Returning a boolean from a Bash function

Question

I want to write a bash function that check if a file has certain properties and returns true or false. Then I can use it in my scripts in the \"if\". But what should I retur

Answer 1

I encountered a point (not explictly yet mentioned?) which I was stumbling over. That is, not how to return the boolean, but rather how to correctly evaluate it!

I was trying to say if [ myfunc ]; then ..., but that's simply wrong. You must not use the brackets! if myfunc; then ... is the way to do it.

As at @Bruno and others reiterated, true and false are commands, not values! That's very important to understanding booleans in shell scripts.

In this post, I explained and demoed using boolean variables: https://stackoverflow.com/a/55174008/3220983 . I strongly suggest checking that out, because it's so closely related.

Here, I'll provide some examples of returning and evaluating booleans from functions:

This:

test(){ false; }                                               
if test; then echo "it is"; fi                                 

Produces no echo output. (i.e. false returns false)

test(){ true; }                                                
if test; then echo "it is"; fi                                 

Produces:

it is                                                        

(i.e. true returns true)

And

test(){ x=1; }                                                
if test; then echo "it is"; fi                                 

Produces:

it is                                                                           

Because 0 (i.e. true) was returned implicitly.

Now, this is what was screwing me up...

test(){ true; }                                                
if [ test ]; then echo "it is"; fi                             

Produces:

it is                                                                           

AND

test(){ false; }                                                
if [ test ]; then echo "it is"; fi                             

ALSO produces:

it is                                                                           

Using the brackets here produced a false positive! (I infer the "outer" command result is 0.)

The major take away from my post is: don't use brackets to evaluate a boolean function (or variable) like you would for a typical equality check e.g. if [ x -eq 1 ]; then... !