Firebird Flight Wiki: Shell Scripting & Automation

Shell Scripting & Automation

Writing shell scripts is a powerful way to automate tasks, combine commands, and create custom workflows. This page introduces core scripting concepts, syntax, and practical examples to help you get started with Bash-based automation.

Contents

Introduction
Basic Syntax
Example Scripts
Best Practices
Conclusion

1. Introduction

Linux shells, like Bash, Zsh, or Dash, allow you to interact with the system via commands. By placing commands in a file (a script), you can automate tasks repeatedly without retyping. Common use cases include:

Regular backups
System updates or cleanup
Log parsing and reporting
Continuous deployment pipelines

We’ll focus on Bash scripting since it’s installed by default on many distributions, but the concepts apply to other shells, too.

2. Basic Syntax

A Bash script typically begins with a shebang line pointing to the shell interpreter:

#!/usr/bin/env bash

Then you include commands, variables, and control structures (if, for, while) as needed. Remember to make the script executable with chmod +x filename.sh or run it via bash filename.sh.

Variables

Variables are assigned without spaces, e.g. MYVAR="Hello". Access them with $MYVAR. For example:

MYVAR="Hello World"
echo $MYVAR

Conditions & Loops

Bash supports if-then-else blocks and loops like for, while, until:

if [ "$MYVAR" == "Hello World" ]; then
  echo "Yes, it matches!"
else
  echo "No, it doesn't."
fi

for FILE in /path/to/*; do
  echo "Found $FILE"
done

3. Example Scripts

3a. Hello World

The simplest script to confirm your environment is set up:

#!/usr/bin/env bash
# hello.sh - a simple Hello World script

echo "Hello, Firebird Flight!"

Make it executable (chmod +x hello.sh), then run ./hello.sh.

3b. Simple Backup Script

Backs up a specified directory to a timestamped tar file:

#!/usr/bin/env bash
# backup.sh - backs up a directory to /tmp

SOURCE_DIR=$1
DEST_DIR="/tmp"
TIMESTAMP=$(date +%Y%m%d-%H%M%S)
BACKUP_FILENAME="backup-$(basename $SOURCE_DIR)-$TIMESTAMP.tar.gz"

if [ -z "$SOURCE_DIR" ]; then
  echo "Usage: $0 /path/to/dir"
  exit 1
fi

# Create the tarball
tar -czf "$DEST_DIR/$BACKUP_FILENAME" "$SOURCE_DIR"
if [ $? -eq 0 ]; then
  echo "Backup of $SOURCE_DIR saved to $DEST_DIR/$BACKUP_FILENAME"
else
  echo "Backup failed!"
  exit 1
fi

Usage: ./backup.sh /var/www, for example. The script ensures you passed a directory path, then creates a compressed archive in /tmp.

3c. Log Parsing Script

A simple script that searches /var/log/syslog for a given keyword, counting occurrences:

#!/usr/bin/env bash
# parse_logs.sh - searches syslog for a keyword

KEYWORD=$1

if [ -z "$KEYWORD" ]; then
  echo "Usage: $0 [keyword]"
  exit 1
fi

COUNT=$(grep -ic "$KEYWORD" /var/log/syslog 2>/dev/null)

if [ $? -eq 0 ]; then
  echo "Found $COUNT occurrences of '$KEYWORD' in /var/log/syslog"
else
  echo "Error reading /var/log/syslog"
  exit 1
fi

Usage: ./parse_logs.sh error might output “Found 12 occurrences of 'error'...”

4. Best Practices

**Use `#!/usr/bin/env bash`** instead of a hard-coded path like `/bin/bash`. This ensures the script picks up the Bash interpreter in the user’s environment.
**Check exit codes** (`$?`) after critical commands to detect failures early.
**Quote variables** (`"$VAR"`) to handle spaces or special characters properly.
**Add usage messages** and argument checks so users know how to run your script.
**Comment your code** to explain tricky logic and keep a record of what each function does.
**Test thoroughly**. Scripts can inadvertently remove files or break systems if not tested carefully.

5. Conclusion

Shell scripting bridges the gap between manual command usage and full-scale programming languages. Even simple scripts can save you hours by automating repetitive tasks. As you gain experience, you’ll create more robust scripts for backups, deployments, container orchestration, and more.

For advanced automation, consider integrating with cron jobs, systemd timers, or configuration management tools (e.g., Ansible) to manage your scripts at scale. Happy scripting!