How do I get command-line Perl to accept shell variables? - bash

Question

I can do math like
perl -e 'print 5253413/39151' -l
But I don't quite get how to take advantage of Perl's ability to do math with my own predefined bash variables. I've tried
var1=$(some wc command that yields a number); var1=$(some wc that yields another number)
perl -e 'print var1/var2' -l
But it doesn't work

Answer 1

There are two main ways to do this.
Within the Perl code you can use the %ENV built-in hash to access environment variables that are exported from the shell
$ export var1=5253413
$ export var2=39151
$ perl -E 'say $ENV{var1}/$ENV{var2}'
134.183366963807
You can use the shell interpolation facility to insert the value of a shell variable into a command
This is best done as parameters to the perl one-liner rather than introducing the values directly into the code
$ var1=5253413
$ var2=39151
$ perl -E '($v1, $v2) = #ARGV; say $v1/$v2' $var1 $var2
134.183366963807

Answer 2

Two less common ways to do this make use of long-standing perl features.
The first is the core module Env, which ties process environment variables to perl variables:
sh$ export VAR1=1000
sh$ export VAR2=33
sh$ perl -MEnv -E 'say $VAR1/$VAR2' # imports all environ vars
333.333333333333
sh$ perl -MEnv=VAR1,VAR2 -E 'say $VAR1/$VAR2' # imports only VAR1, VAR2
333.333333333333
Note that the variables need to be present in the environment inherited by the perl process, for example with export VAR as above, or explicitly for a single command (as by FOO=hello perl -MEnv -E 'say $FOO').
The second and rather more obscure way is to use use perl's -s switch to set arbitrary variables from the command line:
sh$ VAR1=1000
sh$ VAR2=33
sh$ perl -s -E 'say $dividend/$divisor' -- -dividend=$VAR1 -divisor=$VAR2
333.333333333333
awk does something similar with its -v switch.

Answer 3

I believe the spirit of the question is to pass variables without exported ENV vars.
Beside using perl -s -e expression -perlvar=val, below is code that uses two other mechanisms to pass the variable to perl.
a=x; b=N; c=z;
b=y perl -e '$pa='$a';' -e "\$pc=$c;" -e 'print "$pa$ENV{b}$pc\n";'
echo $a$b$c
Passing a and c is same, only the quoting is different. When passing using chained expressions, like this, it is important to end the expression with semi-colon; because, they flow into one expression at the end.
Passing b is done by ENV, but instead of using the exported value, it is passed directly into perl's ENV by giving the assignment before the command on the same command-line.
Last the echo command is to emphasize how the shell's definition of $b is unchanged.
Using the mechanism of b's passing, we arrive at a more secure solution, because the process's ENV data cannot be checked for the value, and it will not be seen in the command-line argument list.