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I would like to have a shell variable that could be dynamically run every time it is refered, for example, i would like to have a variable $countPwd which could return the count of files/dirs in the current directory, it could be defined as:

countPwd=`ls | wc -l`

and if I do echo $countPwd it would only show the value when I define the variable, but it won't update automatically when I change my current directory. So how do I define such a variable in bash that the value of it get updated/calculated on the fly?

Update: The $PWD is a perfect example of a variable get evaluated in the real time. You don't need to use $() or backticks `` to evaluate it. How is it defined in bash?

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    Write a cover function for cd that sets the variable when you have changed directory. Or create a script or function (perhaps fc for 'file count') and simply use that (less typing). Commented May 3, 2011 at 19:38

3 Answers 3

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Make a function:

countPwd() {
    ls | wc -l
}

Then call the function like any other command:

echo "There are $(countPwd) files in the current directory."
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but that's not a variable, so I still can't do echo $countPwd. In another word, I would like to have a variable similar to the $PWD in bash. The $PWD is a perfect example of a variable get evaluated in the real time. How is it defined?
PWD is a "sell variable"; somehow a «builtin». Also it is not evaluated in real time, it is set by the cd command (as per what bash's man page says). IMHO the workaround proposed using a bash function is quite good (+1)
@hmontoliu Thanks. I guess your explaination about cd command solve the mystery, then I guess John's answer is my solution.
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Another option: store the command in a variable, and evaluate it when required:

countPwd='ls | wc -l'
echo $(eval "$countPwd")
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-1 Really terrible idea. I know you can't go wrong with a number, but if someone changes the value of countPwd you're SOL.
@l0b0 You can make countPwd readonly to alleviate that concern.
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You'll need to write some C code: write a bash's loadable buitins to do the heavy lifting in defining variables with dynamic values like $SECONDS or $RANDOM ($PWD is just set by cd).

More details in my answer here (a duplicate of this question).

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If there is a good duplicate, then this question should be closed as duplicate. But at least you spent the time to really answer this question.

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