Shell

Overview

Shell scripting is a method of writing scripts using a shell, which is a command-line interpreter that provides a user interface for the operating system. Shell scripts automate repetitive tasks, manage system operations, and perform complex sequences of commands.

Common Shells

Bourne Shell (sh): Developed by Stephen Bourne at AT&T Bell Laboratories in 1979.
Bourne Again Shell (bash): An enhanced version of the Bourne Shell, widely used in Linux.
C Shell (csh): Developed by Bill Joy with C-like syntax.
Korn Shell (ksh): Combines features of the Bourne and C Shells, developed by David Korn.
Z Shell (zsh): An extended Bourne Shell with many improvements, created by Paul Falstad.

Features

Scripting

Shell scripts are text files containing a sequence of commands. They can include control structures, variables, functions, and more.

Command Substitution

Allows the output of a command to replace the command itself.

current_date=$(date)
echo "Today's date is $current_date"

Control Structures

Conditional Statements

#!/bin/sh
number=10
if [ $number -gt 5 ]; then
  echo "The number is greater than 5"
else
  echo "The number is 5 or less"
fi

Loops

For Loop

#!/bin/sh
for i in 1 2 3 4 5; do
  echo "Iteration $i"
done

While Loop

#!/bin/sh
counter=1
while [ $counter -le 5 ]; do
  echo "Counter: $counter"
  counter=$((counter + 1))
done

Variables

Variables store data that can be used and manipulated within the script.

#!/bin/sh
name="John"
echo "Hello, $name!"

Functions

Functions allow you to group commands into reusable blocks.

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

greet "Alice"
greet "Bob"

Redirection

Redirects input and output of commands.

# Redirect output to a file
echo "This is a new file" > newfile.txt

# Append output to a file
echo "This is another line" >> newfile.txt

# Redirect input from a file
while IFS= read -r line; do
  echo "$line"
done < newfile.txt

Pipelines

Chains commands together using pipes.

# List files and count the number of lines
ls | wc -l

Example: Backup Script

This script compresses the contents of a source directory into a tar.gz file and saves it in the backup directory with a timestamp.

#!/bin/sh
source_dir="/path/to/source"
backup_dir="/path/to/backup"
timestamp=$(date +%Y%m%d%H%M%S)
backup_file="$backup_dir/backup_$timestamp.tar.gz"

tar -czf $backup_file $source_dir
echo "Backup created at $backup_file"

Shell scripting is a versatile and essential skill for system administrators and developers, enabling efficient task automation and system management.

Last modified: 14 December 2024