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.)

Solution 1

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

Solution 2

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

Solution 3

You can try this:

File.foreach('path_to_file').first

Solution 4

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.

Solution 5

first_line = open("filename").gets

View more solutions

Comments

[klochner]

LocalJumpError: no block given

[Chuck]

@klochner: Your Ruby is old. This works fine in 1.8.7 and above.

[klochner]

Sorry Vincent, I can't remove downvote unless you make some minor edit.

[gigimon]

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. :-\

[Mike Woodhouse]

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.

[boulder_ruby]

.last is not working as I planned here. How do I do this for the last line?

[Nathan Long]

Simpler implementation: class File; def self.head(path, n = 1); foreach(path).first(n); end; end

[Saim]

Although it works, but won't it load the whole file in memory?

[skydvr]

(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...

[Joshua Pinter]

Will this close the file afterwards?

[Joshua Pinter]

I also get a Rubocop Security warning for the use of Kernal#open: rubocop.readthedocs.io/en/latest/cops_security/#securityopen

[Blair Anderson]

@skydvr read this stackoverflow.com/questions/14881125/…