How to append some data at the beginning of a file
Uday Thokala
I am newbie to Ruby so please spare me if the question looks silly. My
question is how to append data at the beginning of a file?
Suppose I have a file named test.rb which contains some text, say
"This is first line
This is second line"
Now if I wanted to append some data at the beginning of the file, lets
say
"This line has to be appended at the beginning of the file"
My program
filename = File.open("test","a") do |f|
f.puts "This line should appear at the top of each file";
f.close();
end
is appending at the end of the file. So the output is:
"This is first line
This is second line
This line has to be appended at the beginning of the file"
I searched the forums and found that if I use IO:seek and then try to
append data to the existing file the earlier content which are in the
first lines will get replaced. Is there any easy solution so that I can
get a final output like:
"This line has to be appended at the beginning of the file
This is first line
This is second line"
--
Posted via http://www.ruby-forum.com/.
Ronald Fischer
> question is how to append data at the beginning of a file?
This is not so much a Ruby question, since nearly no operating system
directly
allows appending data to the beginning of a file. The general solution
is to create a new file, putting there the data in the right order,
delete
the old file and rename the new file to the name of the old one.
Renaming a file is done like this:
File.rename("oldname","newname")
This raises the exception SystemCallError, if renaming fails.
HTH
Ronald
--
Ronald Fischer <ronald....@venyon.com>
Phone: +49-89-452133-162
Uday Thokala
>> I am newbie to Ruby so please spare me if the question looks silly. My
>> question is how to append data at the beginning of a file?
>
> This is not so much a Ruby question, since nearly no operating system
> directly
> allows appending data to the beginning of a file.
Thanks Ronald for your comments. I am wondering may be ruby got some way
around it.
> The general solution
> is to create a new file, putting there the data in the right order,
> delete
> the old file and rename the new file to the name of the old one.
>
> Renaming a file is done like this:
>
> File.rename("oldname","newname")
Yeah I implemented the above mentioned solution and it's working fine.
The code is
newfile = File.new("test1","w")
newfile.puts "This line should appear at the top of each file";
oldfile = File.open("test", "r+")
oldfile.each_line { |line| newfile.puts line}
oldfile.close();
newfile.close();
File.delete("test");
File.rename("test1", "test");
Thanks,
Uday.
Ronald Fischer
> got some way
> around it.
It could, but I think it just happens too rare that someone
wants to do this. In more than 2 decades of programming, I
had this need only two or three times, for example.
> newfile = File.new("test1","w")
> newfile.puts "This line should appear at the top of each file";
>
> oldfile = File.open("test", "r+")
> oldfile.each_line { |line| newfile.puts line}
> oldfile.close();
or simply
newfile.puts(File.read("test"))
so you don't need the Ruby variable 'oldfile'.
> newfile.close();
>
> File.delete("test");
> File.rename("test1", "test");
Jano Svitok
1. it's better to use block form of File.open:
File.open("test1","w") do |newfile|
newfile.puts "This line should appear at the top of each file"
File.open("test", "r+") do |oldfile|
oldfile.each_line { |line| newfile.puts line}
end
end
File.delete("test");
File.rename("test1", "test");
The difference is that in case of an exception the file is closed
automatically. Otherwise you have to wait for garbage collector. It's
a good habit to get used to this style.
2. newfile.puts(File.read("test")) will read the entire file into
memory. Don't do this on large files - use the original way (or even
better, loop over the file with File#read(size)). For small files,
this read() is better.
3. newfile.puts(File.read("test")) will put an extra newline at the
end. Use either
newfile << File.read("test")
or
newfile.write(File.read("test"))
Robert Klemme
Just a few remarks: better return to your old habit and use the block
form of File.open (btw, you do not need to close the file, File#open
takes care of that when the block is left - even in case of an
exception).
You don't need to terminate lines with ";".
Also, you can use variables to make your life easier:
file = "test"
tmp = file + "~"
File.open(tmp, "w") do |out|
out.puts "This line should appear at the top of each file"
File.foreach file do |line|
out.puts line
end
end
File.delete file
File.rename tmp, file
Kind regards
robert
Ronald Fischer
> memory. Don't do this on large files - use the original way (or even
> better, loop over the file with File#read(size)). For small files,
> this read() is better.
Good point! (Only that *prepending* data to a file which is so big that
it would be a memory hog, is probably a nightmare anyway.
> 3. newfile.puts(File.read("test")) will put an extra newline at the
> end. Use either
> newfile << File.read("test")
> or
> newfile.write(File.read("test"))
Right, I overlooked this! Thanks for pointing this out.
Ronald
Florian Aßmann
require 'tempfile'
class File
def self.prepend(path, string)
Tempfile.open File.basename(path) do |tempfile|
# shift string to tempfile
tempfile << string
File.open(path, 'r+') do |file|
# append original data to tempfile
tempfile << file.read
# reset file positions
file.pos = tempfile.pos = 0
# copy tempfile back to original file
file << tempfile.read
end
end
end
end
Regards
Florian
Peña, Botp
# newfile = File.new("test1","w")
# newfile.puts "This line should appear at the top of each file";
# oldfile = File.open("test", "r+")
# oldfile.each_line { |line| newfile.puts line}
# oldfile.close();
# newfile.close();
# File.delete("test");
# File.rename("test1", "test");
that is nice, but it would be ideal if your code is immune to dependencies (like static filenames, fixed number of files, etc)
eg, consider the *nix cat command, you can easily insert lines at the beginning or end. In your case eg,
irb(main):006:0> system "ls -la test2.txt"
ls: test2.txt: No such file or directory
=> false
irb(main):007:0> system "cat test.txt"
1
2
3
this
is
a
test=> true
irb(main):008:0> text_insert="this will be inserted"
=> "this will be inserted"
irb(main):009:0> system "echo #{text_insert} | cat - test.txt"
this will be inserted
1
2
3
this
is
a
test=> true
irb(main):010:0> new_file="test2.txt"
=> "test2.txt"
irb(main):011:0> system "echo #{text_insert} | cat - test.txt > #{new_file}"
=> true
irb(main):012:0> system "ls -la test2.txt"
-rw-r--r-- 1 root root 42 Aug 25 09:02 test2.txt
=> true
irb(main):013:0> system "cat test2.txt"
this will be inserted
1
2
3
this
is
a
test=> true
irb(main):014:0>
note that cat is simply flexible. eg,
cat f - g
will concatenate file f, standard input, and file g in that order; thus inserting standard input bw the contents of files f and g.
i think ARGF is flexible enough. just a simple thought.
kind regards -botp
Tom Reilly
which works fine so long as the length of this item doesn't change.
#-----------------------------------------------
#timer to track time I work on medicine
#version 1.3 8/12/07
# sleep added
# total time computed and displayed
rw = "r+"
$stdout.sync = true
begin
fo = File.new("time_study.txt",rw)
rescue
rw = "w+"
retry
end
old_chapter = fo.read(3)
if old_chapter == nil
fo.write("00\t *** \t \n")
p "Start a new file"
else
puts old_chapter
end
fo.seek(0,IO::SEEK_SET)
time_start = Time.now
prev_chapter = fo.read(2)
puts "Start at chapter #{prev_chapter}"
puts "Enter chapter to start"
fo.seek(0,IO::SEEK_SET)
chapter = gets.chomp
fo.write("#{chapter}")
time_end = Time.now
fo.seek(0,IO::SEEK_END)
fo.write("#{time_start.to_i} #{time_end.to_i} #{(time_end -
time_start)}\n")
fo.seek(0,IO::SEEK_SET)
total_time = 0.0
fo.each do |x|
xa = x.split
total_time += xa[2].to_f
end
puts "Total time Hours = #{total_time / 3600.0}"
sleep 10
fo.close