Shell Scripting: Automate Your Work with Bash

Introduction

Anything you type in the terminal can be saved in a file and run again — that is a shell script. Scripts turn repetitive tasks into one command: backups, deployments, cleaning up logs, setting up a new machine. They are also everywhere in real projects, from Docker images to CI/CD pipelines, so reading and writing them is a core skill.

Your First Script

#!/bin/bash
# hello.sh: variables and command substitution
name="Priya"
today=$(date +%A)
echo "Hello, $name! Today is $today."
echo "You are in $(pwd)"

The first line, #!/bin/bash (the “shebang”), tells Linux which program should run the file. Lines starting with # are comments. name="Priya" sets a variable, and $(date +%A) is command substitution: it runs date +%A and puts its output (the day of the week) into the text. Make the script executable and run it:

chmod +x hello.sh
./hello.sh

Arguments and if

Words after the script name are arguments: $1 is the first, $2 the second, $# how many there are, and $0 the script’s own name. if makes decisions, using tests inside [ ]:

#!/bin/bash
# greet.sh: arguments and if/else
if [ $# -eq 0 ]; then
  echo "Usage: $0 <name>"
  exit 1
fi

name="$1"
if [ "$name" = "admin" ]; then
  echo "Welcome back, administrator."
else
  echo "Hello, $name!"
fi

./greet.sh
echo "exit code: $?"

./greet.sh Arjun
./greet.sh admin

Loops and Arithmetic

A for loop repeats commands for each item in a list — here, every .txt file. $(( )) does arithmetic:

#!/bin/bash
# count-lines.sh: loop over files and do arithmetic
total=0
for file in *.txt; do
  lines=$(wc -l < "$file")
  echo "$file: $lines lines"
  total=$((total + lines))
done
echo "Total: $total lines"

./count-lines.sh

Bash also has while loops, which repeat as long as a condition is true — commonly used to read a file line by line with while read -r line; do …; done < file.txt.

A Real Script: Backups

Here is a complete, practical script that saves a folder as a dated archive. It checks its input, uses a function, and reports errors properly:

#!/bin/bash
# backup.sh: save a folder as a dated .tar.gz archive
set -euo pipefail

backup_folder() {
  local source="$1"
  local dest="$HOME/backups"
  local name
  name="$(basename "$source")-$(date +%Y%m%d-%H%M%S).tar.gz"
  mkdir -p "$dest"
  tar -czf "$dest/$name" -C "$(dirname "$source")" "$(basename "$source")"
  echo "Backed up $source to $dest/$name"
}

if [ $# -ne 1 ]; then
  echo "Usage: $0 <folder>" >&2
  exit 1
fi
if [ ! -d "$1" ]; then
  echo "Error: '$1' is not a folder" >&2
  exit 2
fi

backup_folder "$1"

./backup.sh
echo "exit code: $?"

./backup.sh nothing-here
echo "exit code: $?"

./backup.sh website
echo "exit code: $?"
ls backups

tar -tzf lists what is inside an archive without extracting it, so you can check a backup:

tar -tzf backups/website-*.tar.gz

Exit Codes, && and ||

Every command finishes with an exit code: 0 means success, anything else is an error. && runs the next command only if the previous one succeeded, and || runs it only if the previous one failed:

mkdir reports && echo "created reports"
mkdir reports && echo "created again" || echo "could not create reports"

Writing Safer Scripts

By default Bash keeps going after a command fails, which can turn one error into a disaster. The line set -euo pipefail near the top of backup.sh changes that:

Common Mistakes

Frequently Asked Questions

Key Takeaways

Summary

You can now automate tasks with real, robust Bash scripts. Next you will reach beyond your own computer: checking network connections, downloading with curl and wget, and working on remote servers over SSH.