How to define a function inside another function in Bash?

Limit the scope of the inner function

Use function defined with parenthesis () instead of braces {}:

f() (
  g() {
    echo G
  }
  g
)

# Ouputs `G`
f
# Command not found.
g

Parenthesis functions are run in sub-shells, which have the same semantics of () vs {}, see also: Defining bash function body using parenthesis instead of braces

This cannot be used if you want to:

• set variables
• exit
• cd

as those are lost in the created sub-shell.

See also: bash functions: enclosing the body in braces vs. parentheses