Ruby translation for UNIX scripting command

[Thomas Luedeke]

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Quick (and slightly embarassing) newbie question for the Ruby experts. I'm trying to learn Ruby, and move away from antediluvian UNIX scripting.How do I make a sequence of pattern matching lines in a file, then extract a particular field from the line? For example, in UNIX talk if I were interested in finding all lines with a particular match pattern "test", and printing the fourth field, I'd do a grep plus awk command such as:grep "test" file_name | awk ' { print $4 } ' >> output_fileHowever, in my Ruby books, I don't see any discussion about the function equivalent of printing a field of the type "$4" (i.e. fourth field). I can get the lines out of the search file, but I'm not quite sure how to determine fields.Thanks.TPL


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[gabriele renzi]

il Sat, 6 Mar 2004 03:15:08 +0900, "Thomas Luedeke"
<[email protected]> ha scritto::


I'd say:

open('filename').grep(/pattern/).split[4]

 

[Mark Hubbart]

il Sat, 6 Mar 2004 03:15:08 +0900, "Thomas Luedeke"
<[email protected]> ha scritto::


I'd say:

open('filename').grep(/pattern/).split[4]

except that element number four is the fifth element. no, not the movie


open('filename').grep(/pattern/).split[3]

Also, unless I misunderstand, I think there may be many many lines; and
grep returns an array, which can't be split.

if you want to create a filter script, that filters stdin to stdout:

STDIN.read.grep(/pattern/).each{|line| puts line.split[3]}

If, on the other hand, you want a script that you can call like this:

% myscript input_file output_file

then you will be needing to read up a little more in ruby, I think. But
here's some code for an example:

filtered_data = File.open("input_file") do |file|
file.read.grep(/pattern/).map{|line| line.split[3]}.join "\n"
end

File.open("output_file") do |file|
file.write filtered_data
end

--Mark

 

[Florian Gross]

If, on the other hand, you want a script that you can call like this:

% myscript input_file output_file

then you will be needing to read up a little more in ruby, I think. But
here's some code for an example:

filtered_data = File.open("input_file") do |file|
file.read.grep(/pattern/).map{|line| line.split[3]}.join "\n"
end

File.open("output_file") do |file|
file.write filtered_data
end

This can be simplified to:

filtered_data = File.read("input_file").grep(/pattern/) \
.map { |line| line.split[3] }

File.open("output_file") do |file|
file.puts filtered_data
end


It might however be better to do all this in one pass over the data
without using any buffers -- I'd do that like this:

filtered_data = Array.new
File.foreach("input_file") do |line|
if md = /^(#\w+) ([\d:]+) (<\w+>) (.*?)$/.match(line)
channel, time, nick, text = md.captures
filtered_data << text
end
end

File.open("output_file") do |file|
file.puts filtered_data
end

This implementation does also allow one to keep a part of the matching
line not surrounded by whitespace. (Which means that one could also use
it on CSV data or similar input.)

 

[Robert Klemme]

Quick (and slightly embarassing) newbie question for the Ruby experts. I'm

trying to learn Ruby, and move away from antediluvian UNIX scripting.How do
I make a sequence of pattern matching lines in a file, then extract a
particular field from the line? For example, in UNIX talk if I were
interested in finding all lines with a particular match pattern "test", and
printing the fourth field, I'd do a grep plus awk command such as:grep
"test" file_name | awk ' { print $4 } ' &gt;&gt; output_fileHowever, in my
Ruby books, I don't see any discussion about the function equivalent of
printing a field of the type "$4" (i.e. fourth field). I can get the lines
out of the search file, but I'm not quite sure how to determine
fields.Thanks.TPL

$> ruby -a -n -e 'puts $F[3] if /test/' test-file.txt

If you need to split with another pattern, do

$> ruby -a '-F\s+' -n -e 'puts $F[3] if /test/' test-file.txt

Regards

robert