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Basic Shell Scripting Guide

Shell scripting allows for the automation of tasks in Unix-like operating systems. Below are fundamental concepts and constructs commonly used in shell scripts.

Shebang

The shebang (#!) at the beginning of a script specifies the interpreter to execute the script.

#!/bin/bash

Variables

Variables in shell scripts are defined without a type. They can store strings, integers, or arrays. Assign values to variables without spaces around the = sign.

name="Alice"
age=30
numbers=(1 2 3 4 5)

To access the value of a variable, prefix the variable name with a $.

echo $name
echo ${numbers[2]}

Comments

Comments in shell scripts start with a # and continue to the end of the line.

# This is a comment
echo "This is not a comment"

If-else Statements

The if statement evaluates a condition and executes a block of code if the condition is true.

if [ condition ]; then
    # code block
elif [ condition ]; then
    # code block
else
    # code block
fi

Loops

For Loop

The for loop iterates over a list of items.

#!/bin/bash
for i in 1 2 3; do
    echo "Number: $i"
done

While Loop

The while loop executes a block of code as long as a condition is true.

#!/bin/bash
count=1
while [ $count -le 3 ]; do
    echo "Count: $count"
    ((count++))
done

Functions

Functions allow you to group code for reuse.

#!/bin/bash
greet() {
    echo "Hello, $1!"
}

greet "Alice"

Input and Output

Reading User Input

The read command reads input from the user and assigns it to a variable.

#!/bin/bash
echo "Enter your name:"
read name
echo "Hello, $name!"

Output Redirection

The > operator redirects output to a file, overwriting its contents.

echo "Hello, World!" > output.txt

Script Arguments

Access script arguments using $1, $2, and so on.

#!/bin/bash
echo "First argument: $1"
echo "Second argument: $2"

These arguments can be passed when running the script.

./script.sh arg1 arg2

Exit Status

The exit status of the last command executed is stored in the special variable $?.

#!/bin/bash
ls /path/to/file
echo "Exit status: $?"

This value is 0 if the command was successful, and non-zero if there was an error. This can be also used to exit a script with a specific status.

#!/bin/bash
if [ condition ]; then
    exit 0
else
    exit 1
fi

This guide provides an overview of essential shell scripting elements to help you automate tasks effectively.

Sources: