Due to some OS restrictions I am not able to install ruby and mysql on my prod node. I have to process a Log file written on that node. So I planning to write the contents of the log file to a different file as its written to the log and work on that new file.
def watch_for(file)
f = File.open(file,"r")
f.seek(0,IO::SEEK_END)
while true do
select([f])
line = f.gets
open('myfile.out', 'a+') { |s|
s.puts "#{line}"
}
end
end
But this seems to failing; Any help to this is much appreciated.
It's not a great idea to reopen the output file each time through the loop. If you are just planning on appending and not reading the file, use 'a' rather than 'a+' for the IO mode. Also, there's no good reason to use string interpolation like "#{line}". I rewrote the method to open each file just once:
def watch_for(file)
f = File.open(file,"r")
f.seek(0,IO::SEEK_END)
s = File.open('myfile.out', 'a')
while true do
s.write(f.readline)
end
end
Probably better if this isn't a method at all since there's no way to exit except to kill the process. And, of course, you can just use tail -f $file >> myfile.out as mentioned in this comment.
I am learning Ruby by following learnrubythehardway and it seems very easy and straightforward until you are asked to explain what certain things do. I have commented in my code what I believe is happening in the program. Wanted to see if I am on target, need to rethink things or have no clue and should stop trying to learn how to code.
# Sets variables to arguments sent through terminal
from_file, to_file = ARGV
# Saves from_file object into in_file
in_file = open(from_file)
puts in_file
# Saves from_file data into indata
indata = in_file.read
puts indata
# Save to_file object into out_file with writing privileages
out_file = open(to_file, 'w')
puts out_file
# Writes(copies) indata into out_file
out_file.write(indata)
# Closes files so they can not be accessed anymore
out_file.close
in_file.close
This is what the output in the terminal looks like:
#<File:0x0000000201b038>
This is text from the ex17_from.txt file.
Did it copy to the ex17_to.txt file?
#<File:0x0000000201ae58>
We are also given the task to try to reduce the amount of code needed and are told that we can do this entire action in one line of code. I figured I can just erase out all of the comments and puts statements, while everything else is put into one line of code. However, that will be one long line and I don't think that is what the author is asking for. Any ideas on how to shorten this code will be helpful.
I have commented in my code what I believe is happening in the program. Wanted to see if I am on target, need to rethink things or have no clue
You need to learn to see beyond the written words. For example, this comment is pretty useless:
# Saves from_file object into in_file
in_file = open(from_file)
Not only useless, but actually incorrect. What is this from_file object? What kind of object will in_file be? Why don't you mention open in any way?
What actually happens here is that a File/IO object is created by calling open. And from_file, in this case, is a path to file. Not much of an "object", is it? But in_file is a full File object, which you can use to read the file's contents.
The same goes for the rest of your comments. You simply reword the line of code with human words, without describing the intent behind the code.
You can use FileUtils#cp, and do the following:
require "fileutils"
FileUtils.cp *ARGV
* splats the ARGV array into two parameters needed by cp method
Alternatively, below is concise version of your code:
# Sets variables to arguments sent through terminal
from_file, to_file = ARGV
# Saves from_file object into to_file
open(to_file, 'w') do |out|
open(from_file) do |f|
f.each do |line|
out << line
end
end
end
I am writing to a file instance. While the program is still running, the file is always empty. When I check the file after the script has executed, the file has content.
class A
def initialize
#file_ref=File.new("/user/shared/ruby/ruby-example/test.html","w+")
end
def fill
#file_ref.write("whatever\nwhatever\nwhatever\n")
end
end
The Main script:
require_relative 'A'
a=A.new
a.fill
puts File.size("/user/shared/ruby/ruby-example/test.html")
After the A instance has done its job, the puts statement will print "0" as if the file is empty. Indeed it is during program execution, but if I start irb:
puts File.size("/user/shared/ruby/ruby-example/test.html")
# => 27
$ cat test.html
whatever
whatever
whatever
Is my code wrong?
Is it normal that streams are flushed only after the execution of a process?
Ruby flushes IO buffers when you call IO#close or IO#flush. Since you are not calling neither close nor flush the buffers are flushed when the program terminates and the opened file descriptors are released.
Given your simple example a possible solution is:
class A
def initialize
#file_ref_name = '/user/shared/ruby/ruby-example/test.html'
end
def fill
File.open(#file_ref_name, 'w+') do |file|
file.write("whatever\nwhatever\nwhatever\n")
end
end
end
Passing a block to IO#open makes the opened file (the file variable in this example) to be closed (and therefore flushed) once the execution of the block terminates.
Please note that Ruby (to my knowledge since version 1.9) features a one liner shortcut for simple file writes as well, flush included:
File.write('/path/to/file.txt', 'content')
I'm trying to download a file using net/sftp and pass its contents as the stdin for a command-line app. I can do it by first writing the file to disk but I'd rather avoid that step.
Is there any way to control the input to a program invoked with system() in ruby?
Don't use system at all for this sort of thing, system is best for running an external command that you don't need to talk to.
Use Open3.open3 or Open3.open2 to open up some pipes to your external process then write to the stdin pipe just like writing to any other IO channel; if there is any output to deal with, then you can read it straight from the stdout pipe just like reading from any other input IO channel.
Something like this perhaps (using open as mu suggested)?
contents = "Hello, World!"
open('|echo', 'w') { puts contents }
This can also be accomplished with IO.expect
require 'pty'
require 'expect'
str = "RUBY_VERSION"
PTY.spawn("irb") do |reader, writer|
reader.expect(/0> /)
writer.puts(str)
reader.expect(/=> /)
answer = reader.gets
puts "Ruby version from irb: #{answer}"
end
This waits for the spawned process to display "0> " (the end of an irb prompt) and when it sees that prints a defined string. It then looks for the irb to return by waiting for it to display "=> " and grabs the data returned.
I'm looking for a script to search a file (or list of files) for a pattern and, if found, replace that pattern with a given value.
Thoughts?
Disclaimer: This approach is a naive illustration of Ruby's capabilities, and not a production-grade solution for replacing strings in files. It's prone to various failure scenarios, such as data loss in case of a crash, interrupt, or disk being full. This code is not fit for anything beyond a quick one-off script where all the data is backed up. For that reason, do NOT copy this code into your programs.
Here's a quick short way to do it.
file_names = ['foo.txt', 'bar.txt']
file_names.each do |file_name|
text = File.read(file_name)
new_contents = text.gsub(/search_regexp/, "replacement string")
# To merely print the contents of the file, use:
puts new_contents
# To write changes to the file, use:
File.open(file_name, "w") {|file| file.puts new_contents }
end
Actually, Ruby does have an in-place editing feature. Like Perl, you can say
ruby -pi.bak -e "gsub(/oldtext/, 'newtext')" *.txt
This will apply the code in double-quotes to all files in the current directory whose names end with ".txt". Backup copies of edited files will be created with a ".bak" extension ("foobar.txt.bak" I think).
NOTE: this does not appear to work for multiline searches. For those, you have to do it the other less pretty way, with a wrapper script around the regex.
Keep in mind that, when you do this, the filesystem could be out of space and you may create a zero-length file. This is catastrophic if you're doing something like writing out /etc/passwd files as part of system configuration management.
Note that in-place file editing like in the accepted answer will always truncate the file and write out the new file sequentially. There will always be a race condition where concurrent readers will see a truncated file. If the process is aborted for any reason (ctrl-c, OOM killer, system crash, power outage, etc) during the write then the truncated file will also be left over, which can be catastrophic. This is the kind of dataloss scenario which developers MUST consider because it will happen. For that reason, I think the accepted answer should most likely not be the accepted answer. At a bare minimum write to a tempfile and move/rename the file into place like the "simple" solution at the end of this answer.
You need to use an algorithm that:
Reads the old file and writes out to the new file. (You need to be careful about slurping entire files into memory).
Explicitly closes the new temporary file, which is where you may throw an exception because the file buffers cannot be written to disk because there is no space. (Catch this and cleanup the temporary file if you like, but you need to rethrow something or fail fairly hard at this point.
Fixes the file permissions and modes on the new file.
Renames the new file and drops it into place.
With ext3 filesystems you are guaranteed that the metadata write to move the file into place will not get rearranged by the filesystem and written before the data buffers for the new file are written, so this should either succeed or fail. The ext4 filesystem has also been patched to support this kind of behavior. If you are very paranoid you should call the fdatasync() system call as a step 3.5 before moving the file into place.
Regardless of language, this is best practice. In languages where calling close() does not throw an exception (Perl or C) you must explicitly check the return of close() and throw an exception if it fails.
The suggestion above to simply slurp the file into memory, manipulate it and write it out to the file will be guaranteed to produce zero-length files on a full filesystem. You need to always use FileUtils.mv to move a fully-written temporary file into place.
A final consideration is the placement of the temporary file. If you open a file in /tmp then you have to consider a few problems:
If /tmp is mounted on a different file system you may run /tmp out of space before you've written out the file that would otherwise be deployable to the destination of the old file.
Probably more importantly, when you try to mv the file across a device mount you will transparently get converted to cp behavior. The old file will be opened, the old files inode will be preserved and reopened and the file contents will be copied. This is most likely not what you want, and you may run into "text file busy" errors if you try to edit the contents of a running file. This also defeats the purpose of using the filesystem mv commands and you may run the destination filesystem out of space with only a partially written file.
This also has nothing to do with Ruby's implementation. The system mv and cp commands behave similarly.
What is more preferable is to open a Tempfile in the same directory as the old file. This ensures that there will be no cross-device move issues. The mv itself should never fail, and you should always get a complete and untruncated file. Any failures, such as device out of space, permission errors, etc., should be encountered during writing the Tempfile out.
The only downsides to the approach of creating the Tempfile in the destination directory are:
Sometimes you may not be able to open a Tempfile there, such as if you are trying to 'edit' a file in /proc for example. For that reason you might want to fall back and try /tmp if opening the file in the destination directory fails.
You must have enough space on the destination partition in order to hold both the complete old file and the new file. However, if you have insufficient space to hold both copies then you are probably short on disk space and the actual risk of writing a truncated file is much higher, so I would argue this is a very poor tradeoff outside of some exceedingly narrow (and well-monitored) edge cases.
Here's some code that implements the full-algorithm (windows code is untested and unfinished):
#!/usr/bin/env ruby
require 'tempfile'
def file_edit(filename, regexp, replacement)
tempdir = File.dirname(filename)
tempprefix = File.basename(filename)
tempprefix.prepend('.') unless RUBY_PLATFORM =~ /mswin|mingw|windows/
tempfile =
begin
Tempfile.new(tempprefix, tempdir)
rescue
Tempfile.new(tempprefix)
end
File.open(filename).each do |line|
tempfile.puts line.gsub(regexp, replacement)
end
tempfile.fdatasync unless RUBY_PLATFORM =~ /mswin|mingw|windows/
tempfile.close
unless RUBY_PLATFORM =~ /mswin|mingw|windows/
stat = File.stat(filename)
FileUtils.chown stat.uid, stat.gid, tempfile.path
FileUtils.chmod stat.mode, tempfile.path
else
# FIXME: apply perms on windows
end
FileUtils.mv tempfile.path, filename
end
file_edit('/tmp/foo', /foo/, "baz")
And here is a slightly tighter version that doesn't worry about every possible edge case (if you are on Unix and don't care about writing to /proc):
#!/usr/bin/env ruby
require 'tempfile'
def file_edit(filename, regexp, replacement)
Tempfile.open(".#{File.basename(filename)}", File.dirname(filename)) do |tempfile|
File.open(filename).each do |line|
tempfile.puts line.gsub(regexp, replacement)
end
tempfile.fdatasync
tempfile.close
stat = File.stat(filename)
FileUtils.chown stat.uid, stat.gid, tempfile.path
FileUtils.chmod stat.mode, tempfile.path
FileUtils.mv tempfile.path, filename
end
end
file_edit('/tmp/foo', /foo/, "baz")
The really simple use-case, for when you don't care about file system permissions (either you're not running as root, or you're running as root and the file is root owned):
#!/usr/bin/env ruby
require 'tempfile'
def file_edit(filename, regexp, replacement)
Tempfile.open(".#{File.basename(filename)}", File.dirname(filename)) do |tempfile|
File.open(filename).each do |line|
tempfile.puts line.gsub(regexp, replacement)
end
tempfile.close
FileUtils.mv tempfile.path, filename
end
end
file_edit('/tmp/foo', /foo/, "baz")
TL;DR: That should be used instead of the accepted answer at a minimum, in all cases, in order to ensure the update is atomic and concurrent readers will not see truncated files. As I mentioned above, creating the Tempfile in the same directory as the edited file is important here to avoid cross device mv operations being translated into cp operations if /tmp is mounted on a different device. Calling fdatasync is an added layer of paranoia, but it will incur a performance hit, so I omitted it from this example since it is not commonly practiced.
There isn't really a way to edit files in-place. What you usually do when you can get away with it (i.e. if the files are not too big) is, you read the file into memory (File.read), perform your substitutions on the read string (String#gsub) and then write the changed string back to the file (File.open, File#write).
If the files are big enough for that to be unfeasible, what you need to do, is read the file in chunks (if the pattern you want to replace won't span multiple lines then one chunk usually means one line - you can use File.foreach to read a file line by line), and for each chunk perform the substitution on it and append it to a temporary file. When you're done iterating over the source file, you close it and use FileUtils.mv to overwrite it with the temporary file.
Another approach is to use inplace editing inside Ruby (not from the command line):
#!/usr/bin/ruby
def inplace_edit(file, bak, &block)
old_stdout = $stdout
argf = ARGF.clone
argf.argv.replace [file]
argf.inplace_mode = bak
argf.each_line do |line|
yield line
end
argf.close
$stdout = old_stdout
end
inplace_edit 'test.txt', '.bak' do |line|
line = line.gsub(/search1/,"replace1")
line = line.gsub(/search2/,"replace2")
print line unless line.match(/something/)
end
If you don't want to create a backup then change '.bak' to ''.
This works for me:
filename = "foo"
text = File.read(filename)
content = text.gsub(/search_regexp/, "replacestring")
File.open(filename, "w") { |file| file << content }
Here's a solution for find/replace in all files of a given directory. Basically I took the answer provided by sepp2k and expanded it.
# First set the files to search/replace in
files = Dir.glob("/PATH/*")
# Then set the variables for find/replace
#original_string_or_regex = /REGEX/
#replacement_string = "STRING"
files.each do |file_name|
text = File.read(file_name)
replace = text.gsub!(#original_string_or_regex, #replacement_string)
File.open(file_name, "w") { |file| file.puts replace }
end
require 'trollop'
opts = Trollop::options do
opt :output, "Output file", :type => String
opt :input, "Input file", :type => String
opt :ss, "String to search", :type => String
opt :rs, "String to replace", :type => String
end
text = File.read(opts.input)
text.gsub!(opts.ss, opts.rs)
File.open(opts.output, 'w') { |f| f.write(text) }
If you need to do substitutions across line boundaries, then using ruby -pi -e won't work because the p processes one line at a time. Instead, I recommend the following, although it could fail with a multi-GB file:
ruby -e "file='translation.ja.yml'; IO.write(file, (IO.read(file).gsub(/\s+'$/, %q('))))"
The is looking for white space (potentially including new lines) following by a quote, in which case it gets rid of the whitespace. The %q(')is just a fancy way of quoting the quote character.
Here an alternative to the one liner from jim, this time in a script
ARGV[0..-3].each{|f| File.write(f, File.read(f).gsub(ARGV[-2],ARGV[-1]))}
Save it in a script, eg replace.rb
You start in on the command line with
replace.rb *.txt <string_to_replace> <replacement>
*.txt can be replaced with another selection or with some filenames or paths
broken down so that I can explain what's happening but still executable
# ARGV is an array of the arguments passed to the script.
ARGV[0..-3].each do |f| # enumerate the arguments of this script from the first to the last (-1) minus 2
File.write(f, # open the argument (= filename) for writing
File.read(f) # open the argument (= filename) for reading
.gsub(ARGV[-2],ARGV[-1])) # and replace all occurances of the beforelast with the last argument (string)
end
EDIT: if you want to use a regular expression use this instead
Obviously, this is only for handling relatively small text files, no Gigabyte monsters
ARGV[0..-3].each{|f| File.write(f, File.read(f).gsub(/#{ARGV[-2]}/,ARGV[-1]))}
I am using the tty-file gem
Apart from replacing, it includes append, prepend (on a given text/regex inside the file), diff, and others.