echo does not work for PATH variable!

When I run the ls command this runs fine. But echo $PATH does not give me any output from perl. When I run ec开发者_运维技巧ho from the shell prompt it gives me output. Can you explain this behavior?

#!usr/bin/perl

$\="\n";

$out = `ls`;
print $out;

$out=`echo $PATH`;
print $out;

Please note that while the technically correct answer to your question is the $ interpolation, you should also note that you should not treat Perl like a shell script and call external commands via backticks instead of using Perl built-in or library functions designed for the purpose:

$out = join("\n", glob("*")); # same as `ls`
$out = $ENV{PATH}; # same as `echo $PATH`

This has several significant advantages:

• speed (no call to system)
• portability
• More security (no shell attack vector)
• Most built ins cover proper error handling for you better than your own system call implementation
• Nearly always a better, cleaner, shorter and easier to read/maintain code

Backticks interpolate like double quotes, so you need to escape the $.

$out=`echo \$PATH`;

$PATH is shell variable, from perl you should use it as perl variable $ENV{PATH}

Still try to read some basic docs too, like perldoc perlintro. No need for executing echo at all.

Perl is interpolating $PATH in the backticks as a Perl variable, and you've not set a $PATH anywhere in your script, so the command is coming out as

$out = `echo `

which is basically a null-op. Try

$out = `echo \$PATH`

instead, which would force Perl to ignore the $ and pass it intact to the shell.

You need to escape $ in $PATH because the backticks operator interpolates any variables.

$out=`echo \$PATH`;

You could also use qx// with single quotes. From perldoc perlop:

Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing it on to the shell instead:

1. $perl_info = qx(ps $$); # that's Perl's $$
2. $shell_info = qx'ps $$'; # that's the new shell's $$

Others have already explained the reason - variables inside backticks are interpolated, so your echo $PATH is actually becoming echo since there's no $PATH variable declared.

However, always put use strict; at the top of every Perl script you write.

Had you done so, Perl would have told you what was happening, e.g.:

Global symbol "$PATH" requires explicit package name at myscript.pl line 9

To stop variables being interpolated, either escape them (e.g. \$PATH), or, more cleanly, use e.g. qx'echo $PATH'.

Also, as others have pointed out, calling echo $PATH makes no real-world sense; if you're trying to get the contents of the PATH environment variable, just use $ENV{PATH} - however, you may have just been using it as a simple reduced demonstration case.