An Introduction to Shell Scripting: Automating Tasks on Linux

Shell scripts allow automating workflows on Linux. This friendly guide covers shell history, syntax basics, real-world scripts, best practices, tools and in-depth topics to empower you with shell scripting skills!

The Many Shells of Linux

The original Unix shell came on the scene in 1971. But today there are many options:

Popular Shells by Usage Share:

Shell Percentage
Bash 94%
Zsh 4%
Fish 1%
Others 1%

As this breakdown shows, Bash dominates as the default interactive shell and for scripting due to being widely available and full-featured. But alternatives like Zsh and Fish also have strong followings.

Over 28% of Linux system administrators report using shell scripts for automating tasks according to the Tecmint 2021 survey, and that number climbs every year. The flexibility and raw power of the Unix shell continues propelling it forward!

Now let‘s dive into unlocking that potential for yourself…

Shell Scripting Syntax Basics

The first step is learning the building blocks of crafting a shell script.

While syntax differs slightly across shells, the concepts are similar. We‘ll use standard Bash syntax since that transfers widely.

Let‘s start scripting!

Shebang

Scripts start with a "shebang" pointing at the interpreter:

#!/bin/bash

Other options could include /bin/sh, /bin/zsh, etc. This tells the OS how to execute what follows.

Comments

Code comments use the # symbol:

# This initializes variables
name="John" # Set name

Comments help document and disable sections of code.

Echo

Printing to screen uses the echo command:

echo "Hello $name" # Hello John

Echo is useful for displaying output, variables, and messages.

Variables

Store reusable values in variables like:

count=5
echo "$count Accesses" # 5 Accesses

Var syntax is name=value. Access with $name.

Conditionals

Support logic and flow control using if statements:

if [[ $count > 10 ]]; then
  echo "Warning: High Access Count" 
fi

Check conditions and run code selectively.

That covers the basics – let‘s look at how to work with other programs.

Integrating External Commands

The abilities of other Linux utilities and apps can be integrated directly into your scripts thanks to pipes, redirection, and substitution.

For example, file transformations:

cat files.txt | grep -i ".log" | cut -d"/" -f3 > results.txt

System maintenance:

df -h | grep "/dev/sda1" | { read a b c d e size rest; 
  if [[ $size > 90% ]]; then 
    echo "Warning! Disk usage high!" | mail [email protected]
  fi
}

This demonstrates leveraging the Linux toolchain for processing text, monitoring system state, and automating workflows.

Let‘s examine some of those concepts closer…

Piping |

The pipe | connects stdout to stdin between programs:

ls -l | grep ".doc"

This streams data allowing complex combinations.

Command Substitution $()

Substitute the output from a command as a variable value:

files=$(find . -type f | wc -l) # Set files count

Allows assigning program output to variables.

Redirection > >> <

Redirects output streams to files rather than stdout:

echo "Data loaded $(date)" >> log.txt # Append log

Insert detail logs without tampering normal output.

This makes it easy to leverage other Linux commands in powerful ways.

Now that we‘ve covered the scripting fundamentals, let‘s discuss some best practices!

Writing Robust Shell Scripts

Like any programming, following best practices ensures your shell scripts are maintainable, portable and less buggy.

Here are some tips:

File Naming

  • Use .sh extension for shell scripts
  • Name files based on functionality

Ex: setup_docker.sh

Formatting

  • Indent code under conditionals/loops
  • Break long lines for readability
  • Use consistent spacing around = $ ( )

Organization matters!

Comments

  • Add header summarizing script purpose
  • Document sections of complex code
  • Comment out debug prints rather than delete

Code tells how, comments tell why.

Error Handling

  • Display user-friendly messages on failure
  • Return proper exit codes from script
  • Validate input rather than assume valid
  • Handle exceptions gracefully

Failures happen – make them easy to debug!

Help Text

  • Include help option with -h/--help
  • List available options and usage
  • Give examples for common scenarios

Scripts should self-document.

Using these practices will ensure your solutions remain maintainable and robust over time!

Now let‘s walk through some real-world examples.

Automating with Shell Scripts

To give you a taste of common automation scenarios, here are some examples of using shell scripts:

1. System Administration

Checking disk usage, updating software, notifications – these repetitive sysadmin tasks are perfect for scripts:

#!/bin/bash

# Monitor disk usage
df -h | grep "/dev/sda1" | awk ‘{print $5}‘ > space.txt 

# Warn if low space  
if [[ $(cat space.txt) > 90% ]]; then
  echo "Disk usage high!" | mail -s "Alert" [email protected]
fi

# Patch system to latest 
yum update -y

Here a single script handles monitoring, notifications and patching.

2. Data Processing

In data science, you often chain tools for ETL and analysis:

#!/bin/bash

# Extract - download data 
curl https://data.source > raw.csv  

# Transform - clean and filter 
cut -d, -f1-5,7-10 raw.csv | grep -v "NA" > clean.csv 

# Load - summarize for reporting  
echo "Total Records: $(wc -l < clean.csv)"

This demonstrates a common ETL paradigm – made easy with piping and substituion!

3. Application Deployment

Deploying web apps involves coordinating tools like git, config managers, databases and more – orchestration made easy with shell scripts:

#!/bin/bash

# Provision resource 
terraform apply loadbalancer.tf  

# Configure settings
ansible-playbook playbook.yaml 

# Deploy application
git pull origin main
docker-compose up -d

# Validation checks
curl --retry 10 http://myapp.com

This standardizes and automates the deployment process reliably.

As you can see, scripts apply to a wide variety of workflow automation scenarios – they are a versatile tool!

Now that you have the basics down, let‘s cover some more advanced shell concepts…

Taking Shell Skills to the Next Level

We‘ve covered the basics – but the shell has many more powerful capabilities! Here‘s a taste to spur your curiosity:

Functions & Libraries

Break code into reusable functions:

greeting() {
  echo "Hello $1!"
}

greeting "Shelly" # Hello Shelly!

Functions allow easy re-use and composition. Load from scripts with source.

Job Control

Push processes to background and foreground:

sleep 100 & # background process
fg %1 # bring last stopped job to foreground

Manage concurrent tasks from shell.

Aliases

Create command shortcuts by assigning aliases: 

alias update=‘sudo apt update && sudo apt upgrade‘

Set custom names for commonly used sequences.

Dotfiles

Shell startup configurations stored in ~/.bashrc and ~/.bash_profile:

# .bashrc
export EDITOR=vim  
alias ll=‘ls -lah‘

Load aliases, settings, startup commands from these dotfiles.

This is still just scratching the surface of built-in shell capabilities – there is always more to learn!

Comparing Shell to Other Automation Tools

It‘s important to note that while shell scripting is powerful, it differs from other popular automation technologies:

Feature Shell Script Python Ansible
Allows custom code Yes Yes Limited
Data structures Basic Full featured Jinja2 templates
Enforces structure No Somewhat Yes – declarative
Execution speed Medium Fast Slow
Learning curve Shallow Moderate Shallow
Integrates commands Tightly Loosely Tightly
Open source Yes Yes No

Each approach has tradeoffs – generally shells are more flexible and direct, while Python and Ansible encourage modular reusable code.

Understanding these technical differences allows selecting the best tool for your use cases between shell scripts, Python, Ansible and more.

Resources for Leveling Up Shell Wizardry

With this intro, hopefully you feel empowered to start scripting – but don‘t stop here!

Here are excellent resources to take your shell mastery to new heights:

Tools

Books

  • "The Linux Command Line" by William Shotts
  • "Linux Shell Scripting Cookbook" by Shantanu Tushar
  • "Linux Command Line and Shell Scripting Bible" by Richard Blum

Tutorials

Leveraging tutorials/books, tools like shellcheck, and patterns from code like this will transform you into an unstoppable shell wizard!

The end goal is being able to script pipelines and solutions faster than doing the task manually.

Let‘s get building!

Let‘s Recap

In this detailed guide you covered:

  • Origin of Unix shells and prominence today
  • Shell scripting basics – variables, conditionals
  • Integrating external Linux commands
  • Best practices for maintainable scripts
  • Real world automation examples
  • Introduction to advanced built-in features
  • How other technologies compare
  • Resources for mastering shell scripting

You‘re now equipped to start on your automation journey with Linux shell scripting.

The skills will unlock the ability to quickly orchestrate complex workflows that would take 10X effort by hand. This lets you focus time on big picture goals rather than repetitive tasks.

Script ALL the things!

Let me know in the comments if you have any other questions as you being your shell adventure. Happy automating!

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