Shell Scripting Guide¶

This guide covers essential concepts and techniques for shell scripting, including customizing shell variables, loops, conditional statements, functions, and more. It’s designed to help you write efficient and maintainable scripts.

Customizing and Using Shell Variables¶

Key Points¶

Set environment variables.
Write Bash functions for frequently used commands.
Maintain skeleton directories for new user accounts.
Set the command search path with the proper directory.

Shell Overview¶

The shell is a powerful tool in Linux, providing an interface for managing the operating system. It takes user input as commands and outputs results to the terminal.

Shell Types¶

Login Shells: Initial shell started when a user logs in (e.g., via terminal or SSH). Uses ~/.profile or ~/.bashrc. Indicated by -.
Non-Login Shells: Started without a login process (e.g., from another shell or program). Executes ~/.bashrc.
Interactive Shells: Respond to user commands (e.g., Bash, Zsh).
Non-Interactive Shells: Do not allow user input (e.g., running a command via SSH).

Startup Scripts¶

/etc/profile: Sets environment variables for all users.
~/.bash_profile: Configures the user’s environment.
~/.bashrc: Runs for interactive non-login shells.
~/.bash_logout: Executes cleanup tasks on logout.

Executing Files¶

Use either of the following commands:

source <filename>
. <filename>

SKEL Directory¶

The /etc/skel directory contains template files for new user accounts. View its contents with:

cd /etc/skel

Extended Tests¶

Tests evaluate expressions using square brackets:

if [[ -f "$filename" ]]; then
  echo "File exists."
else
  echo "File does not exist."
fi

Loop Constructs¶

Types of Loops¶

For Loop
Iterates through a list of items:

bash for VARNAME in LIST; do COMMANDS done

Until Loop
Runs until a condition becomes true:

bash until [ "$count" -eq 5 ]; do echo "Count: $count" count=$((count + 1)) done

While Loop
Runs while a condition is true:

bash while [ $count -le 5 ]; do echo "Count: $count" ((count++)) done

Variables in Shell Scripting¶

Variable Basics¶

Syntax: <variable_name>=<variable_value> (no spaces around =).
Display: Use $ and echo:

bash echo $<variable_name>

Variable Types¶

Global Variables: Exist only within the shell they are created.
Local Variables: Exist in all new shells. Use export to make a local variable global:

bash export <variable_name>

Special Variables¶

$?: Exit status of the last command.
$$: PID of the current shell.
$!: PID of the last background job.
$#: Number of arguments passed to the script.
$@: All arguments passed to the script.

Case Statement¶

Executes code blocks based on a variable’s value:

case $fruit in
  apple) echo "You chose an apple!" ;;
  banana) echo "You chose a banana!" ;;
  *) echo "Unknown fruit!" ;;
esac

Script Structure and Extensions¶

Shebang: #!/bin/bash specifies the interpreter.
Permissions: Use chmod to set executable permissions:

bash chmod u+x script.sh

Execution: Run with ./script.sh.

Arrays¶

Store multiple values in a single variable:

declare -a size=(1 2 3 4 5)
echo ${size[1]}  # Output: 2

Arithmetic Expressions¶

Perform calculations:

sum=$((val1 + val2))

Conditional Expressions¶

AND Statement¶

if [[ "$age" -gt 18 && "$age" -lt 30 ]]; then
  echo "Valid age."
fi

OR Statement¶

if [ "$age" -gt 18 ] || [ "$age" -lt 30 ]; then
  echo "Valid age."
fi

Functions and Aliases¶

Creating Functions¶

function greet {
  echo "Hello, $1!"
}
greet "John"

Aliases¶

alias ll='ls -la'

Task List Script¶

A simple task manager:

#!/bin/bash

add_task() {
  echo "$1" >> tasks.txt
  echo "Task added."
}

remove_task() {
  sed -i "/$1/d" tasks.txt
  echo "Task removed."
}

view_tasks() {
  cat tasks.txt
}

while true; do
  echo "1. Add task"
  echo "2. Remove task"
  echo "3. View tasks"
  echo "4. Exit"
  read -p "Enter choice: " choice

  case $choice in
    1) read -p "Enter task: " task; add_task "$task" ;;
    2) read -p "Enter task to remove: " task; remove_task "$task" ;;
    3) view_tasks ;;
    4) exit 0 ;;
    *) echo "Invalid choice." ;;
  esac
done