8

I know that Bash and Zsh support local variables, but there are systems only have POSIX-compatible shells. And local is undefined in POSIX shells.

So I want to ask which shells support local keyword for defining local variables?

Edit: About shells I mean the default /bin/sh shell.

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    dash supports them, ksh supports them but I think you need to use typeset instead of local – jesse_b Jan 10 '19 at 14:18
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    @Jesse_b ksh doesn't support the local keyword; it does support local variables, but via typeset not via the local, and only in funcs defined as function foo { }, not in funcs defined as foo(){ }. – pizdelect Jan 10 '19 at 14:21
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    @Jesse_b did you miss the part about typeset not working in functions defined as foo(){ ... }? Example: foo(){ typeset v=3; }; function bar { typeset v=7; }; v=1; foo; echo $v; bar; echo $v; => 3, 3. – pizdelect Jan 10 '19 at 14:24
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    @Jesse_b I’ve seen many AIX systems without bash, even new ones set up last year. – Stephen Kitt Jan 10 '19 at 15:51
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    @StephenKitt @Jesse_b @pizdelect Current AIX systems have ksh88 as /bin/ksh, which has incompatibilities around this topic with ksh93. In ksh88 you don't need function foo {...} to get locals using typeset, but in ksh93 you do. That's bad news, if you used the foo() {...} syntax, for shell scripts written for ksh88 (AIX) and then ported to ksh93 (Linux), – jrw32982 supports Monica Feb 21 '19 at 17:14
22

It's not as simple as supporting local or not. There is a lot of variation on the syntax and how it's done between shells that have one form or other of local scope.

That's why it's very hard to come up with a standard that agrees with all. See http://austingroupbugs.net/bug_view_page.php?bug_id=767 for the POSIX effort on that front.

local scope was added first in ksh in the early 80s.

The syntax to declare a local variable in a function was with typeset:

function f {
  typeset var=value
  set -o noglob # also local to the function
  ...
}

(function support was added to the Bourne shell later, but with a different syntax (f() command) and ksh added support for that one as well later; the Bourne shell never had local scope (except of course via subshells))

The local builtin AFAIK was added first to the Almquist shell (used in BSDs, dash, busybox sh) in 1989, but works significantly differently from ksh's typeset. ash derivatives don't support typeset as an alias to local, but you can always define one by hand.

bash and zsh added typeset aliased to local in 1989 and 1991 respectively.

ksh88 added local as an undocumented alias to typeset circa 1990 and pdksh and its derivatives in 1994. posh (based on pdksh) removed typeset (for strict compliance to the Debian Policy that requires local, but not typeset).

POSIX initially objected to specifying typeset on the ground that it was dynamic scoping. So ksh93 (a rewrite of ksh in 1993 by David Korn) switched to static scoping instead. Also in ksh93, as opposed to ksh88, local scoping is only done for functions declared with the ksh syntax (function f {...}), not the Bourne syntax (f() {...}) and the local alias was removed.

However the ksh93v- beta and final version from AT&T can be compiled with an experimental "bash" mode (actually enabled by default) that does dynamic scoping (in bother forms of functions, including with local and typeset) when ksh93 is invoked as bash. local differs from typeset in that case in that it can only be invoked from within a function. That bash mode will be disabled by default in ksh2020 though the local/declare aliases to typeset will be retained even when the bash mode is not compiled in (though still with static scoping).

yash (written much later), has typeset (à la ksh88), but has only had local as an alias to it since version 2.48 (December 2018).

@Schily maintains a Bourne shell descendant which has been recently made mostly POSIX compliant, called bosh that supports local scope since version 2016-07-06 (with local, similar to ash).

So the Bourne-like shells that have some form of local scope for variables today are:

  • ksh, all implementations and their derivatives (ksh88, ksh93, pdksh and derivatives like posh, mksh, OpenBSD sh).
  • ash and all its derivatives (NetBSD sh, FreeBSD sh, dash, busybox sh)
  • bash
  • zsh
  • yash
  • bosh

As far as the sh of different systems go, note that there are systems where the POSIX sh is in /bin (most), and others where it's not (like Solaris where it's in /usr/xpg4/bin). For the sh implementation on various systems we have:

  • ksh88: most SysV-derived commercial Unices (AIX, HP/UX, Solaris¹...)
  • bash: most GNU/Linux systems, Cygwin, macOS
  • ash: by default on Debian and most derivatives (including Ubuntu, Linux/Mint) though can be changed by the admin to bash or mksh. NetBSD, FreeBSD and some of their derivatives (not macOS).
  • busybox sh: many if not most embedded Linux systems
  • pdksh or derivatives: OpenBSD, MirOS

Now, where they differ:

  • typeset (ksh, pdksh, bash, zsh, yash) vs local (ksh88, pdksh, bash, zsh, ash, yash 2.48+).
  • static (ksh93, in function f {...} function), vs dynamic scoping (all other shells). For instance, whether function f { typeset v=1; g; echo "$v"; }; function g { v=2; }; f outputs 1 or 2. See also how the export attribute affects scoping in ksh93.
  • whether local/typeset just makes the variable local (ash, bosh), or creates a new instance of the variable (other shells). For instance, whether v=1; f() { local v; echo "${v:-empty}"; }; f outputs 1 or empty (see also the localvar_inherit option in bash 5.0 and above).
  • with those that create a new variable, whether the new one inherits the attributes (like export) and/or type and which ones from the variable in the parent scope. For instance, whether export V=1; f() { local V=2; printenv V; }; f prints 1, 2 or nothing.
  • whether that new variable has an initial value (empty, 0, empty list, depending on type, zsh) or is initially unset.
  • whether unset V on a variable in a local scope leaves the variable unset, or just peels one level of scoping (mksh, yash, bash under some circumstances). For instance, whether v=1; f() { local v=2; unset v; echo "$v"; } outputs 1 or nothing (see also the localvar_unset option in bash 5.0 and above)
  • like for export, whether it's a keyword or only a mere builtin or both and under what condition it's considered as a keyword.
  • like for export, whether the arguments are parsed as normal command arguments or as assignments (and under what condition).
  • whether you can declare local a variable that was readonly in the parent scope.
  • the interactions with v=value myfunction where myfunction itself declares v as local or not.

That's the ones I'm thinking of just now. Check the austin group bug above for more details.

As far as local scoping for shell options (as opposed to variables), shells supporting it are:

  • ksh88 (with both function definition syntax): done by default, I'm not aware of any way to disable it.
  • ash (since 1989): with local -. It makes the $- parameter (which stores the list of options) local.
  • ksh93: now only done for function f {...} functions.
  • zsh (since 1995). With setopt localoptions. Also with emulate -L for the emulation mode (and its set of options) to be made local to the function.
  • bash (since 2016) with local - like in ash.

¹ the POSIX sh on Solaris is /usr/xpg4/bin/sh (though it has many conformance bugs including those options local to functions). /bin/sh up to Solaris 10 was the Bourne shell (so no local scope), and since Solaris 11 is ksh93

| improve this answer | |
  • Bourne (heirloom version) supports functions as f() { list; }. – Isaac Jan 11 '19 at 12:25
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    @Isaac, it's f() any-command (except that if any-command is a simple command, it can't have redirections). The POSIX syntax is f() any-compound-command. {...} is both a command and compound command. The Bourne shell initially didn't have functions. Function support was added in SVR2 in 1984 with a different syntax from that of ksh (1983) which is what I'm saying here. – Stéphane Chazelas Jan 11 '19 at 14:13
  • @Isaac, the best source of information about the Bourne shell is at in-ulm.de/~mascheck/bourne/index.html – Stéphane Chazelas Jan 11 '19 at 14:20
  • See also web.archive.org/web/20000816225337if_/http://… where David Korn mentioned that support for functions in the Bourne shell was added in 1982 (1984 was when SVR2 was released) and the ksh function f { ....} predated that (ksh first announced in 1983 AFAIK). – Stéphane Chazelas Jan 11 '19 at 14:26
  • I came here hunting for 'POSIX compatible way to use local' and found this: stchaz.free.fr/locvar.sh – Nehal J Wani May 25 at 22:57
4

To follow up on a hint in Stéphane's answer, using subshells gets you the local effect. I don't have access to a true POSIX shell, but this works in busybox ash -- declare your functions with () parentheses instead of {} braces. That forces the function to run in a subshell.

func() (
    echo "in func, before declaring: x=$x"
    x=10
    echo "in func, after declaring: x=$x"
)

x=5
echo "before func: x=$x"
func
echo "after func: x=$x"

outputs

before func: x=5
in func, before declaring: x=5
in func, after declaring: x=10
after func: x=5

That shows that the function has access to global variables, and setting variables in the function does not alter the globals. This is a technique I sometimes use when I want to alter $IFS or cd to a different directory, but I don't want those actions to affect the rest of the program.

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