🐚 Bash Shell Substitutions – Using $(...) and Backticks `...` in Scripts

Introduction to Shell Substitution in Bash – Command Substitution Explained

Shell substitution in Bash allows you to capture the output of a command and use it as part of another command or assign it to a variable. This feature is fundamental for building dynamic, data-driven scripts.

Bash provides two primary syntaxes for command substitution:

• Modern syntax: $(...)
• Legacy syntax: `...` (backticks)

Understanding these syntaxes helps you write clear, nested, and efficient scripts.

In this article, you’ll learn:

• What command substitution is in Bash
• The difference between $(...) and `...`
• How to assign command output to variables
• How to nest commands using substitution

What Is Command Substitution in Bash?

Command substitution lets you run a command, capture its output, and use it as:

• An argument to another command
• A value for a variable
• Inline output in strings or conditions

Syntax Examples

Modern Command Substitution: $(...)

today=$(date)
echo "Today is: $today"

Output:

Today is: Sat Jun 14 10:05:37 IST 2025

Legacy Syntax: `...`

now=`date`
echo "Current time: $now"

Output:

Current time: Sat Jun 14 10:05:37 IST 2025

The backtick method is older and harder to read when nesting commands.

Why Prefer $(...) Over Backticks?

Real-World Examples of Bash Shell Substitution

Assign Output to Variable

user_count=$(who | wc -l)
echo "Logged in users: $user_count"

Use in Filename or Log

filename="backup-$(date +%F).tar.gz"
echo "Filename: $filename"

Output:

Filename: backup-2025-06-14.tar.gz

Nesting Commands

echo "Current directory: $(basename $(pwd))"

Output:

Current directory: myproject

Clean, readable nesting with $(...).

With Loops or Conditions

if [ "$(whoami)" = "root" ]; then
  echo "You are root!"
else
  echo "You are not root."
fi

Common Mistakes to Avoid

Summary – Bash Command Substitution

Command substitution is a powerful Bash feature that lets you dynamically embed command output inside scripts. The modern $(...) syntax is preferred over backticks for clarity, readability, and maintainability—especially when nesting.

Key Takeaways:

• $(...) runs a command and substitutes its output
• Backticks (`...`) still work, but are outdated
• Always quote command substitution to avoid word splitting

Real-world Uses:

• Automating backup filenames with dates
• Dynamic decision-making using system commands
• Combining outputs from multiple utilities

FAQ – Bash Command Substitution

What is the difference between $(...) and `...`?
Both perform command substitution. However, $(...) is easier to read, supports nesting, and is recommended for all modern Bash scripts.

Can I use command substitution in a conditional statement?
Yes. Example:

if [ "$(whoami)" = "root" ]; then echo "You're root"; fi

How do I nest command substitutions?
Use:

echo "$(basename $(pwd))"

Backticks would require escaping, which is messy.

Do I need to quote $(...) in Bash?
Yes, it’s best practice to quote it:

"$(command)"

This prevents issues with whitespace or globbing.

Is $(...) POSIX compliant?
Yes, and it is supported across all modern Unix/Linux systems and shells.