Re: command line perl reading from STDIN

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Re^2: command line perl reading from STDIN by perlhappy (Novice) on Jan 22, 2013 at 17:54 UTC
`cat input_file.fq \| perl -ne '$s=<>;<>;<>;chomp($s);print length($s)."\n";' > output.txt` Ok, but for the above, if we were to step-by-step describe what is going on. How would it be described? It just that from the above I thought it would assign $s on every line OR skip 2 lines and then assign $s to the third line and repeat.	[reply] [d/l]
Re^3: command line perl reading from STDIN by choroba (Cardinal) on Jan 22, 2013 at 18:02 UTC
The fourth line (i.e. the first line, in fact) is consumed by `-n`. See perlrun - how to execute the Perl interpreter. لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l]
Re^4: command line perl reading from STDIN by perlhappy (Novice) on Jan 22, 2013 at 18:14 UTC
At the start, its the second line and then every fourth file after. ie line 2, 6, 8, 12 .. eof This is what was confusing me but I think I worked it out. See below. Thanks	[reply]
Re^3: command line perl reading from STDIN by perlhappy (Novice) on Jan 22, 2013 at 18:12 UTC
Actually, I think I've just worked it out my head. Took a bit of thinking but this is what i think it is doing first line gets sent, but because <> is unassigned (before $s), it skips; then reads the second line and assigns $s to the line and does whatever; then reads third line but because <> is unassigned, it skips; reads fourth line and the same happens; restarts with 5 line but again because <> is unassigned before the $s it skips; reads 6th line and assigns it to $s and does whatever again this continues until end of file If this is incorrect, let me know. Otherwise I hope this helps anyone else who might look at it. Thanks for your help choroba	[reply]
Re^4: command line perl reading from STDIN by Anonymous Monk on Jan 23, 2013 at 23:32 UTC
`perl -ne ' $s=<>; <>; <>; do_something_with_$s'` is equivalent to `perl -e ' while (<>) { $s=<>; <>; <>; do_something_with_$s} '` so 4 lines will be read each pass, but only the second line will be used for anything Personally, I'd go with something like `perl -ne 'chomp; print length($_)."\n" if (/^@/);'` instead.	[reply] [d/l] [select]