Does variable=$(…) store the command or the result of the command in POSIX sh

Question

I was wondering: When I e.g. call

list=$(ps -ax)

is ps -ax executed once and I can read the value multiple times or is the command executed e

Answer 1

POSIX sh guarantees that the command will not be re-executed, see below.

Be careful about concluding things about POSIX sh through empirical tests. For example, if you time sleep 60 | true, you'd find that bash, ksh, zsh and dash all take 60 seconds. However, POSIX sh still allows it to finish immediately.

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Spec for $():

The shell shall expand the command substitution by executing command in a subshell environment (see Shell Execution Environment) and replacing the command substitution (the text of command plus the enclosing "$()" or backquotes) with the standard output of the command.

and when it's applied:

When a given simple command is required to be executed [...] the following expansions, assignments, and redirections shall all be performed from the beginning of the command text to the end:

[...]

4. Each variable assignment shall be expanded for tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal prior to assigning the value.

[...]

Variable assignments shall be performed as follows:

If no command name results, variable assignments shall affect the current execution environment.

Since $() is replaced with the text of the command, and this happens before the variable is assigned, the command is guaranteed not to be re-executed in a conforming POSIX sh implementation.