Creating a Simple Command-Line Interface (CLI) Tool with Bash

In this tutorial, we will be walking through the process of creating a simple CLI tool using the Bash scripting language. The tool will provide a help option, a version information option, and a custom output file option. It's a great start to automating tasks and creating your own tools.

The Script Here is the Bash script we will be discussing:

#!/bin/bash

# A basic boilerplate for a CLI tool using Bash

function show_help() {
cat << EOF
Usage: ${0##*/} [-h] [-v] [-o OUTFILE] [FILE]...
This is a CLI tool template.
    -h          display this help and exit
    -v          display version information and exit
    -o OUTFILE  define the output file. Default is 'output.txt'.
FILE is the name of the file to process.
EOF
}

# Initialize our own variables:
output_file="output.txt"
version="1.0.0"

while getopts "hvo:" opt; do
    case "$opt" in
    h)
        show_help
        exit 0
        ;;
    v)  echo "Version: $version"
        exit 0
        ;;
    o)  output_file=$OPTARG
        ;;
    esac
done

shift $((OPTIND-1))

# The rest of the script can go here. For now, let's just echo the output file and remaining arguments:
echo "output_file='$output_file', Remaining args: $@"

Script Breakdown

Shebang (#!/bin/bash)

The first line is called a shebang. It tells the system this script should be executed using Bash.

Help Function

Next, we define a function show_help that displays the usage of the tool when called. This is typically invoked by the -h or --help flags.

Variable Initialization

We then initialize some variables that will be used in the script. output_file is the name of the output file, which defaults to output.txt. version is the current version of the script.

Option Processing

The while loop uses the getopts function to process command-line options. In this case, -h displays the help message, -v shows the version information, and -o sets the output file.

Shift Arguments

The shift command removes the processed options from the argument list, leaving the remaining arguments, which in our case, could be files to process.

Script Body

Lastly, we have the main body of the script. In this case, it simply echoes the name of the output file and any remaining arguments.

Using the Tool

To use this tool, save it as a .sh file and give it execute permissions with chmod +x scriptname.sh. Then, you can run it with ./scriptname.sh. You can pass in -h to see the help message, -v to see the version information, and -o outputfile to set the name of the output file.

Remember to replace "scriptname.sh" with the name of your script.