ls piped tpo perl -pe runs indefintely

ISAI student has asked for the wisdom of the Perl Monks concerning the following question:

Hello all. I have tried to do something simple. Do ls, pipe it to perl -ne (using xargs) and print. And it runs indefintely. I have tried something simple, like printing out only the directories. See code below

ls | xargs perl -ne ' map { print $_ , "\n" if ( -d $_ ) ; } @ARGV '

This runs almost indefintely. However this gets me what I need:

ls | xargs perl -ne ' $a = shift ; print $a "\n" if (-d $a) ; '

why?

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Re: ls piped tpo perl -pe runs indefintely by zwon (Abbot) on May 01, 2012 at 13:47 UTC
You don't need xargs: `ls \| perl -nE'chomp; say $_ if -d $_'` [download] neither you need ls: `perl -E'for (<*>) {say $_ if -d $_}'` [download]	[reply] [d/l] [select]
Re^2: ls piped tpo perl -pe runs indefintely by ISAI student (Scribe) on May 01, 2012 at 16:26 UTC
Many thanks. It works, and has taught me a lot.	[reply]
Re^2: ls piped tpo perl -pe runs indefintely by Anonymous Monk on May 01, 2012 at 16:30 UTC
Maybe a bit more clear: `perl -E'for (@ARGV) { say $_ if -d $_}' *`	[reply] [d/l]
Re: ls piped tpo perl -pe runs indefintely by JavaFan (Canon) on May 01, 2012 at 13:45 UTC
The second one modifies @ARGV, the first one doesn't. Due to you use of `-n` (why?), the first one executes the body for every line of the files listed by `ls`. It should actually terminate, but if you have a lot of large files, this my take a while. The second one intends to do the same, that is, execute the body for each of the lines of the files listed by `ls`, but here you are actually modifying `@ARGV`, which causes the implicit `<ARGV>` to terminate after the first iteration. My guess is that if you remove the `-n`, both will do what you want them to do.	[reply] [d/l] [select]
Re: ls piped tpo perl -pe runs indefintely by Anonymous Monk on May 01, 2012 at 13:46 UTC
`shift` keeps on reducing the number of elements in `@ARGV` in the second instance. Thus the `while` loop (via `-n` option) ends eventually. See `perldoc perlrun`, and `Deparse` ... `perl -MO=Deparse -ne ' map { print $_ , "\n" if ( -d $_ ) ; } @ARGV +' LINE: while (defined($_ = <ARGV>)) { map {print $_, "\n" if -d $_;} @ARGV; } -e syntax OK` [download]	[reply] [d/l] [select]
Re^2: ls piped tpo perl -pe runs indefintely by Anonymous Monk on May 01, 2012 at 13:59 UTC
With respect to the first code instance, if I have (2 directories, 3 files) ... `../ ./ p/ q/ r s t` [download] ... then nothing is printed (perl 5.8.8, CentOS 6, zsh 4). If I remove the while loop (-n option), expected output is printed.	[reply] [d/l]