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comp.lang.ruby

Ruby-esque approaches to adding a line at the beginning of a file.

Randy Kramer

3/26/2006 8:56:00 PM

On our local lug mail list a discussion came up about using Ruby (as opposed
to Perl) for an application that involved inserting a new line at the front
of an existing file.

(As a newbie), I proposed an approach like this (with help from the Ruby
Cookbook which I'm working my way through).

t = "New first line of file.\n"
t = t + open("sample_file") { |f| f.read }
open("sample_file", "w") { |f| f.write(t)}

However, it doesn't appear (to me) to be very Ruby-esque--I'm wondering how
others on this list might approach such a task.

Randy Kramer
3 Answers

Stephen Waits

3/26/2006 9:21:00 PM

0


On Mar 26, 2006, at 12:55 PM, Randy Kramer wrote:

> t = "New first line of file.\n"
> t = t + open("sample_file") { |f| f.read }
> open("sample_file", "w") { |f| f.write(t)}
>
> However, it doesn't appear (to me) to be very Ruby-esque--I'm
> wondering how
> others on this list might approach such a task.

I think it's as good as anything. You could scrunch it up a bit like
this:

t = "New First Line\n" + File.read("sample_file")
...

--Steve

Gerardo Santana Gómez Garrido

3/26/2006 9:40:00 PM

0

2006/3/26, Randy Kramer <rhkramer@gmail.com>:
> On our local lug mail list a discussion came up about using Ruby (as opposed
> to Perl) for an application that involved inserting a new line at the front
> of an existing file.
>
> (As a newbie), I proposed an approach like this (with help from the Ruby
> Cookbook which I'm working my way through).
>
> t = "New first line of file.\n"
> t = t + open("sample_file") { |f| f.read }
> open("sample_file", "w") { |f| f.write(t)}
>
> However, it doesn't appear (to me) to be very Ruby-esque--I'm wondering how
> others on this list might approach such a task.
>
> Randy Kramer


If the intention is to do it in the command line, you can write:

ruby -i -pe 'puts "New first line" if ARGF.file.lineno == 1' file

and that will work even with multiple files
--
Gerardo Santana
"Between individuals, as between nations, respect for the rights of
others is peace" - Don Benito Juárez
http://santanatechnotes.blo...

Andrew Johnson

3/26/2006 10:05:00 PM

0

On Mon, 27 Mar 2006 05:55:30 +0900, Randy Kramer <rhkramer@gmail.com> wrote:
> On our local lug mail list a discussion came up about using Ruby (as opposed
> to Perl) for an application that involved inserting a new line at the front
> of an existing file.

Well, assuming reading the entire file into memory is acceptable,
we can use a fairly standard approach without a temporary file:

#!/usr/bin/ruby -w

file = 'somefile'
first = "new start\n"

File.open(file,'r+') do |f|
contents = f.read
f.seek 0, IO::SEEK_SET
f.truncate 0
f.puts first, contents
end

And, if you like, you can wrap the manipulations with #flock

File.open(file,'r+') do |f|
f.flock File::LOCK_EX
contents = f.read
f.seek 0, IO::SEEK_SET
f.truncate 0
f.puts first, contents
f.flock File::LOCK_UN
end

Of course, this is a generalized version for any content manipulation you
might want to do on the contents, not just adding a new first line.

Otherwise, loop over lines, making changes and writing to a temp file, and
afterwards, #rename the temp file back to the original.

Or, use the command line -i switch:

$ cat somefile
one
two
three

$ ruby -pi -e 'puts "zero\n" if $. == 1' somefile

$ cat somefile
zero
one
two
three

regards,
andrew

--
Andrew L. Johnson http://www.s...
What have you done to the cat? It looks half-dead.
-- Schroedinger's wife