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

To cite the bash manpage:

Command Substitution
    Command substitution allows the output of a com-
    mand to replace the command name. There are two forms:
        $(command)
    or
        `command`

So the output of the command is stored, not the command itself. If you want to store a command that is evaluated each time, you need to use eval:

#!/bin/bash

command="date"
startTime=$(date)
for i in `seq 1 4`
do
    echo "$i: $startTime - " $(eval $command)
    sleep 1
done