Shell – Why isn’t $RANDOM included in the output of ‘env’

environment-variablesshell

I know env is a shell command, it can be used to print a list of the current environment variables. And as far as I understand, RANDOM is also
a environment variable.

So why, when I launch env on Linux, does the output not include RANDOM?

Best Answer

RANDOM is not an environment variable. It's a shell variable maintained by some shells. It is generally not exported by default. This is why it doesn't show up in the output of env.

Once it's been used at least once, it would show up in the output of set, which, by itself, lists the shell variables (and functions) and their values in the current shell session. This behaviour is dependent on the shell and using pdksh on OpenBSD, RANDOM would be listed by set even if not previously used.

The rest of this answer concerns what could be expected to happen if RANDOM was exported (i.e. turned into an environment variable).

Exporting it with export RANDOM would make it an environment variable but its use would be severely limited as its value in a child process would be "random but static" (meaning it would be an unchanging random number). The exact behaviour differs between shells.

I'm using pdksh on OpenBSD in the example below and I get a new random value in each awk run (but the same value every time within the same awk instance). Using bash, I would get exactly the same random value in all invocations of awk.

$ awk 'BEGIN { print ENVIRON["RANDOM"], ENVIRON["RANDOM"] }'
25444 25444

$ awk 'BEGIN { print ENVIRON["RANDOM"], ENVIRON["RANDOM"] }'
30906 30906

In bash, the exported value of RANDOM would remain static regardless of the use of RANDOM in the shell (where each use of $RANDOM would still give a new value).

This is because each reference to the shell variable RANDOM in bash makes the shell access its internal get_random() function to give the variable a new random value, but the shell does not update the environment variable RANDOM. This is similar in behaviour as with other dynamic bash variables, such as LINENO, SECONDS, BASHPID etc.

To update the environment variable RANDOM in bash, you would have to assign it the value of the shell variable RANDOM and re-export it:

export RANDOM="$RANDOM"

It is unclear to me if this would have the additional side effect of re-seeding the random number generator in bash or not (but an educated guess would be that it doesn't).

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