$ENV{$variable} in perl

Question

Is that anyway in Perl to expand the variable by in $ENV{$variable}?

I exported "a=T" and "T=b" in shell, and I run the Perl script in which print "$ENV{$a}\n", but nothing printed. I want to "b" can be printed, then how should I do in Perl?

Answer 1

Those environment variables should be chained you say, so

$ENV{ $ENV{a} };

Note: not $a but a, like $ENV{USER} etc. This uses the hash %ENV (see perlvar), which has the current environment, so with keys being names of environment variables.

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It is apparently of interest to use a Perl variable (for the shell variable's name^†) in %ENV, and not a string literal as above. In that case we need to pass that shell variable, its name or the value, to the Perl program somehow so to have it stored in a variable; can't just use it directly.

Incidentally, one of the ways to pass a variable from shell to Perl is precisely by exporting it, what then makes it available via %ENV. However, it can also be passed as usual, via command line. Assuming the use of a Perl one-liner (common in shell scripts), we have two options for how to pass

• As an argument, perl -we'...' "$var", in which case it is available in @ARGV

• Via the -s command switch, perl -s -we'...' -- -shv="$var", what sets up $shv variable in the one-liner, with the value $var. The -- mark the start of arguments.

See this post for details, and perhaps this one for another, more involved, example.

Note  A comment asks how to pass variable's name (string a), not its value ($a). This doesn't seem as the best design to me; if the name of a variable for some reason need be passed around then it makes sense to store that in a variable (var="a") and pass that variable, as above.

But if the idea is indeed to pass the name itself around, then do that instead, so either of

perl -we'...' "a"
perl -we'...' -s -- -shv="a"

The rest is the same and %ENV uses the variable that got assigned the input.

If a full Perl script is used (not a one-liner) then use Getopt::Long to nicely handle arugments.

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^† A comment asks about passing the shell variable's name to a Perl variable — so a from the OP, not its value $a. I am a little uncertain of the utility of that but it is of course possible.

The two ways for how to pass a variable from shell to Perl then differ in what is passed.