Perl one-liner for parsing host names from ssh config file

Question

Here is the working code (intended to be executed with perl -e '<code>' ~/.ssh/config):

Actual version:

while(<>){if(/^Host (.+)/){$_=$1;foreach $i(/([^ ]+)/g){$h{$i}=1}}}print "$_ " for keys %h;

Identical version with whitespace for reading:

while(<>) {
    if(/^Host (.+)/) {
        $_=$1;
        foreach $i(/([^ ]+)/g) {
            $h{$i}=1
        }
    }
}
print "$_ " for keys %h;

I'm sort of a perl newbie. Is $_=$1 kosher? Is there a straight-forward way to do this with a single regex? Is there anything else I should be doing better here?

Example input output:

For a config file with contents like this:

Host build
    HostName build.myserver.com
Host build build.myserver.com
    User ubuntu
    IdentityFile ~/.ssh/build
Host tunnel
    LocalForward 6397 redis.production.com:6379

The output should be (order is not required, trailing space is not required but acceptable):

build build.myserver.com tunnel 

Answer 1

I would use something like

perl -lane '@h{@F[1..$#F]}=()if/^Host\b/;END{$,=" ";print keys %h}' -- file

or

perl -lane '@h{ @F[ 1 .. $#F ] } = () if /^Host\b/;
            END {
                $, = " ";
                print keys %h;
            }' -- file

• -l removes newlines from input and adds them to prints
• -n runs the code for each line of the input
• -a splits each line on whitespace into the @F array
• the hash %h is populated by all the non-whitepsace strings following Host
• at the end, all the keys are printed separated by space