Recursive functions in Bash Scripting - Step-by-Step Execution

Concept Flow - Recursive functions

Call function with n

↓

Check base case: n == 0?

Yes→Return base value

No↓

Call function with n-1

↓

Receive result from smaller call

↓

Combine result with current n

↓

Return combined result

↩Back to previous call or end

The function calls itself with a smaller input until it reaches a base case, then returns values back up the call chain.

Execution Sample

Bash Scripting

#!/bin/bash
factorial() {
  if [ $1 -eq 0 ]; then
    echo 1
  else
    local prev=$(factorial $(( $1 - 1 )))
    echo $(( $1 * prev ))
  fi
}
factorial 3

Calculates factorial of 3 by calling itself with decreasing numbers until 0, then multiplies results back up.

Execution Table

Step	Function Call	Parameter n	Base Case?	Action	Output
1	factorial 3	3	No	Call factorial 2
2	factorial 2	2	No	Call factorial 1
3	factorial 1	1	No	Call factorial 0
4	factorial 0	0	Yes	Return 1	1
5	factorial 1	1	No	Calculate 1 * 1	1
6	factorial 2	2	No	Calculate 2 * 1	2
7	factorial 3	3	No	Calculate 3 * 2	6
8	End			All calls returned	6

💡 Reached base case at n=0, then returned results up the call stack until original call completed.

Variable Tracker

Variable	Start	After Step 4	After Step 5	After Step 6	After Step 7	Final
n	3	0 (in deepest call)	1	2	3	3
prev		1	1	2	6	6
Output		1	1	2	6	6

Key Moments - 3 Insights

Why does the function call itself with n-1 instead of n+1?

Because the function needs to reach the base case where n=0 by decreasing n each time, as shown in execution_table steps 1-4.

What happens when the base case is reached?

The function returns 1 immediately without further calls, as seen in step 4, which stops the recursion from continuing infinitely.

How does the function combine results from recursive calls?

Each call waits for the smaller call to return, then multiplies the returned value by its own n, shown in steps 5-7.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when factorial is called with n=1 (step 5)?

A 0

B 2

C 1

D 3

💡 Hint

Check the Output column at step 5 in the execution_table.

At which step does the base case occur in the execution table?

A Step 4

B Step 7

C Step 1

D Step 3

💡 Hint

Look for the row where Base Case? is Yes.

If the base case returned 0 instead of 1, what would be the final output for factorial 3?

A 6

B 0

C 3

D 1

💡 Hint

Consider how multiplication with 0 affects the final result in the variable_tracker.

Concept Snapshot

Recursive functions call themselves with smaller inputs.
They must have a base case to stop recursion.
Each call waits for the smaller call to return.
Results combine as calls return back up.
In bash, use local variables to store recursive results.
Always test base case to avoid infinite loops.

Full Transcript

This example shows a bash recursive function calculating factorial. The function calls itself with n-1 until it reaches 0, the base case, where it returns 1. Then each call multiplies its n by the returned value from the smaller call. The execution table traces each call and return step. Variables n and prev change as recursion deepens and unwinds. Key moments clarify why n decreases, what base case means, and how results combine. The quiz tests understanding of outputs and base case location. The snapshot summarizes recursion essentials in bash scripting.