Reading the first line of a file in Ruby

Question

I want to read only the first line of a file using Ruby in the fastest, simplest, most idiomatic way possible. What's the best approach?

(Specifically: I want to read the git commit UUID out of the REVISION file in my latest Capistrano-deployed Rails directory, and then output that to my tag. This will let me see at an http-glance what version is deployed to my server. If there's an entirely different & better way to do this, please let me know.)

score 125 · Accepted Answer · edited Mar 27 '18 at 23:57

125

This will read exactly one line and ensure that the file is properly closed immediately after.

strVar = File.open('somefile.txt') {|f| f.readline}
# or, in Ruby 1.8.7 and above: #
strVar = File.open('somefile.txt', &:readline)
puts strVar

edited Mar 27 '18 at 23:57

Julien Lamarche

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answered Sep 29 '09 at 01:39

Chuck

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Blake Taylor · Answer 2 · 2011-01-19T00:24:32.413

26

Here's a concise idiomatic way to do it that properly opens the file for reading and closes it afterwards.

File.open('path.txt', &:gets)

If you want an empty file to cause an exception use this instead.

File.open('path.txt', &:readline)

Also, here's a quick & dirty implementation of head that would work for your purposes and in many other instances where you want to read a few more lines.

# Reads a set number of lines from the top.
# Usage: File.head('path.txt')
class File
  def self.head(path, n = 1)
     open(path) do |f|
        lines = []
        n.times do
          line = f.gets || break
          lines << line
        end
        lines
     end
  end
end

edited Jan 19 '11 at 00:24

answered Jan 18 '11 at 17:16

Blake Taylor

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Simpler implementation: `class File; def self.head(path, n = 1); foreach(path).first(n); end; end` – Nathan Long Nov 11 '14 at 14:23
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(6 years later....) What is happening in those first 2 examples with the symbol as the second argument to open? I can't find any doc that explains what that's doing. Thx for any info... – skydvr Dec 14 '17 at 16:51
@skydvr read this https://stackoverflow.com/questions/14881125/what-does-to-proc-method-mean – Blair Anderson May 13 '20 at 15:13

Vincent · Answer 3 · 2009-09-29T02:57:08.103

7

You can try this:

File.foreach('path_to_file').first

edited Sep 29 '09 at 02:57

answered Sep 29 '09 at 01:36

Vincent

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LocalJumpError: no block given – klochner Sep 29 '09 at 01:43
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@klochner: Your Ruby is old. This works fine in 1.8.7 and above. – Chuck Sep 29 '09 at 01:55
Sorry Vincent, I can't remove downvote unless you make some minor edit. – klochner Sep 29 '09 at 02:09
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I upvoted this one because I like the "first"ness of it. Unfortunately, my Rails host (DreamHost) is only on 1.8.5, so it isn't the "correct" one for me. :-\ – Craig Walker Sep 29 '09 at 03:33
I like the expressiveness of "first" but I don't like the "foreach", which is misleading. I suppose the "perfect" (?) answer is to monkey-patch a File#first_line(path) method. – Mike Woodhouse Sep 29 '09 at 07:58

score 6 · Answer 4 · answered Sep 29 '09 at 01:39

6

How to read the first line in a ruby file:

commit_hash = File.open("filename.txt").first

Alternatively you could just do a git-log from inside your application:

commit_hash = `git log -1 --pretty=format:"%H"`

The %H tells the format to print the full commit hash. There are also modules which allow you to access your local git repo from inside a Rails app in a more ruby-ish manner although I have never used them.

answered Sep 29 '09 at 01:39

jkupferman

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.last is not working as I planned here. How do I do this for the last line? – boulder_ruby Jul 20 '12 at 22:15

score 5 · Answer 5 · answered Sep 29 '09 at 01:36

5

first_line = open("filename").gets

answered Sep 29 '09 at 01:36

Paige Ruten

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Will this close the file afterwards? – Joshua Pinter Dec 08 '18 at 01:47
I also get a Rubocop Security warning for the use of `Kernal#open`: https://rubocop.readthedocs.io/en/latest/cops_security/#securityopen – Joshua Pinter Dec 08 '18 at 14:51

score 3 · Answer 6 · edited Aug 07 '14 at 16:00

3

I think the jkupferman suggestion of investigating the git --pretty options makes the most sense, however yet another approach would be the head command e.g.

ruby -e 'puts `head -n 1 filename`'  #(backtick before `head` and after `filename`)

edited Aug 07 '14 at 16:00

the Tin Man

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answered Oct 14 '09 at 04:58

Andrew Atkinson

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score 2 · Answer 7 · edited Aug 12 '19 at 08:42

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first_line = File.readlines('file_path').first.chomp

edited Aug 12 '19 at 08:42

Ulysse BN

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answered Aug 14 '13 at 13:51

boblin

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Although it works, but won't it load the whole file in memory? – Saim May 20 '16 at 22:39

score 2 · Answer 8 · answered May 20 '17 at 20:24

Improving on the answer posted by @Chuck, I think it might be worthwhile to point out that if the file you are reading is empty, an EOFError exception will be thrown. Catch and ignore the exception:

def readit(filename)
 text = ""
 begin
   text = File.open(filename, &:readline)
 rescue EOFError
 end
 text
end

Reading the first line of a file in Ruby

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