HPR2807: Are bash local variables local?


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In hpr2739, Dave talked briefly about local variables. But what are they?

In most modern languages, especially in compiled languages, "local" means that the value of a variable cannot be directly known, by looking up the name, outside the bounds of that function, but that’s not how it works in bash.

Languages like C and Python have lexical scope. Lexical scope means local variables are local in the text. The names are local.

If I’m writing code that is textually located outside the function, I cannot even describe how to access the variables within the function, because myvariable in my function is not the same variable, not the same place, as myvariable in your function.

Languages like Bash and Elisp have dynamic scope. That means local variables are local in time. The names are global.

What happens when you declare a variable local in bash is that the existing value of that variable is stowed away, to be brought back when your function exits.

#!/usr/bin/env bash function sayscope() { echo The scope is $whatsmyscope } function globalscope() { whatsmyscope=global } function dynamicscope() { whatsmyscope=dynamic } function localscope() { local whatsmyscope=local sayscope dynamicscope sayscope } globalscope sayscope localscope sayscope
The scope is global The scope is local The scope is dynamic The scope is global

Perl has both, and it calls them local (dynamic scope, like bash) and my (lexical scope):

#!/usr/bin/env perl use v5.10; sub sayscope { say "Dynamic scope is $whatsmyscope"; } sub globalscope { $whatsmyscope="global"; } sub dynamicscope { $whatsmyscope="dynamic"; } sub lexicalscope { my $whatsmyscope="lexical"; say "Lexical scope is $whatsmyscope"; sayscope; } sub localscope { local $whatsmyscope="local"; sayscope; dynamicscope; sayscope; lexicalscope; } globalscope; sayscope; localscope; sayscope;
Dynamic scope is global Dynamic scope is local Dynamic scope is dynamic Lexical scope is lexical Dynamic scope is dynamic Dynamic scope is global

You almost never want to use local in Perl, it’s mostly there for historical reasons — lexical scope is a Perl 5 feature. https://perl.plover.com/local.html covers well the remaining few and narrow exceptions where local might be useful.

As dynamic scope has some valid use, it’s available in some otherwise lexically scoped languages. For example, Common LISP has the special form, and several Schemes and Racket have parameter objects:

To dig fully into the history and flora of dynamic and lexical scope merits another episode.

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