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Is there some reason why bash 'variables' are different from variables in other 'normal' programming languages?

Is it due to the fact that they are set by the output of previous programs or have to be set by some kind of literal text, ie they have to be set by the output of some program or something outputting text through standard input/output or the console or such like?

I am at a loss to use the right vocabulary, but can anyone who can understands what I trying to say and perhaps use the right words or point me some docs where I can understand bash variable concepts better.

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  • I'd like to mention something about this question. I stated in the question that I Commented Jan 29, 2013 at 13:09
  • I want to comment about voting attitudes and apparent 'me too' attitudes on SO. When I posted this question within a short time there were about 4 comments (which have all vanished now) asking me what I meant by a 'variable' when I clearly stated that I couldn't explain it properly only that bash variables were different. As of this comment there are two answers which go some way towards my understanding. Is it right for people who don't understand the subject matter to vote to close without giving reasons why or even telling who they are? I also think they acted in concert to gather points. Commented Jan 29, 2013 at 13:16

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In most languages, variables can contain different kinds of values. For example, in Python a variable can be a number that you can do arithmetics on (a-1), an array or string you can split (a[3:]), or a custom, nested object (person.name.first_name).

In bash, you can't do any of this directly. If I understood you right, you asked why this is.

There are two reasons why you can't really do the same in bash.

One: environment variables are (conventionally) simple key=value strings, and the original sh was a pretty thin wrapper on top of the Unix process model. Bash works the same, for technical and compatibility reasons. Since all variables are (based on) strings, you can't really have rich, nested types.

This also means that you can't set a variable in a subshell/subscript you call. The variable won't be set in the parent script, because that's not how environment variables work.

Two: Original sh didn't separate code and data, since this makes it easier to work with interactively. Sh treated all non-special characters as literal. I.e. find / -name foo was considered four literal strings: a command and three arguments.

Bash can't just decide that find / -name now means "the value of the variable find divided by the negated value of variable name", since that would mean everyone's find commands would start breaking. This is why you can't have the simple dereferencing syntax other languages do.

Even $name-1 can't be used to substract, because it could just as easily be intended as part of $name-1-12-2012.tar.gz, a filename with a timestamp.

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I would say it has to do with Bash functions. Bash functions cannot return a value, only a status code.

So with Bash you can have a function

foo ()
{
  grep bar baz
}

But if you try to "save" the return value of the function

quux=$?

It is merely saving the exit status, not any value. Contrast this with a language such as Javascript, functions can actually return values.

foo ()
{
  return document.getElementById("dog").getAttribute("cat");
}

and save like this

quux = foo();
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