"

Output:

First argument: foo
Second argument: bar
Third argument: baz

echo "User provided: $1"

Practical Example: Cloning and Counting Files

Suppose you need to clone a Git repository and count its files. A hard-coded approach looks like this:

#!/bin/bash

git clone [email protected]:kodedkloud/kodekloud-advanced-shell-scripting.git
find . -type f | wc -l

This works but requires editing the script for each repository URL.

Parameterizing Your Script

By using $1, you can pass the repository URL when running the script:

#!/bin/bash

# Clone the repository passed as the first argument
git clone "$1"

# Count files in the cloned repo
find . -type f | wc -l

Run it as:

$ ./clone-and-count.sh [email protected]:kodedkloud/kodekloud-advanced-shell-scripting.git

Now the script clones any repository you specify.

Common Special Variables

Example:

#!/bin/bash
echo "Script name: $0"
echo "Number of arguments: $#"
echo "All arguments (\$@): $@"

$ ./myscript.sh apple banana cherry
Script name: ./myscript.sh
Number of arguments: 3
All arguments ($@): apple banana cherry

Handling Maximum Argument Size (ARG_MAX)

Unix-like systems impose a limit on the total size of command-line arguments. Check it with:

$ getconf ARG_MAX
1048576

On most Linux distributions, ARG_MAX is around 1 MiB, which is sufficient for tens of thousands of small arguments.

Iterating with shift

The shift command discards $1 and shifts all other parameters down by one. This is useful when you don’t know the number of arguments in advance:

#!/bin/bash

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

$ ./shift-example.sh arg1 arg2 arg3
Current argument: arg1
Current argument: arg2
Current argument: arg3

Command-line arguments empower you to build dynamic, user-driven shell scripts. Next up: advanced option parsing with getopts and long-form flags.

References

• Bash Reference Manual
• getconf
• find(1) — Find Files
• wc(1) — Word, Line, Character, and Byte Count