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ドル;foreach $i(/([^ ]+)/g){$h{$i}=1}}}print "$_ " for keys %h;

Identical version with whitespace for reading:

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

I'm sort of a perl newbie. Is $_=1ドル 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{,ドル=" ";print keys %h}' -- file

or

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

• GrandOpener: Commenting some other things I learned from this answer that I didn't know previously: $#array returns the last index of an array, ,ドル is the "output field separator" printed automatically between fields, END{} gives a block of code that is executed when exiting, useful for doing final one-time code while still using the -n option.
• Oh My Goodness: can shave a few characters there by switching to print "@{[ EXPR ]}" and using eof in place of an END block. And the spaces after -e and keys are optional. perl -lane'@h{@F[1..$#F]}=()if/^Host\b/;eof&&print"@{[keys%h]}"' -- file