How to define a function inside another function in Bash?

Solution 1:

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

Solution 2:

Function definitions in bash don't work the way function definitions work in many other languages. In bash, a function definition is an executable command which defines the effect of a function (replacing any previous definition), in much the same way that a variable assignment command defines the value of a variable (replacing any previous definition). Perhaps this example will clarify what I mean:

$ outerfunc1() {
> innerfunc() { echo "Running inner function #1"; }
> echo "Running outer function #1"
> }
$ outerfunc2() {
> innerfunc() { echo "Running inner function #2"; }
> echo "Running outer function #2"
> }
$
$ # At this point, both outerfunc1 and outerfunc2 contain definitions of
$ # innerfunc, but since neither has been executed yet, the definitions
$ # haven't "happened".
$ innerfunc
-bash: innerfunc: command not found
$
$ outerfunc1
Running outer function #1
$ # Now that outerfunc1 has executed, it has defined innerfunc:
$ innerfunc
Running inner function #1
$
$ outerfunc2
Running outer function #2
$ # Running outerfunc2 has redefined innerfunc:
$ innerfunc
Running inner function #2

Now, if you didn't already know this, I'm pretty sure this wasn't your reason for nesting function definitions. Which brings up the question: why are you nesting function definitions at all? Whatever effect you expected nested definitions to have, that's not what they do in bash; so 1) unnest them and 2) find some other way to accomplish whatever you were trying to get the nesting to do for you.

Solution 3:

In the question case I suppose that you were trying to call function2 before it is defined, "some function thing" should have been after the function2 definition.

For the sake of discussion, I have a case where using such definitions can be of some use.

Suppose you want to provide a function that might be complex, its readability could be helped by splitting the code in smaller functions but you don't want that such functions are made accessible.

Running the following script (inner_vs_outer.sh)

#!/bin/bash
function outer1 {
    function inner1 {
       echo '*** Into inner function of outer1'
    }
    inner1;
    unset -f inner1
}

function outer2 {
    function inner2 {
       echo '*** Into inner function of outer2'
    }
    inner2;
    unset -f inner2
}
export PS1=':inner_vs_outer\$ '
export -f outer1 outer2

exec bash -i

when executed a new shell is created. Here outer1 and outer2 are valid commands, but inner is not, since it has been unset exiting from where you have outer1 and outer2 defined but inner is not and will not be because you unset it at the end of the function.

$ ./inner_vs_outer.sh
:inner_vs_outer$ outer1
*** Into inner function of outer1
:inner_vs_outer$ outer2
*** Into inner function of outer2
:inner_vs_outer$ inner1
bash: inner1: command not found
:inner_vs_outer$ inner2
bash: inner2: command not found

Note that if you define the inner functions at the outer level and you don't export them they will not be accessible from the new shell, but running the outer function will result in errors because they will try executing functions no longer accessible; instead, the nested functions are defined every time the outer function is called.

Solution 4:

Don't nest function definitions. replace with:

$ cat try.bash 
function one {
  echo "One"
}

function two {
  echo "Two"
}

function three {
   one
   two
}

three
$ bash try.bash 
One
Two
$ 

Solution 5:

If you're nesting a function, say function2 inside function1, it doesn't become available until function1 is called. Some people might consider this a feature, as you can do something like "unset function2" at the end of function1 and its scope is completely local to that function (can't be called from elsewhere). If you want to call the function elsewhere, there's probably no need to nest it anyway.