Why are shell variables invisible in my Ruby script?

Question

Working with Ruby shell executions by a backquote or %x syntax I found that when I declare a new variable in bash it is not visible by ruby script, until I make it environment by export command.

Please explain mechanism why do Shell Variables is not visible by a script?

# Shell and Environment variables.
V123='This is an variable. VAR HERE.'
echo $V123

# Read shell variable via Ruby
ruby -e 'p `echo $V123`' # "$V123\n"
ruby -e 'p %x[echo $V123]' # "$V123\n"

# Check SET for this var
set | grep V123
ruby -e 'p `set | grep V123`' # ""
ruby -e 'p %x[set | grep V123]' # ""

# Put this var itno the Environment
echo "--- Export"
printenv V123
export V123
printenv V123
echo "--- Ruby printenv"
ruby -e 'p `printenv V123`' # This is an variable. VAR HERE.\n"
ruby -e 'p %x[printenv V123]' # This is an variable. VAR HERE.\n"

This might help: Difference between single and double quotes in bash — Cyrus
– Cyrus, Commented May 1, 2018 at 7:27
Why do you expect the subshell to derive the environment of the parent shell in the first place? That’s not how bash works. One might V123=FOO ruby -e 'p `echo $V123`' to explicitly pass the env var to another shell script. Sidenote: that behaviour has nothing to do with ruby, it’s shell subprocess default. — Aleksei Matiushkin
– Aleksei Matiushkin, Commented May 1, 2018 at 7:32

Tom Lord · Accepted Answer · 2018-05-01 09:27:51Z

This question does not really have anything to do with ruby; it's just about the behaviour of bash variables.

I found that when I declare a new variable in bash it is not visible by ruby script, until I make it environment by export command.

Sub-shells do not inherit the environment of their parent shell. You can explicitly call a sub-shell with variables, or you can export them. (That's the whole point of the export function!)

For example, consider the following - which, again, has got nothing to do with ruby.

Suppose test.txt has the following lines:

line one
line two

Observe the following outputs:

When the variable is declared as a separate command, it is not passed to the sub-shell:
```
GREP_OPTIONS='-v'
grep one test.txt

// Result:
line one
```
When the variable is set as part of the same command, it is passed to the sub-shell:
```
GREP_OPTIONS='-v' grep one test.txt

// Result:
line two
```

When the variable is exported, it is passed to the sub-shell:

export GREP_OPTIONS='-v'
grep one test.txt

// Result:
line two

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