complicated bash functions returning boolean - bash

Question

I want to do something like this:
function one {
if [ "$1" == "hello" ]; then
return true
fi
return false
}
if [ one hello -o one goodbye ]; then
echo "Well, that's weird."
fi
As written, I know this doesn't work. Bash won't let you return boolean values. You can return 0 or 1. But bash doesn't seem to like using 0 or 1 as boolean expressions. So instead I have to return 0 instead of true and 1 for false. I can live with that. But then my if becomes:
if [ one hello -eq 0 -o one goodbye -eq 0 ] ...
Which I can do, but it's awkward. But THAT doesn't work, either, because test doesn't want to call my function with an argument.
if [ `one hello` -eq 0 -o `one goodbye` -eq 0 ] ...
Finally, I think that version works. But it's fugly.
Is there an elegant way to:
Have a bash function that returns true/false
And it takes arguments
And then it gets used in a complex if-statement (there's some combination of -o / -a between the [] )
I'd really like to have some way of writing:
if [ myfunc1 $somearg -a $myfunc2 $someotherarg ]; then ...
For now, I'm doing it the fugly way.

Answer 1

You don't really write predicates like this. Instead, you set the exit status to 0 if a function succeeds, or non-zero value if it fails. The if statement then checks the exit status directly.
one () {
if [ "$1" = "hello" ]; then
return 0
fi
return 1
}
if one hello || one goodbye; then
echo "Well, that's weird."
fi
Since [ itself is a command that has a 0 exit status when the test is true and 1 if false, you can define one as simply
one () {
[ "$1" = "hello" ]
}
If no return statement is encountered, the exit status of a function is the exit status of the last command to execute.

Answer 2

Functions don't return true or false. Instead, they succeed or fail. if checks to see if a command succeeded or not. [ is a very common command that is used in shell scripts, but it misleads many into believing it is part of the grammar. It is not. You can simply write:
if one hello || one goodbye; then
echo "Well, that's weird."
fi
You don't want to use -a or -o. (Those are typically passed as arguments to [, but are effectively deprecated in that usage.) Instead, use the shell operators && and ||.

Answer 3

The fun thing about the strings "true" and "false" is that they are also shell builtin commands that have the expected exit status:
function one {
[[ "$1" == "hello" ]] && echo true || echo false
}
a=$(one hello)
b=$(one goodbye)
if $a || $b; then ...
But, don't do this. Keep it simple as other answers advice.

Answer 4

Like this?
one () {
if [ "$1" = "hello" ]; then
echo true; return 0
fi
echo false; return 1
}
$ one hello && one bye || one hello
true
false
true

Answer 5

Use the builtin commands, /bin/true and /bin/false:
/bin/true always returns 0.
/bin/false always returns 1.
function one {
[[ "$1" == "hello" ]] && /bin/true || /bin/false
}
if one hello || one goodbye; then
# do stuff…
fi;
Because they’re builtins under the /bin (/usr/bin) directory, which will likely be in your $PATH, you can simply call true or false:
function one {
[[ "$1" == "hello" ]] && true || false
}
if one hello || one goodbye; then
# do stuff…
fi;
Some of the answers above almost got it. Though, you wouldn’t want to echo the words, “true” or “false,” as strings—that does nothing in relation to the question. You want to call the commands that were made for this very purpose.