Linux Shell Programming with Bash and the vi Editor

🐚 Introduction to Shell Programming in Linux

Shell programming is the process of writing shell scripts—sequences of commands executed by the command-line interpreter (the shell, typically Bash) to automate tasks. It leverages command-line utilities, variables, and control flow structures to create powerful programs.

✍️ The vi Editor

The vi (or vim) editor is a text-based, modal editor crucial for writing shell scripts and editing configuration files in Linux. It operates in distinct modes:

📝 Shell Variables

Shell variables are named entities used to store data within a shell environment or a script.

• Defining/Assigning: NAME="value" (Note: no spaces around the equals sign).
• Accessing: Use the dollar sign ($) prefix.
• Example:

  MY_VAR="Linux"; echo "The OS is $MY_VAR"

• Special Variables (Positional Parameters): Used to access arguments passed to a script or function.

Special Variables (Positional Parameters)

🔄 I/O in Shell

Shell scripts manage Input/Output (I/O) through commands and redirection mechanisms.

• Reading Input: The read command pauses the script and waits for user input, storing it in a variable.

  read -p "Enter username: " USERNAME

• Output Redirection:
  • >: Redirects output to a file, overwriting it. Example: ls -l > output.txt
  • >>: Appends output to the end of a file. Example: echo "More data" >> output.txt
• Input Redirection:
  • <: Takes input for a command from a file instead of the keyboard. Example: wc -l < data.txt
• Piping (|): Connects the standard output of one command to the standard input of another.

  cat log.txt | grep ERROR | sort

🚦 Control Structures and Loops

These structures introduce logic flow, allowing scripts to make decisions and perform repetitive tasks.

Control Structures (Decision Making)

The if-elif-else structure executes commands based on the result of a test condition (often inside [ ] or [[ ]]).

if [ "$1" = "start" ]; then
    echo "Starting service..."
elif [ "$1" = "stop" ]; then
    echo "Stopping service..."
else
    echo "Usage: script {start|stop}"
fi

Loops (Repetition)

Common loop types:

📦 Subprograms (Functions)

Functions (subprograms) are reusable blocks of code defined within a script. They improve script organization and modularity.

# Define the function

backup_file () {
    # $1 here is the first argument passed to the function
    cp "$1" "$1.bak"
    echo "Backed up $1"
}

# Call the function
backup_file "config.conf"

* Arguments passed to the function are accessed using the local positional variables $1, $2, etc.

▶️ Creating & Executing Shell Scripts

1. Creating the Script
   • Use an editor (e.g., vi) to create a file (e.g., check_status.sh).
   • The first line must be the Shebang line, which specifies the interpreter.
   • Example:

     #!/bin/bash

2. Granting Execution Permission

   Scripts must have the execute permission set before they can be run directly.

   chmod +x check_status.sh

3. Executing the Script

Execution Methods