a subshell is a child shell process

bash, represented by a box with a smiley face: hey, can you run this bash code for me?
other bash process: sure thing!

some ways to create a subshell

1. put code in parentheses (...)
(cd $DIR; ls)
   (runs in subshell)

2. put code in $(...)
var=$(cat file.txt)
   (runs in subshell)

3. pipe/redirect to a code block
   cat x.txt while read line...
   (piping to a loop makes the loop run in a subshell)

4. and lots more
   for example, process substitution <() creates a subshell

cd in a subshell doesn’t cd in the parent shell

(
cd subdir/
mv x.txt y.txt
)

I like to do this so I don’t have to remember to cd back at the end!

setting a variable in a subshell doesn’t update it in the main shell

var=3 
(var=2) 
echo $var

(this prints 3, not 2)

it’s easy to create a subshell and not notice

x=$(some_function)

sad stick person: I changed directories in some_function, why I didn’t it work?
happy stick person: it’s running in a subshell!