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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 return?

function myfun(){ ... return 0; else return 1; fi;}

then I use it like this:

if myfun filename.txt; then ...

of course this doesn't work. How can this be accomplished?

Accepted Answer

Use 0 for true and 1 for false.

Sample:

#!/bin/bash

isdirectory() {
  if [ -d "$1" ]
  then
    # 0 = true
    return 0 
  else
    # 1 = false
    return 1
  fi
}


if isdirectory $1; then echo "is directory"; else echo "nopes"; fi

Edit

From @amichair's comment, these are also possible

isdirectory() {
  if [ -d "$1" ]
  then
    true
  else
    false
  fi
}


isdirectory() {
  [ -d "$1" ]
}

Answer 2

Why you should care what I say in spite of there being a 350+ upvote answer

It's not that 0 = true and 1 = false. It is: zero means no failure (success) and non-zero means failure (of type N).

While the selected answer is technically "true" please do not put return 1** in your code for false. It will have several unfortunate side effects.

Experienced developers will spot you as an amateur (for the reason below).
Experienced developers don't do this (for all the reasons below).
It is error prone.

Even experienced developers can mistake 0 and 1 as false and true respectively (for the reason above).

It requires (or will encourage) extraneous and ridiculous comments.
It's actually less helpful than implicit return statuses.

Learn some bash

The bash manual says (emphasis mine)

return [n]

Cause a shell function to stop executing and return the value n to its caller. If n is not supplied, the return value is the exit status of the last command executed in the function.

Therefore, we don't have to EVER use 0 and 1 to indicate True and False. The fact that they do so is essentially trivial knowledge useful only for debugging code, interview questions, and blowing the minds of newbies.

The bash manual also says

otherwise the function’s return status is the exit status of the last command executed

The bash manual also says

($?) Expands to the exit status of the most recently executed foreground pipeline.

Whoa, wait. Pipeline? Let's turn to the bash manual one more time.

A pipeline is a sequence of one or more commands separated by one of the control operators ‘|’ or ‘|&’.

Yes. They said 1 command is a pipeline. Therefore, all 3 of those quotes are saying the same thing.

$? tells you what happened last.
It bubbles up.

My answer

While @Kambus demonstrated that with such a simple function, no return is needed at all. I think it was unrealistically simple compared to the needs of most people who will read this.