zsh: pass variable by reference, and modify its value (like bash's local -n namerefs)?

typeset -n is actually originally from ksh93 (1993). bash added something similar in 4.3 released in 2014. In ksh93, it's much more useful as ksh93 does static scoping (where you can't access local variables from the caller unless they're exported) instead of dynamic in bash/zsh (where a function always sees the local variables of its caller¹). See also:

$ bash -c 'function f { typeset -n var=$1; var=x;}; var=0; f var; echo "$var"' environment: line 0: typeset: warning: var: circular name reference environment: warning: var: circular name reference environment: warning: var: circular name reference x $ ksh -c 'function f { typeset -n var=$1; var=x;}; var=0; f var; echo "$var"' x 

zsh has no nameref support though it has variable dereference operators which can be used both for expansion and assignment.

$ expand() print -r -- ${(P)1} $ assign() : ${(P)1::=$2} $ var=foo $ expand var foo $ assign var bar $ print -r -- $var bar 

Or you can always use the standard approach using eval which should work in any Bourne-like shell:

expand() { eval 'printf "%s\n" "${'"$1"'}"' } assign() { eval "$1=\$2" } 

and before you ask, no, that doesn't make it more or less unsafe than using the P flag or namerefs in bash provided it's done properly like here. Regardless of the approach, it's important the name of the variable is properly controlled.

assign var "$external_input" is safe, assign "$external_input" value or expand "$external_input" is not whether you use P, namerefs or eval. Same as with [ -v "$external_input" ] or read "$external_input" which are all arbitrary command execution vulnerabilities in bash/ksh/zsh.

See also that discussion on the zsh-workers mailing list on the merits of the various approaches at doing indirect assignment.

And that discussion from 2001 where adding ksh93-style nameref support to zsh was being considered. And again in 2015 and in 2023, showing that there's interest in adding that feature, and it will likely end up being added in the next version after 5.9 whether that's 5.10 or 6.0.

^{¹ Though see the zsh/param/private module in zsh for variables to become private to a function.}