Deleting first line from a file

ruby-talk
1

Hi

I have a HUGE data file multiple lines of data. I want to delete just
the first line from it. How to do it efficiently?

thanks
suresh

2

Would this work for you?

  tail -n +2 original.file > modified.file

HTH,
Chris

···

On Jun 3, 1:59 pm, suresh <suresh.amritap...@gmail.com> wrote:

Hi

I have a HUGE data file multiple lines of data. I want to delete just
the first line from it. How to do it efficiently?

thanks
suresh

3

Depends what you mean by "efficiently".

Removing data from the end of a file takes close to no time at all, if you do
it right -- it just requires truncating the file.

Removing data from the beginning of a file, or the middle of a file, isn't
something most filesystems will let you do. As Chris said, you're going to
have to read the entire file in (minus the line you want removed) and output
it to another file. (Or rather, that's what his tail command does.)

So, if we're talking about a multi-gigabyte file, it's going to take a few
minutes.

···

On Tuesday 03 June 2008 14:59:17 suresh wrote:

Hi

I have a HUGE data file multiple lines of data. I want to delete just
the first line from it. How to do it efficiently?

4

# I have a HUGE data file multiple lines of data. I want to delete just
# the first line from it. How to do it efficiently?

i think you want something fast yet clean ruby solution.

maybe something like,

botp@botp-desktop:~$ cat test.txt
this line will be deleted
2nd line
3rd asdfasdf line
4th qwerty line
5th
6th six
7th 777777777777

botp@botp-desktop:~$ irb
irb(main):001:0> File.open("newfile","w") do |fw|
irb(main):002:1* File.open("test.txt") do |fr|
irb(main):003:2* fr.gets
irb(main):004:2> fw.write fr.read
irb(main):005:2> end
irb(main):006:1> end
=> 72

botp@botp-desktop:~$ cat newfile
2nd line
3rd asdfasdf line
4th qwerty line
5th
6th six
7th 777777777777

just add some checks/rescues so that it will work on all your cases.

kind regards -botp

···

From: suresh [mailto:suresh.amritapuri@gmail.com]

5

suresh wrote:
Hi

I have a HUGE data file multiple lines of data. I want to delete just
the first line from it. How to do it efficiently?

thanks
suresh

csplit file 1 '{1}'

sed -i "" '1d' file # in-place without backup

Cheers,

j.k.

···

--
Posted via http://www.ruby-forum.com/\.

6

Hi Chris

Thanks, I was not aware of the +2 option...

suresh

···

On Jun 4, 1:48 am, Chris Shea <cms...@gmail.com> wrote:

On Jun 3, 1:59 pm, suresh <suresh.amritap...@gmail.com> wrote:

> Hi

> I have a HUGE data file multiple lines of data. I want to delete just
> the first line from it. How to do it efficiently?

> thanks
> suresh

Would this work for you?

  tail -n +2 original.file > modified.file

HTH,
Chris

7

Hi

BTW is there any equivalent method equivalent to linux tail in ruby?

suresh

···

On Jun 4, 1:48 am, Chris Shea <cms...@gmail.com> wrote:

On Jun 3, 1:59 pm, suresh <suresh.amritap...@gmail.com> wrote:

> Hi

> I have a HUGE data file multiple lines of data. I want to delete just
> the first line from it. How to do it efficiently?

> thanks
> suresh

Would this work for you?

  tail -n +2 original.file > modified.file

HTH,
Chris

8

Chris Shea wrote:

···

On Jun 3, 1:59 pm, suresh <suresh.amritap...@gmail.com> wrote:

I have a HUGE data file multiple lines of data. I want to delete just
the first line from it. How to do it efficiently?

  tail -n +2 original.file > modified.file

For the sake of trivia, sed (stream editor) also does the job:

  sed 1d original.file > modified.file # 1d means delete line 1
--
Posted via http://www.ruby-forum.com/\.

9

If that's all you want (and you're not frightened by the command-line options):

ruby -i -n -e 'puts $_ if $. > 1' lines.txt

Of if you want to keep the original in lines.txt.orig

ruby -i.orig -n -e 'puts $_ if $. > 1' lines.txt

-Rob

···

On Jun 4, 2008, at 9:16 PM, Peña, Botp wrote:

From: suresh [mailto:suresh.amritapuri@gmail.com]
# I have a HUGE data file multiple lines of data. I want to delete just
# the first line from it. How to do it efficiently?

i think you want something fast yet clean ruby solution.

maybe something like,

botp@botp-desktop:~$ cat test.txt
this line will be deleted
2nd line
3rd asdfasdf line
4th qwerty line
5th
6th six
7th 777777777777

botp@botp-desktop:~$ irb
irb(main):001:0> File.open("newfile","w") do |fw|
irb(main):002:1* File.open("test.txt") do |fr|
irb(main):003:2* fr.gets
irb(main):004:2> fw.write fr.read
irb(main):005:2> end
irb(main):006:1> end
=> 72

botp@botp-desktop:~$ cat newfile
2nd line
3rd asdfasdf line
4th qwerty line
5th
6th six
7th 777777777777

just add some checks/rescues so that it will work on all your cases.

kind regards -botp

Rob Biedenharn http://agileconsultingllc.com
Rob@AgileConsultingLLC.com

10

Jimmy Kofler wrote:
...
csplit file 1 '{1}'
...

csplit -k file 1 '{1}'

···

--
Posted via http://www.ruby-forum.com/\.

11

ruby -n -e 'print $_ if $.>1' original.file > modified.file

Regards,

Park Heesob

···

2008/6/4 suresh <suresh.amritapuri@gmail.com>:

On Jun 4, 1:48 am, Chris Shea <cms...@gmail.com> wrote:

On Jun 3, 1:59 pm, suresh <suresh.amritap...@gmail.com> wrote:

> Hi

> I have a HUGE data file multiple lines of data. I want to delete just
> the first line from it. How to do it efficiently?

> thanks
> suresh

Would this work for you?

  tail -n +2 original.file > modified.file

HTH,
Chris

Hi

BTW is there any equivalent method equivalent to linux tail in ruby?

12

Not really, but it shouldn't be difficult to build. Maybe trickier to build
efficiently, though.

···

On Wednesday 04 June 2008 04:59:12 suresh wrote:

BTW is there any equivalent method equivalent to linux tail in ruby?

13

Hi Park Heesob

Thanks. But how can this be done inside a .rb file? The above must be
from command line right?

suresh

···

On Jun 4, 3:19 pm, Heesob Park <pha...@gmail.com> wrote:

2008/6/4 suresh <suresh.amritap...@gmail.com>:

> On Jun 4, 1:48 am, Chris Shea <cms...@gmail.com> wrote:
>> On Jun 3, 1:59 pm, suresh <suresh.amritap...@gmail.com> wrote:

>> > Hi

>> > I have a HUGE data file multiple lines of data. I want to delete just
>> > the first line from it. How to do it efficiently?

>> > thanks
>> > suresh

>> Would this work for you?

>> tail -n +2 original.file > modified.file

>> HTH,
>> Chris

> Hi

> BTW is there any equivalent method equivalent to linux tail in ruby?

ruby -n -e 'print $_ if $.>1' original.file > modified.file

Regards,

Park Heesob

14

There's no core method, but there is a library:

http://raa.ruby-lang.org/project/file-tail/

Regards,

Dan

···

On Jun 4, 11:57 am, David Masover <ni...@slaphack.com> wrote:

On Wednesday 04 June 2008 04:59:12 suresh wrote:

> BTW is there any equivalent method equivalent to linux tail in ruby?

Not really, but it shouldn't be difficult to build. Maybe trickier to build
efficiently, though.

15

Hi,

···

2008/6/4 suresh <suresh.amritapuri@gmail.com>:

On Jun 4, 3:19 pm, Heesob Park <pha...@gmail.com> wrote:

2008/6/4 suresh <suresh.amritap...@gmail.com>:

> On Jun 4, 1:48 am, Chris Shea <cms...@gmail.com> wrote:
>> On Jun 3, 1:59 pm, suresh <suresh.amritap...@gmail.com> wrote:

>> > Hi

>> > I have a HUGE data file multiple lines of data. I want to delete just
>> > the first line from it. How to do it efficiently?

>> > thanks
>> > suresh

>> Would this work for you?

>> tail -n +2 original.file > modified.file

>> HTH,
>> Chris

> Hi

> BTW is there any equivalent method equivalent to linux tail in ruby?

ruby -n -e 'print $_ if $.>1' original.file > modified.file

Regards,

Park Heesob

Hi Park Heesob

Thanks. But how can this be done inside a .rb file? The above must be
from command line right?

It is equivalent to

while gets
  print $_ if $.>1
end

Regards,

Park Heesob

16

Thanks Park Heesob, thank you
suresh

···

On Jun 4, 5:20 pm, Heesob Park <pha...@gmail.com> wrote:

Hi,

2008/6/4 suresh <suresh.amritap...@gmail.com>:

> On Jun 4, 3:19 pm, Heesob Park <pha...@gmail.com> wrote:
>> 2008/6/4 suresh <suresh.amritap...@gmail.com>:

>> > On Jun 4, 1:48 am, Chris Shea <cms...@gmail.com> wrote:
>> >> On Jun 3, 1:59 pm, suresh <suresh.amritap...@gmail.com> wrote:

>> >> > Hi

>> >> > I have a HUGE data file multiple lines of data. I want to delete just
>> >> > the first line from it. How to do it efficiently?

>> >> > thanks
>> >> > suresh

>> >> Would this work for you?

>> >> tail -n +2 original.file > modified.file

>> >> HTH,
>> >> Chris

>> > Hi

>> > BTW is there any equivalent method equivalent to linux tail in ruby?

>> ruby -n -e 'print $_ if $.>1' original.file > modified.file

>> Regards,

>> Park Heesob

> Hi Park Heesob

> Thanks. But how can this be done inside a .rb file? The above must be
> from command line right?

It is equivalent to

while gets
  print $_ if $.>1
end

Regards,

Park Heesob