Creating Your First Shell Script

This tutorial demonstrates how to build a basic interactive shell script covering essential programming concepts including conditional statemants, fille operations, and permission management.

Setting Up the Script Environment

Create and open a new script file using Vim:

vim ./demo.sh

Vim operates in two primary modes: Command mode for executing instructions and Insert mode for typing content. Press i to enter Insert mode, then type your script. Save and exit by pressing Esc followed by :wq.

Quick Vim tips:

• Esc yy copies a line
• Esc p pastes a line

Make the script executable:

chmod +x ./demo.sh
./demo.sh

Understanding the If Statement

The if statement controls program flow based on conditions. Each conditional test using [ ] requires spaces around the brackets:

if [ "$variable" = "value" ]; then
    echo "Condition met"
elif [ "$variable" = "other" ]; then
    echo "Alternative condition"
else
    echo "Default action"
fi

Script Features

The following script implements four menu options:

1. Display hostname and IP address
2. Create directories or files
3. Modify file permissions
4. Exit the script

Displaying System Information

hostname          # Shows the system hostname
hostname -I       # Shows the IP address

Creating Directories and Files

The script prompts for a working directory, validates it exists, then creates either a file or directory based on whether the name contains a file extension:

# Check if path exists
if [ -d "$workspace" ]; then
    # Create file if name matches pattern (contains dot)
    if [[ "$name" =~ \..+ ]]; then
        touch "$name"
    else
        mkdir "$name"
    fi
fi

Modifying File Permissions

The chmod command changes access permissions using numeric or symbolic notation. Linux permissions consist of nine characters representing three permission sets:

• Owner permissions (characters 1-3): read, write, execute
• Group permissions (characters 4-6): read, write, execute
• Other permissions (characters 7-9): read, write, execute

Numeric values:

• Read (r): 4
• Write (w): 2
• Execute (x): 1

Examples:

• chmod 755 file.sh sets rwxr-xr-x
• chmod 721 file.sh sets rwx-w---x
• chmod 777 file.sh sets rwxrwxrwx

Verify permissions with ls -l.

Complete Script Example

#!/bin/bash
echo $(date)
printf "%-8s %-20s\n" 'Option' 'Description'
printf "%-8s %-20s\n" '1' 'Show Hostname and IP'
printf "%-8s %-20s\n" '2' 'Create Directory or File'
printf "%-8s %-20s\n" '3' 'Change Permissions'
printf "%-8s %-20s\n" '4' 'Exit'
echo "Enter your selection:"
read choice

if [ "$choice" -eq 1 ]; then
    printf "%-12s %s\n" 'Hostname:' $(hostname)
    printf "%-12s %s\n" 'IP Address:' $(hostname -I)
elif [ "$choice" -eq 2 ]; then
    echo "Enter the absolute path for your workspace:"
    read workspace
    if [ -d "$workspace" ]; then
        cd "$workspace"
        echo "Working directory: $(pwd)"
        ls
        echo "Enter name for directory or file to create:"
        read itemname
        if [[ "$itemname" =~ \..+ ]]; then
            touch "$itemname"
        else
            mkdir "$itemname"
        fi
        echo "Updated workspace contents:"
        ls
    else
        echo "Error: Specified path does not exist"
    fi
elif [ "$choice" -eq 3 ]; then
    echo "Enter the absolute path for your workspace:"
    read workspace
    if [ -d "$workspace" ]; then
        cd "$workspace"
        echo "Working directory: $(pwd)"
        ls -l
        echo "Enter directory or file name:"
        read itemname
        if [ -e "$itemname" ]; then
            echo "Enter permission number (e.g., 755, 644):"
            read permission
            chmod "$permission" "$itemname"
            echo "Permissions updated:"
            ls -al "$itemname"
        else
            echo "Error: Specified item does not exist"
        fi
    else
        echo "Error: Specified path does not exist"
    fi
else
    echo "Exiting script"
    exit 0
fi

Running the Script

Execute the script and select options from the menu. Each option performs the corresponding operation:

• Option 1 displays system network information
• Option 2 creates new filesystem objects
• Option 3 changes access permissions
• Option 4 terminates the script