I am trying to open a non existent file and write to it, however when I run the script, no file is being created.
Here is the line of code
File.open("valid_policies.txt", 'a+').write(policy_number.to_s + "\n")
Instead of using .write try this instead:
File.open("valid_policies.txt", 'a+') {|f| f.write(policy_number.to_s + "\n") }
You're using:
File.open("valid_policies.txt", 'a+').write(policy_number.to_s + "\n")
That's a non-block form of open which doesn't automatically close the file. That means the data is most likely not being written to the file but is sitting in the IO buffer waiting to be flushed/synced. You could add a close but that only propagates non-idiomatic code.
Instead you can use:
File.write("valid_policies.txt", policy_number.to_s + "\n")
File.write automatically creates then writes to the file then closes it. It will overwrite existing files though.
If you aren't sure whether the file exists and want to create it if it doesn't, or append to it, then you use File.open with the a mode instead of a+. From the mode documentation:
"a" Write-only, each write call appends data at end of file.
Creates a new file for writing if file does not exist.
Using a+ will work but it unnecessarily opens the file for reading also. Don't do that unless you're sure that's what you have to do.
If I needed to append I'd use:
File.open('valid_policies.txt', 'a') do |fa|
fa.puts policy_number
end
That's idiomatic Ruby. puts will automatically "stringify" policy_number if it has a to_s method, which it should have since you're already calling it, and it'll also automatically add the trailing "\n" if it doesn't exist at the end of the string. Also, using the block form of open will automatically close the file when the block exists, which is smart house-keeping.
I'm in the process of learning Ruby and reading through Chris Pine's book. I'm learning how to read (and write) files, and came upon this example:
require 'yaml'
test_array = ['Give Quiche A Chance',
'Mutants Out!',
'Chameleonic Life-Forms, No Thanks']
test_string = test_array.to_yaml
filename = 'whatever.txt'
File.open filename, 'w' do |f|
f.write test_string
end
read_string = File.read filename
read_array = YAML::load read_string
puts(read_string == test_string)
puts(read_array == test_array )
The point of the example was to teach me about YAML, but my question is, if you can read a file with:
File.read filename
Can you write to a file in a similar way?:
File.write filename test_string
Sorry if it's a dumb question. I was just curious why it's written the way it was and if it had to be written that way.
Can you write to a file in a similar way?
Actually, yes. And it's pretty much exactly as you guessed:
File.write 'whatever.txt', test_array.to_yaml
I think it is amazing how intuitive Ruby can be.
See IO.write for more details. Note that IO.binwrite is also available, along with IO.read and IO.binread.
The Ruby File class will give you new and open but it inherits from the IO class so you get the read and write methods too.
I think the right way to write into a file is the following:
File.open(yourfile, 'w') { |file| file.write("your text") }
To brake this line down:
We first open the file setting the access mode ('w' to overwrite, 'a' to append, etc.)
We then actually write into the file
You can read or write to a file by specifying the mode you access it through. The Ruby File class is a subclass of IO.
The File class open or new methods take a path and a mode as arguments:
File.open('path', 'mode') alternatively: File.new('path','mode')
Example: to write to an existing file
somefile = File.open('./dir/subdirectory/file.txt', 'w')
##some code to write to file, eg:
array_of_links.each {|link| somefile.puts link }
somefile.close
See the source documentation as suggested above for more details, or similar question here: How to write to file in Ruby?
Is there a way to edit each line in a file, without involving 2 files? Say, the original file has,
test01
test02
test03
I want to edit it like
test01,a
test02,a
test03,a
Tried something as show in the code block, but it replaces some of the characters.
Writing it to a temporary file and then replace the original file works, However, I need to edit the file quite often and therefore prefer to do it within the file itself .Any pointers are appreciated.
Thank you!
File.open('mytest.csv', 'r+') do |file|
file.each_line do |line|
file.seek(-line.length, IO::SEEK_CUR)
file.puts 'a'
end
end
f = open 'mytest.csv', 'r+'
r = f.readlines.map { |e| e.strip << ',a' }
f.rewind
f.puts r
f.close # you can leave out this line if it's the last one that runs
Here is a one-liner variation, note that in this case 2 descriptors are left open until the program exits.
open(F='mytest.csv','r+').puts open(F,'r').readlines.map{|e|e.strip<<',a'}
Writing to a file doesn't insert; it always overwrites. This makes it awkward to modify text in-place, because you have to rewrite the entire rest of the contents of the file every time you add something new.
If the file is small enough to fit in memory, you can read it in, modify it, and write it back out. Otherwise, you really are better off with the temporary file.
What is the best way to create an empty file in Ruby?
Something similar to the Unix command, touch:
touch file.txt
FileUtils.touch looks like what it does, and mirrors* the touch command:
require 'fileutils'
FileUtils.touch('file.txt')
* Unlike touch(1) you can't update mtime or atime alone. It's also missing a few other nice options.
If you are worried about file handles:
File.open("foo.txt", "w") {}
From the docs:
If the optional code block is given, it will be passed the opened file
as an argument, and the File object will automatically be closed when
the block terminates.
In Ruby 1.9.3+, you can use File.write (a.k.a IO.write):
File.write("foo.txt", "")
For earlier version, either require "backports/1.9.3/file/write" or use File.open("foo.txt", "w") {}
And also, less advantageous, but very brief:
`touch file.txt`
Just an example:
File.open "foo.txt", "w"
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.