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.)

Answer 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

Answer 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

Answer 3

你可以试试这个：

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 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.

Answer 5

first_line = open("filename").gets

Answer 6

我认为调查git --pretty选项的jkupferman建议最有意义，但是另一种方法是head命令，例如

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

Answer 7

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

Answer 8

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