WHY

In Bash scripting, the creation of child shells, often through pipelines or certain command structures, can lead to unexpected behavior in variable management.

When having command like find ... | ..., a child shell is created. Child processes gets readonly copies from the parent shell's memory. Any variable changes in the child shell trigger Copy-on-Write (CoW), creating a separate memory copy for that child.

However, unmodified variables remain in shared memory.

#!/bin/bash

# ...
processed_count=0
updated_count=0


find "$directory" -name "*.jpeg" -type f | while read -r file; do
    # ...
    ((processed_count++))
done

echo "Total "$processed_count" files, Update "$updated_count"  files"

The last line shows processed_count and updated_count still remain the same as initial value. And I don’t want that result.

Solution

This approach ensures that the loop runs in the current shell, allowing variables to be updated correctly.

#!/bin/bash

# ...

while read -r file; do
    # ...
    ((processed_count++))
done < <(find "$directory" -name "*.jpeg" -type f)

(..) v.s. ((..))

Single Parentheses ()

• Creates a child shell (child process)

• ; works as commands separator

Double Parentheses (())

• It only does arithmetic expression

• works in the current shell

• ; causes error

• Cannot contain many commands together command1; command2; command3;
  • but can do math expression 1, math expression 2

# works
(var=20; echo "In subshell: $var")

# failed, syntax error, it only does "arithmetic expression"
((var=20; echo "In subshell: $var"))

# works, just many arithmetic expressions
((var=20, temp=var*2))

 ((x = y + 1))
 # equal to 
 x=$((y + 1))

Why need (()) to specify an expression even when it's already on the current shell

# no child
while read -r file; do
    # ...
    ((processed_count++))
done < <(find "$directory" -name "*.html" -type f)

prevents ambiguity

make it elegant

Child Shell vs. Child Process

• every child shell is a child process

• not every child process is necessarily a child shell

• Child shells are a specific type of child process.