Backticks and $() for command substitution in Bash Scripting - Step-by-Step Execution

Concept Flow - Backticks and $() for command substitution

Start script

↓

Run command inside backticks or $()

↓

Shell executes command

↓

Command outputs text

↓

Shell replaces substitution with output

↓

Use output in script

↓

Continue script execution

The shell runs the command inside backticks or $(), captures its output, and replaces the substitution with that output for use in the script.

Execution Sample

Bash Scripting

current_date=`date`
echo "Today is $current_date"

This script stores the output of the date command in a variable and then prints it.

Execution Table

Step	Action	Command/Substitution	Output Captured	Variable State	Echo Output
1	Execute command substitution	`date`	Fri Jun 14 12:00:00 UTC 2024	current_date = 'Fri Jun 14 12:00:00 UTC 2024'
2	Echo variable	echo "Today is $current_date"		current_date unchanged	Today is Fri Jun 14 12:00:00 UTC 2024
3	End script			current_date unchanged

💡 Script ends after printing the date stored in current_date.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
current_date	undefined	Fri Jun 14 12:00:00 UTC 2024	Fri Jun 14 12:00:00 UTC 2024	Fri Jun 14 12:00:00 UTC 2024

Key Moments - 3 Insights

Why do we use backticks or $() around a command?

Because the shell runs the command inside them and replaces it with the command's output, as shown in execution_table step 1.

What happens if the command inside backticks produces multiple lines?

All output lines are captured as a single string with newlines preserved, and assigned to the variable, just like in step 1.

Is there a difference between using backticks and $()?

Both do command substitution, but $() is easier to read and nest, which helps avoid confusion in complex scripts.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of current_date after step 1?

A "date"

B "Fri Jun 14 12:00:00 UTC 2024"

C "Today is Fri Jun 14 12:00:00 UTC 2024"

D undefined

💡 Hint

Check the 'Variable State' column in row for step 1 in execution_table.

At which step does the script print the final output?

A Step 2

B Step 3

C Step 1

D No output is printed

💡 Hint

Look at the 'Echo Output' column in execution_table.

If we replace backticks with $(), how does the execution table change?

A Echo output changes to command name

B Variable current_date becomes empty

C No change in output or variable state

D Script fails to run

💡 Hint

Both backticks and $() perform command substitution similarly, as explained in key_moments.

Concept Snapshot

Command substitution runs a command inside backticks (`command`) or $() and replaces it with the output.
Use it to capture command output into variables.
Example: current_date=`date` or current_date=$(date).
$() is preferred for readability and nesting.
The shell replaces the substitution before running the rest of the script.

Full Transcript

In bash scripting, backticks (`command`) and $() are used for command substitution. This means the shell runs the command inside them and replaces the whole expression with the command's output. For example, current_date=`date` runs the date command and stores its output in the variable current_date. Then echo "Today is $current_date" prints the stored date. Both backticks and $() work the same, but $() is easier to read and nest. The execution table shows step-by-step how the command runs, output is captured, and the variable is set. This helps beginners see how command substitution works in practice.