Reading the first line of a file in Ruby

Question

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

(In particular: I want to read the UUID with git code from the REVISION file in my last Rails directory with the Capistrano extension, and then output it to my tag. This will allow me to take a look at http-glance, which version is deployed on my server. If you have there is a completely different and better way to do this, please let me know.)

Answer 1

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

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

Answer 2

Here's a brief idiomatic way to do this, which correctly opens the file for reading and closes it later.

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

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

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

In addition, there is a quick and dirty implementation of the head that will work for your purposes and in many other cases when 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 

Answer 3

You can try the following:

 File.foreach('path_to_file').first 

Answer 4

How to read the first line in a ruby ​​file:

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

Alternatively, you can simply make git -log from your application:

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

% H specifies the format for printing the full commit hash. There are also modules that allow you to access the local git repository from inside the Rails application in more ruby ​​mode, although I have never used them.

Answer 5

 first_line = open("filename").gets 

Answer 6

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

Answer 7

Improving the response posted by @Chuck, I think it would be wise to indicate 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 

Answer 8

I think jkupferman’s suggestion for exploring git --pretty options makes the most sense, however, the head command, for example, would be another approach.

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