Bash Script to Parse Command Line Arguments Easily

Use getopts in Bash to parse command line arguments like options and flags; for example, while getopts ":a:b" opt; do case $opt in a) echo "Option a with value $OPTARG" ;; b) echo "Option b set" ;; esac; done.

Examples

Input./script.sh -a hello -b

OutputOption a with value hello Option b set

Input./script.sh -b

OutputOption b set

Input./script.sh -a

OutputOption -a requires an argument.

How to Think About It

To parse command line arguments in Bash, think of options as flags that may or may not have values. Use getopts to loop through each option and handle its value if required. This helps your script understand what the user wants without manual string checks.

Algorithm

1

Start reading the command line arguments one by one.

2

Check if the current argument is an option (starts with a dash).

3

If the option requires a value, read the next argument as its value.

4

Store or act on the option and its value accordingly.

5

Repeat until all options are processed.

Code

bash

#!/bin/bash
while getopts ":a:b" opt; do
  case $opt in
    a) echo "Option a with value $OPTARG" ;; 
    b) echo "Option b set" ;;
    :) echo "Option -$OPTARG requires an argument."; exit 1 ;; 
    \?) echo "Invalid option: -$OPTARG"; exit 1 ;;
  esac
done

Output

Option a with value hello
Option b set

Dry Run

Let's trace './script.sh -a hello -b' through the code

1

Start getopts loop

opt = 'a', OPTARG = 'hello'

2

Handle option a

Print 'Option a with value hello'

3

Next getopts iteration

opt = 'b', no OPTARG

4

Handle option b

Print 'Option b set'

Iteration	opt	OPTARG	Action
1	a	hello	Print 'Option a with value hello'
2	b		Print 'Option b set'

Why This Works

Step 1: Using getopts

The getopts command reads options one by one from the command line arguments.

Step 2: Option handling

Inside the case statement, each option is matched and handled accordingly.

Step 3: Error handling

If an option requires an argument but none is given, getopts triggers the : case to show an error.

Alternative Approaches

Manual parsing with a while loop

bash

#!/bin/bash
while [[ $# -gt 0 ]]; do
  case "$1" in
    -a)
      echo "Option a with value $2"
      shift 2
      ;;
    -b)
      echo "Option b set"
      shift
      ;;
    *)
      echo "Unknown option $1"
      exit 1
      ;;
  esac
done

This method is simple but requires manual shifting and careful checks; less robust than getopts.

Using getopt command

bash

#!/bin/bash
ARGS=$(getopt -o a:b -- "$@")
eval set -- "$ARGS"
while true; do
  case "$1" in
    -a) echo "Option a with value $2"; shift 2;;
    -b) echo "Option b set"; shift;;
    --) shift; break;;
    *) echo "Invalid option"; exit 1;;
esac
done

getopt supports long options and more complex parsing but is less portable and more complex.

Complexity: O(n) time, O(1) space

Time Complexity

Parsing command line arguments with getopts is linear in the number of arguments, so O(n).

Space Complexity

The script uses constant extra space, O(1), as it processes arguments one by one without storing all.

Which Approach is Fastest?

getopts is efficient and built-in, manual parsing is simple but error-prone, getopt is powerful but less portable.

Approach	Time	Space	Best For
getopts	O(n)	O(1)	Simple option parsing
Manual parsing	O(n)	O(1)	Very simple scripts, custom logic
getopt command	O(n)	O(1)	Complex options, long options