Bash: create a function that returns a Boolean value

Scene description

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Writing Shell Script time , Sometimes I will encounter more complex logical judgments , Only when these complex logical judgments return true or false Only when , At this time, if these complex logical judgments are written directly in if Back , It will look messy , Poor readability , And it is not easy to maintain later .

resolvent

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For the scenario described above , We can write complex logical judgments into a function , According to the results of these complex logical judgments , We hope this function can return true or false.

however ,Bash There is no Boolean value in , Reference resources bash Of man manual , Function return Only one value can be returned :

return [n]

       ...

       The return status is non-zero if return is supplied a non-numeric argument, or is used outside  a function  and  not  during execution of a script by . or source.

       ...

man bash

however , stay Shell in , Whether a command is successfully executed can be used exit code To judge ,0 It means success , Not 0 Value to indicate failure :

For  the shell's purposes, a command which exits with a zero exit status has succeeded.  An exit status of zero indicates success.  A non-zero exit status indicates failure.

man bash

And in the function return value , In fact, it is the exit state of the function , So we can use return 0 To express true, return 1 To express false.

Example

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Here is an example , In this case , We determine whether a given path is a directory ：

#!/bin/bash

function isDir() {
    path="$1"

    if [ -d "$path" ]
    then
        # 0  Express  true
        return 0
    else
        # 1  Express  false
        return 1
    fi
}

path="/tmp"

if isDir "$path"
then
    echo "$path is dir"
else
    echo "$path is not dir"
fi

In the example above , We wrote the conditional judgment isDir()  Function , But when judging that the given path is a directory , return 0 (true); Otherwise return to 1 (false).

as everyone knows ,Linux In the system /tmp Is a directory , Therefore, the running result of the above program is ：

/tmp is dir

Of course , The above example is relatively simple , There is no need to use functions , But when you encounter scenes with complex logical judgments , It would be a better way to write logical judgments into functions .

summary

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Shell There is no boolean type in , But we can use in functions return 0 Said to return to true;return 1 Express false. Writing Shell Script time , Write complex logical judgments into functions , And then by going back 0 or 1 To express true or false, Will make Shell Scripts are more readable , Later maintenance is also more convenient .

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