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

Answer 2

Solution is, that the result is stored. To test that, I used the following script

    #!/bin/sh
    myTime=$(date)
    for a in 1 2 3 4
    do
        echo 'Stored command: '$myTime
        echo 'Fresh command: '$(date)
        sleep 2
    done 

Result looks like this:

Stored commandTue Sep 29 17:39:44 +02 2020
Fresh commandTue Sep 29 17:39:44 +02 2020
Stored commandTue Sep 29 17:39:44 +02 2020
Fresh commandTue Sep 29 17:39:46 +02 2020
Stored commandTue Sep 29 17:39:44 +02 2020
Fresh commandTue Sep 29 17:39:48 +02 2020
Stored commandTue Sep 29 17:39:44 +02 2020
Fresh commandTue Sep 29 17:39:50 +02 2020

Feel free to add missing tags; not sure about how to make stuff like $() most search engine friendly

Answer 3

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.

---

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.