Class: IO

Creates and returns a new IO object (file stream) from a file descriptor.

IO.new may be useful for interaction with low-level libraries. For higher-level interactions, it may be simpler to create the file stream using File.open.

Argument fd must be a valid file descriptor (integer):

path = 't.tmp'
fd = IO.sysopen(path) # => 3
IO.new(fd)            # => #<IO:fd 3>

The new IO object does not inherit encoding (because the integer file descriptor does not have an encoding):

fd = IO.sysopen('t.rus', 'rb')
io = IO.new(fd)
io.external_encoding # => #<Encoding:UTF-8> # Not ASCII-8BIT.

Optional argument mode (defaults to ‘r’) must specify a valid mode; see Access Modes:

IO.new(fd, 'w')         # => #<IO:fd 3>
IO.new(fd, File::WRONLY) # => #<IO:fd 3>

Optional keyword arguments opts specify:

• Open Options.

• {Encoding options}.

Examples:

IO.new(fd, internal_encoding: nil) # => #<IO:fd 3>
IO.new(fd, autoclose: true)        # => #<IO:fd 3>

Behaves like .write, except that the stream is opened in binary mode with ASCII-8BIT encoding.

When called from class IO (but not subclasses of IO), this method has potential security vulnerabilities if called with untrusted input; see Command Injection.

Copies from the given src to the given dst, returning the number of bytes copied.

• The given src must be one of the following:

  • The path to a readable file, from which source data is to be read.

  • An IO-like object, opened for reading and capable of responding to method :readpartial or method :read.

• The given dst must be one of the following:

  • The path to a writable file, to which data is to be written.

  • An IO-like object, opened for writing and capable of responding to method :write.

The examples here use file t.txt as source:

File.read('t.txt')
# => "First line\nSecond line\n\nThird line\nFourth line\n"
File.read('t.txt').size # => 47

If only arguments src and dst are given, the entire source stream is copied:

# Paths.
IO.copy_stream('t.txt', 't.tmp')  # => 47

# IOs (recall that a File is also an IO).
src_io = File.open('t.txt', 'r') # => #<File:t.txt>
dst_io = File.open('t.tmp', 'w') # => #<File:t.tmp>
IO.copy_stream(src_io, dst_io)   # => 47
src_io.close
dst_io.close

With argument src_length a non-negative integer, no more than that many bytes are copied:

IO.copy_stream('t.txt', 't.tmp', 10) # => 10
File.read('t.tmp')                   # => "First line"

With argument src_offset also given, the source stream is read beginning at that offset:

IO.copy_stream('t.txt', 't.tmp', 11, 11) # => 11
IO.read('t.tmp')                         # => "Second line"

Synonym for .new.

Calls the block with each successive line read from the stream.

When called from class IO (but not subclasses of IO), this method has potential security vulnerabilities if called with untrusted input; see Command Injection.

The first argument must be a string that is one of the following:

• Path: if self is a subclass of IO (File, for example), or if the string does not start with the pipe character ('|'), the string is the path to a file.

• Command: if self is the class IO, and if the string starts with the pipe character, the rest of the string is a command to be executed as a subprocess. This usage has potential security vulnerabilities if called with untrusted input; see Command Injection.

With only argument #path given, parses lines from the file at the given #path, as determined by the default line separator, and calls the block with each successive line:

File.foreach('t.txt') {|line| p line }

Output: the same as above.

For both forms, command and path, the remaining arguments are the same.

With argument sep given, parses lines as determined by that line separator (see Line Separator):

File.foreach('t.txt', 'li') {|line| p line }

Output:

"First li"
"ne\nSecond li"
"ne\n\nThird li"
"ne\nFourth li"
"ne\n"

Each paragraph:

File.foreach('t.txt', '') {|paragraph| p paragraph }

Output:

"First line\nSecond line\n\n"
"Third line\nFourth line\n"

With argument limit given, parses lines as determined by the default line separator and the given line-length limit (see Line Limit):

File.foreach('t.txt', 7) {|line| p line }

Output:

"First l"
"ine\n"
"Second "
"line\n"
"\n"
"Third l"
"ine\n"
"Fourth l"
"line\n"

With arguments sep and limit given, parses lines as determined by the given line separator and the given line-length limit (see Line Separator and Line Limit):

Optional keyword arguments opts specify:

• Open Options.

• {Encoding options}.

• Line Options.

Returns an ::Enumerator if no block is given.

Alias for File.open. Creates a new IO object, via .new with the given arguments.

With no block given, returns the IO object.

With a block given, calls the block with the IO object and returns the block’s value.

Creates a pair of pipe endpoints, read_io and write_io, connected to each other.

If argument enc_string is given, it must be a string containing one of:

• The name of the encoding to be used as the external encoding.

• The colon-separated names of two encodings to be used as the external and internal encodings.

If argument int_enc is given, it must be an ::Encoding object or encoding name string that specifies the internal encoding to be used; if argument ext_enc is also given, it must be an ::Encoding object or encoding name string that specifies the external encoding to be used.

The string read from read_io is tagged with the external encoding; if an internal encoding is also specified, the string is converted to, and tagged with, that encoding.

If any encoding is specified, optional hash arguments specify the conversion option.

Optional keyword arguments opts specify:

• Open Options.

• {Encoding Options}.

With no block given, returns the two endpoints in an array:

IO.pipe # => [#<IO:fd 4>, #<IO:fd 5>]

With a block given, calls the block with the two endpoints; closes both endpoints and returns the value of the block:

IO.pipe {|read_io, write_io| p read_io; p write_io }

Output:

#<IO:fd 6>
#<IO:fd 7>

Not available on all platforms.

In the example below, the two processes close the ends of the pipe that they are not using. This is not just a cosmetic nicety. The read end of a pipe will not generate an end of file condition if there are any writers with the pipe still open. In the case of the parent process, the rd.read will never return if it does not first issue a wr.close:

rd, wr = IO.pipe

if fork
  wr.close
  puts "Parent got: <#{rd.read}>"
  rd.close
  Process.wait
else
  rd.close
  puts 'Sending message to parent'
  wr.write "Hi Dad"
  wr.close
end

produces:

Sending message to parent
Parent got: <Hi Dad>

Executes the given command cmd as a subprocess whose $stdin and $stdout are connected to a new stream io.

This method has potential security vulnerabilities if called with untrusted input; see Command Injection.

If no block is given, returns the new stream, which depending on given mode may be open for reading, writing, or both. The stream should be explicitly closed (eventually) to avoid resource leaks.

If a block is given, the stream is passed to the block (again, open for reading, writing, or both); when the block exits, the stream is closed, and the block’s value is assigned to global variable $? and returned.

Optional argument mode may be any valid IO mode. See Access Modes.

Required argument cmd determines which of the following occurs:

• The process forks.

• A specified program runs in a shell.

• A specified program runs with specified arguments.

• A specified program runs with specified arguments and a specified argv0.

Each of these is detailed below.

The optional hash argument env specifies name/value pairs that are to be added to the environment variables for the subprocess:

IO.popen({'FOO' => 'bar'}, 'ruby', 'r+') do |pipe|
  pipe.puts 'puts ENV["FOO"]'
  pipe.close_write
  pipe.gets
end => "bar\n"

Optional keyword arguments opts specify:

• Open options.

• {Encoding options}.

• Options for Kernel.spawn.

Forked Process

When argument cmd is the 1-character string '-', causes the process to fork:

IO.popen('-') do |pipe|
  if pipe
    $stderr.puts "In parent, child pid is #{pipe.pid}\n"
  else
    $stderr.puts "In child, pid is #{$$}\n"
  end
end

Output:

In parent, child pid is 26253
In child, pid is 26253

Note that this is not supported on all platforms.

Shell Subprocess

When argument cmd is a single string (but not '-'), the program named cmd is run as a shell command:

IO.popen('uname') do |pipe|
  pipe.readlines
end

Output:

["Linux\n"]

Another example:

IO.popen('/bin/sh', 'r+') do |pipe|
  pipe.puts('ls')
  pipe.close_write
  $stderr.puts pipe.readlines.size
end

Output:

213

Program Subprocess

When argument cmd is an array of strings, the program named cmd[0] is run with all elements of cmd as its arguments:

IO.popen(['du', '..', '.']) do |pipe|
  $stderr.puts pipe.readlines.size
end

Output:

1111

Program Subprocess with argv0

When argument cmd is an array whose first element is a 2-element string array and whose remaining elements (if any) are strings:

• cmd[0][0] (the first string in the nested array) is the name of a program that is run.

• cmd[0][1] (the second string in the nested array) is set as the program’s argv[0].

• cmd[1..-1] (the strings in the outer array) are the program’s arguments.

Example (sets $0 to ‘foo’):

IO.popen([['/bin/sh', 'foo'], '-c', 'echo $0']).read # => "foo\n"

Some Special Examples

# Set IO encoding.
IO.popen("nkf -e filename", :external_encoding=>"EUC-JP") {|nkf_io|
  euc_jp_string = nkf_io.read
}

# Merge standard output and standard error using Kernel#spawn option. See Kernel#spawn.
IO.popen(["ls", "/", :err=>[:child, :out]]) do |io|
  ls_result_with_error = io.read
end

# Use mixture of spawn options and IO options.
IO.popen(["ls", "/"], :err=>[:child, :out]) do |io|
  ls_result_with_error = io.read
end

 f = IO.popen("uname")
 p f.readlines
 f.close
 puts "Parent is #{Process.pid}"
 IO.popen("date") {|f| puts f.gets }
 IO.popen("-") {|f| $stderr.puts "#{Process.pid} is here, f is #{f.inspect}"}
 p $?
 IO.popen(%w"sed -e s|^|<foo>| -e s&$&;zot;&", "r+") {|f|
   f.puts "bar"; f.close_write; puts f.gets
 }

Output (from last section):

["Linux\n"]
Parent is 21346
Thu Jan 15 22:41:19 JST 2009
21346 is here, f is #<IO:fd 3>
21352 is here, f is nil
#<Process::Status: pid 21352 exit 0>
<foo>bar;zot;

Raises exceptions that .pipe and Kernel.spawn raise.

Opens the stream, reads and returns some or all of its content, and closes the stream; returns nil if no bytes were read.

When called from class IO (but not subclasses of IO), this method has potential security vulnerabilities if called with untrusted input; see Command Injection.

The first argument must be a string; its meaning depends on whether it starts with the pipe character ('|'):

• If so (and if self is IO), the rest of the string is a command to be executed as a subprocess.

• Otherwise, the string is the path to a file.

With only argument command given, executes the command in a shell, returns its entire $stdout:

IO.read('| cat t.txt')
# => "First line\nSecond line\n\nThird line\nFourth line\n"

With only argument #path given, reads in text mode and returns the entire content of the file at the given path:

IO.read('t.txt')
# => "First line\nSecond line\n\nThird line\nFourth line\n"

On Windows, text mode can terminate reading and leave bytes in the file unread when encountering certain special bytes. Consider using .binread if all bytes in the file should be read.

For both forms, command and path, the remaining arguments are the same.

With argument length, returns length bytes if available:

IO.read('t.txt', 7) # => "First l"
IO.read('t.txt', 700)
# => "First line\r\nSecond line\r\n\r\nFourth line\r\nFifth line\r\n"

With arguments length and offset, returns length bytes if available, beginning at the given offset:

IO.read('t.txt', 10, 2)   # => "rst line\nS"
IO.read('t.txt', 10, 200) # => nil

Optional keyword arguments opts specify:

• Open Options.

• {Encoding options}.

Returns an array of all lines read from the stream.

When called from class IO (but not subclasses of IO), this method has potential security vulnerabilities if called with untrusted input; see Command Injection.

The first argument must be a string; its meaning depends on whether it starts with the pipe character ('|'):

• If so (and if self is IO), the rest of the string is a command to be executed as a subprocess.

• Otherwise, the string is the path to a file.

With only argument command given, executes the command in a shell, parses its $stdout into lines, as determined by the default line separator, and returns those lines in an array:

IO.readlines('| cat t.txt')
# => ["First line\n", "Second line\n", "\n", "Third line\n", "Fourth line\n"]

With only argument #path given, parses lines from the file at the given #path, as determined by the default line separator, and returns those lines in an array:

IO.readlines('t.txt')
# => ["First line\n", "Second line\n", "\n", "Third line\n", "Fourth line\n"]

For both forms, command and path, the remaining arguments are the same.

With argument sep given, parses lines as determined by that line separator (see Line Separator):

# Ordinary separator.
IO.readlines('t.txt', 'li')
# =>["First li", "ne\nSecond li", "ne\n\nThird li", "ne\nFourth li", "ne\n"]
# Get-paragraphs separator.
IO.readlines('t.txt', '')
# => ["First line\nSecond line\n\n", "Third line\nFourth line\n"]
# Get-all separator.
IO.readlines('t.txt', nil)
# => ["First line\nSecond line\n\nThird line\nFourth line\n"]

With argument limit given, parses lines as determined by the default line separator and the given line-length limit (see Line Limit):

IO.readlines('t.txt', 7)
# => ["First l", "ine\n", "Second ", "line\n", "\n", "Third l", "ine\n", "Fourth ", "line\n"]

With arguments sep and limit given, parses lines as determined by the given line separator and the given line-length limit (see Line Separator and Line Limit):

Optional keyword arguments opts specify:

• Open Options.

• {Encoding options}.

• Line Options.

Alias for Kernel.select. Invokes system call select(2), which monitors multiple file descriptors, waiting until one or more of the file descriptors becomes ready for some class of I/O operation.

Not implemented on all platforms.

Each of the arguments read_ios, write_ios, and error_ios is an array of IO objects.

Argument #timeout is an integer timeout interval in seconds.

The method monitors the IO objects given in all three arrays, waiting for some to be ready; returns a 3-element array whose elements are:

• An array of the objects in read_ios that are ready for reading.

• An array of the objects in write_ios that are ready for writing.

• An array of the objects in error_ios have pending exceptions.

If no object becomes ready within the given #timeout, nil is returned.

IO.select peeks the buffer of IO objects for testing readability. If the IO buffer is not empty, IO.select immediately notifies readability. This “peek” only happens for IO objects. It does not happen for IO-like objects such as OpenSSL::SSL::SSLSocket.

The best way to use IO.select is invoking it after non-blocking methods such as #read_nonblock, #write_nonblock, etc. The methods raise an exception which is extended by ::IO::WaitReadable or ::IO::WaitWritable. The modules notify how the caller should wait with IO.select. If ::IO::WaitReadable is raised, the caller should wait for reading. If ::IO::WaitWritable is raised, the caller should wait for writing.

So, blocking read (#readpartial) can be emulated using #read_nonblock and IO.select as follows:

begin
  result = io_like.read_nonblock(maxlen)
rescue IO::WaitReadable
  IO.select([io_like])
  retry
rescue IO::WaitWritable
  IO.select(nil, [io_like])
  retry
end

Especially, the combination of non-blocking methods and IO.select is preferred for IO like objects such as OpenSSL::SSL::SSLSocket. It has #to_io method to return underlying IO object. select calls #to_io to obtain the file descriptor to wait.

This means that readability notified by IO.select doesn’t mean readability from OpenSSL::SSL::SSLSocket object.

The most likely situation is that OpenSSL::SSL::SSLSocket buffers some data. IO.select doesn’t see the buffer. So IO.select can block when OpenSSL::SSL::SSLSocket#readpartial doesn’t block.

However, several more complicated situations exist.

SSL is a protocol which is sequence of records. The record consists of multiple bytes. So, the remote side of SSL sends a partial record, select notifies readability but OpenSSL::SSL::SSLSocket cannot decrypt a byte and OpenSSL::SSL::SSLSocket#readpartial will block.

Also, the remote side can request SSL renegotiation which forces the local SSL engine to write some data. This means OpenSSL::SSL::SSLSocket#readpartial may invoke #write system call and it can block. In such a situation, OpenSSL::SSL::SSLSocket#read_nonblock raises ::IO::WaitWritable instead of blocking. So, the caller should wait for ready for writability as above example.

The combination of non-blocking methods and IO.select is also useful for streams such as tty, pipe socket socket when multiple processes read from a stream.

Finally, Linux kernel developers don’t guarantee that readability of select(2) means readability of following read(2) even for a single process; see select(2)

Invoking IO.select before #readpartial works well as usual. However it is not the best way to use IO.select.

The writability notified by select(2) doesn’t show how many bytes are writable. #write method blocks until given whole string is written. So, IO#write(two or more bytes) can block after writability is notified by IO.select. #write_nonblock is required to avoid the blocking.

Blocking write (#write) can be emulated using #write_nonblock and select as follows: ::IO::WaitReadable should also be rescued for SSL renegotiation in OpenSSL::SSL::SSLSocket.

while 0 < string.bytesize
  begin
    written = io_like.write_nonblock(string)
  rescue IO::WaitReadable
    IO.select([io_like])
    retry
  rescue IO::WaitWritable
    IO.select(nil, [io_like])
    retry
  end
  string = string.byteslice(written..-1)
end

Example:

rp, wp = IO.pipe
mesg = "ping "
100.times {
  # IO.select follows IO#read.  Not the best way to use IO.select.
  rs, ws, = IO.select([rp], [wp])
  if r = rs[0]
    ret = r.read(5)
    print ret
    case ret
    when /ping/
      mesg = "pong\n"
    when /pong/
      mesg = "ping "
    end
  end
  if w = ws[0]
    w.write(mesg)
  end
}

Output:

ping pong
ping pong
ping pong
(snipped)
ping

Opens the file at the given path with the given mode and permissions; returns the integer file descriptor.

If the file is to be readable, it must exist; if the file is to be writable and does not exist, it is created with the given permissions:

File.write('t.tmp', '')  # => 0
IO.sysopen('t.tmp')      # => 8
IO.sysopen('t.tmp', 'w') # => 9

Attempts to convert object into an IO object via method #to_io; returns the new IO object if successful, or nil otherwise:

IO.try_convert(STDOUT)   # => #<IO:<STDOUT>>
IO.try_convert(ARGF)     # => #<IO:<STDIN>>
IO.try_convert('STDOUT') # => nil

Opens the stream, writes the given data to it, and closes the stream; returns the number of bytes written.

When called from class IO (but not subclasses of IO), this method has potential security vulnerabilities if called with untrusted input; see Command Injection.

The first argument must be a string; its meaning depends on whether it starts with the pipe character ('|'):

• If so (and if self is IO), the rest of the string is a command to be executed as a subprocess.

• Otherwise, the string is the path to a file.

With argument command given, executes the command in a shell, passes data through standard input, writes its output to $stdout, and returns the length of the given data:

IO.write('| cat', 'Hello World!') # => 12

Output:

Hello World!

With argument #path given, writes the given data to the file at that path:

IO.write('t.tmp', 'abc')    # => 3
File.read('t.tmp')          # => "abc"

If offset is zero (the default), the file is overwritten:

IO.write('t.tmp', 'A')      # => 1
File.read('t.tmp')          # => "A"

If offset in within the file content, the file is partly overwritten:

IO.write('t.tmp', 'abcdef') # => 3
File.read('t.tmp')          # => "abcdef"
# Offset within content.
IO.write('t.tmp', '012', 2) # => 3
File.read('t.tmp')          # => "ab012f"

If offset is outside the file content, the file is padded with null characters "\u0000":

IO.write('t.tmp', 'xyz', 10) # => 3
File.read('t.tmp')           # => "ab012f\u0000\u0000\u0000\u0000xyz"

Optional keyword arguments opts specify:

• Open Options.

• {Encoding options}.

Returns true if the underlying file descriptor of ios will be closed automatically at its finalization, otherwise false.

Sets the stream’s data mode as binary (see {Data Mode}).

A stream’s data mode may not be changed from binary to text.

Returns true if the stream is on binary mode, false otherwise. See {Data Mode}.

Sets a close-on-exec flag.

f = open("/dev/null")
f.close_on_exec = true
system("cat", "/proc/self/fd/#{f.fileno}") # cat: /proc/self/fd/3: No such file or directory
f.closed?                #=> false

Ruby sets close-on-exec flags of all file descriptors by default since Ruby 2.0.0. So you don’t need to set by yourself. Also, unsetting a close-on-exec flag can cause file descriptor leak if another thread use fork() and exec() (via system() method for example). If you really needs file descriptor inheritance to child process, use spawn()‘s argument such as fd=>fd.

Returns true if the stream will be closed on exec, false otherwise:

f = File.open('t.txt')
f.close_on_exec? # => true
f.close_on_exec = false
f.close_on_exec? # => false
f.close

Returns true if the stream is closed for both reading and writing, false otherwise. See Open and Closed Streams.

IO.popen('ruby', 'r+') do |pipe|
  puts pipe.closed?
  pipe.close_read
  puts pipe.closed?
  pipe.close_write
  puts pipe.closed?
end

Output:

false
false
true

Related: #close_read, #close_write, #close.

Returns true if the stream is positioned at its end, false otherwise; see Position:

f = File.open('t.txt')
f.eof           # => false
f.seek(0, :END) # => 0
f.eof           # => true
f.close

Raises an exception unless the stream is opened for reading; see Mode.

If self is a stream such as pipe or socket, this method blocks until the other end sends some data or closes it:

r, w = IO.pipe
Thread.new { sleep 1; w.close }
r.eof? # => true # After 1-second wait.

r, w = IO.pipe
Thread.new { sleep 1; w.puts "a" }
r.eof?  # => false # After 1-second wait.

r, w = IO.pipe
r.eof?  # blocks forever

Note that this method reads data to the input byte buffer. So #sysread may not behave as you intend with #eof?, unless you call #rewind first (which is not available for some streams).

#eof? is an alias for eof.

Alias for #eof.

Alias for #tty?.

Returns the current line number for the stream; see Line Number.

Sets and returns the line number for the stream; see Line Number.

Returns the current position (in bytes) in self (see Position):

f = File.open('t.txt')
f.tell # => 0
f.gets # => "First line\n"
f.tell # => 12
f.close

Related: #pos=, #seek.

pos is an alias for #tell.

Seeks to the given new_position (in bytes); see Position:

f = File.open('t.txt')
f.tell     # => 0
f.pos = 20 # => 20
f.tell     # => 20
f.close

Related: #seek, #tell.

Returns the current sync mode of the stream. When sync mode is true, all output is immediately flushed to the underlying operating system and is not buffered by Ruby internally. See also #fsync.

f = File.open('t.tmp', 'w')
f.sync # => false
f.sync = true
f.sync # => true
f.close

Sets the sync mode for the stream to the given value; returns the given value.

Values for the sync mode:

• true: All output is immediately flushed to the underlying operating system and is not buffered internally.

• false: Output may be buffered internally.

Example;

f = File.open('t.tmp', 'w')
f.sync # => false
f.sync = true
f.sync # => true
f.close

Related: #fsync.

Alias for #pos.

Get the internal timeout duration or nil if it was not set.

Set the internal timeout to the specified duration or nil. The timeout applies to all blocking operations where possible.

This affects the following methods (but is not limited to): #gets, #puts, #read, #write, #wait_readable and #wait_writable. This also affects blocking socket operations like Socket#accept and Socket#connect.

Some operations like File#open and #close are not affected by the timeout. A timeout during a write operation may leave the IO in an inconsistent state, e.g. data was partially written. Generally speaking, a timeout is a last ditch effort to prevent an application from hanging on slow I/O operations, such as those that occur during a slowloris attack.

Returns true if the stream is associated with a terminal device (tty), false otherwise:

f = File.new('t.txt').isatty    #=> false
f.close
f = File.new('/dev/tty').isatty #=> true
f.close

tty? is an alias for #isatty.

Invokes Posix system call posix_fadvise(2), which announces an intention to access data from the current file in a particular manner.

The arguments and results are platform-dependent.

The relevant data is specified by:

• offset: The offset of the first byte of data.

• len: The number of bytes to be accessed; if len is zero, or is larger than the number of bytes remaining, all remaining bytes will be accessed.

Argument advice is one of the following symbols:

• :normal: The application has no advice to give about its access pattern for the specified data. If no advice is given for an open file, this is the default assumption.

• :sequential: The application expects to access the specified data sequentially (with lower offsets read before higher ones).

• :random: The specified data will be accessed in random order.

• :noreuse: The specified data will be accessed only once.

• :willneed: The specified data will be accessed in the near future.

• :dontneed: The specified data will not be accessed in the near future.

Not implemented on all platforms.

Closes the stream for both reading and writing if open for either or both; returns nil. See Open and Closed Streams.

If the stream is open for writing, flushes any buffered writes to the operating system before closing.

If the stream was opened by .popen, sets global variable $? (child exit status).

Example:

IO.popen('ruby', 'r+') do |pipe|
  puts pipe.closed?
  pipe.close
  puts $?
  puts pipe.closed?
end

Output:

false
pid 13760 exit 0
true

Related: #close_read, #close_write, #closed?.

Closes the stream for reading if open for reading; returns nil. See Open and Closed Streams.

If the stream was opened by .popen and is also closed for writing, sets global variable $? (child exit status).

Example:

IO.popen('ruby', 'r+') do |pipe|
  puts pipe.closed?
  pipe.close_write
  puts pipe.closed?
  pipe.close_read
  puts $?
  puts pipe.closed?
end

Output:

false
false
pid 14748 exit 0
true

Related: #close, #close_write, #closed?.

Closes the stream for writing if open for writing; returns nil. See Open and Closed Streams.

Flushes any buffered writes to the operating system before closing.

If the stream was opened by .popen and is also closed for reading, sets global variable $? (child exit status).

IO.popen('ruby', 'r+') do |pipe|
  puts pipe.closed?
  pipe.close_read
  puts pipe.closed?
  pipe.close_write
  puts $?
  puts pipe.closed?
end

Output:

false
false
pid 15044 exit 0
true

Related: #close, #close_read, #closed?.

Calls the block with each remaining line read from the stream; returns self. Does nothing if already at end-of-stream; See Line IO.

With no arguments given, reads lines as determined by line separator $/:

f = File.new('t.txt')
f.each_line {|line| p line }
f.each_line {|line| fail 'Cannot happen' }
f.close

Output:

"First line\n"
"Second line\n"
"\n"
"Fourth line\n"
"Fifth line\n"

With only string argument sep given, reads lines as determined by line separator sep; see Line Separator:

f = File.new('t.txt')
f.each_line('li') {|line| p line }
f.close

Output:

"First li"
"ne\nSecond li"
"ne\n\nFourth li"
"ne\nFifth li"
"ne\n"

The two special values for sep are honored:

f = File.new('t.txt')
# Get all into one string.
f.each_line(nil) {|line| p line }
f.close

Output:

"First line\nSecond line\n\nFourth line\nFifth line\n"

f.rewind
# Get paragraphs (up to two line separators).
f.each_line('') {|line| p line }

Output:

"First line\nSecond line\n\n"
"Fourth line\nFifth line\n"

With only integer argument limit given, limits the number of bytes in each line; see Line Limit:

f = File.new('t.txt')
f.each_line(8) {|line| p line }
f.close

Output:

"First li"
"ne\n"
"Second l"
"ine\n"
"\n"
"Fourth l"
"ine\n"
"Fifth li"
"ne\n"

With arguments sep and limit given, combines the two behaviors:

• Calls with the next line as determined by line separator sep.

• But returns no more bytes than are allowed by the limit.

Optional keyword argument chomp specifies whether line separators are to be omitted:

f = File.new('t.txt')
f.each_line(chomp: true) {|line| p line }
f.close

Output:

"First line"
"Second line"
""
"Fourth line"
"Fifth line"

Returns an ::Enumerator if no block is given.

each is an alias for #each_line.

Calls the given block with each byte (0..255) in the stream; returns self. See Byte IO.

f = File.new('t.rus')
a = []
f.each_byte {|b| a << b }
a # => [209, 130, 208, 181, 209, 129, 209, 130]
f.close

Returns an ::Enumerator if no block is given.

Related: #each_char, #each_codepoint.

Calls the given block with each character in the stream; returns self. See Character IO.

f = File.new('t.rus')
a = []
f.each_char {|c| a << c.ord }
a # => [1090, 1077, 1089, 1090]
f.close

Returns an ::Enumerator if no block is given.

Related: #each_byte, #each_codepoint.

Calls the given block with each codepoint in the stream; returns self:

f = File.new('t.rus')
a = []
f.each_codepoint {|c| a << c }
a # => [1090, 1077, 1089, 1090]
f.close

Returns an ::Enumerator if no block is given.

Related: #each_byte, #each_char.

Alias for #each.

Returns the ::Encoding object that represents the encoding of the stream, or nil if the stream is in write mode and no encoding is specified.

See Encodings.

Invokes Posix system call fcntl(2), which provides a mechanism for issuing low-level commands to control or query a file-oriented I/O stream. Arguments and results are platform dependent.

If +argument is a number, its value is passed directly; if it is a string, it is interpreted as a binary sequence of bytes. (Array#pack might be a useful way to build this string.)

Not implemented on all platforms.

Immediately writes to disk all data buffered in the stream, via the operating system’s: fdatasync(2), if supported, otherwise via fsync(2), if supported; otherwise raises an exception.

Alias for #to_i.

Flushes data buffered in self to the operating system (but does not necessarily flush data buffered in the operating system):

$stdout.print 'no newline' # Not necessarily flushed.
$stdout.flush              # Flushed.

Immediately writes to disk all data buffered in the stream, via the operating system’s fsync(2).

Note this difference:

• #sync=: Ensures that data is flushed from the stream’s internal buffers, but does not guarantee that the operating system actually writes the data to disk.

• fsync: Ensures both that data is flushed from internal buffers, and that data is written to disk.

Raises an exception if the operating system does not support fsync(2).

Reads and returns the next byte (in range 0..255) from the stream; returns nil if already at end-of-stream. See Byte IO.

f = File.open('t.txt')
f.getbyte # => 70
f.close
f = File.open('t.rus')
f.getbyte # => 209
f.close

Related: #readbyte (may raise ::EOFError).

Reads and returns the next 1-character string from the stream; returns nil if already at end-of-stream. See Character IO.

f = File.open('t.txt')
f.getc     # => "F"
f.close
f = File.open('t.rus')
f.getc.ord # => 1090
f.close

Related: #readchar (may raise ::EOFError).

Reads and returns a line from the stream; assigns the return value to $_. See Line IO.

With no arguments given, returns the next line as determined by line separator $/, or nil if none:

f = File.open('t.txt')
f.gets # => "First line\n"
$_     # => "First line\n"
f.gets # => "\n"
f.gets # => "Fourth line\n"
f.gets # => "Fifth line\n"
f.gets # => nil
f.close

With only string argument sep given, returns the next line as determined by line separator sep, or nil if none; see Line Separator:

f = File.new('t.txt')
f.gets('l')   # => "First l"
f.gets('li')  # => "ine\nSecond li"
f.gets('lin') # => "ne\n\nFourth lin"
f.gets        # => "e\n"
f.close

The two special values for sep are honored:

f = File.new('t.txt')
# Get all.
f.gets(nil) # => "First line\nSecond line\n\nFourth line\nFifth line\n"
f.rewind
# Get paragraph (up to two line separators).
f.gets('')  # => "First line\nSecond line\n\n"
f.close

With only integer argument limit given, limits the number of bytes in the line; see Line Limit:

# No more than one line.
File.open('t.txt') {|f| f.gets(10) } # => "First line"
File.open('t.txt') {|f| f.gets(11) } # => "First line\n"
File.open('t.txt') {|f| f.gets(12) } # => "First line\n"

With arguments sep and limit given, combines the two behaviors:

• Returns the next line as determined by line separator sep, or nil if none.

• But returns no more bytes than are allowed by the limit.

Optional keyword argument chomp specifies whether line separators are to be omitted:

f = File.open('t.txt')
# Chomp the lines.
f.gets(chomp: true) # => "First line"
f.gets(chomp: true) # => "Second line"
f.gets(chomp: true) # => ""
f.gets(chomp: true) # => "Fourth line"
f.gets(chomp: true) # => "Fifth line"
f.gets(chomp: true) # => nil
f.close

Returns a string representation of self:

f = File.open('t.txt')
f.inspect # => "#<File:t.txt>"
f.close

Returns the ::Encoding object that represents the encoding of the internal string, if conversion is specified, or nil otherwise.

See Encodings.

Invokes Posix system call ioctl(2), which issues a low-level command to an I/O device.

Issues a low-level command to an I/O device. The arguments and returned value are platform-dependent. The effect of the call is platform-dependent.

If argument argument is an integer, it is passed directly; if it is a string, it is interpreted as a binary sequence of bytes.

Not implemented on all platforms.

Returns the path associated with the IO, or nil if there is no path associated with the IO. It is not guaranteed that the path exists on the filesystem.

$stdin.path # => "<STDIN>"

File.open("testfile") {|f| f.path} # => "testfile"

Returns the process ID of a child process associated with the stream, which will have been set by IO#popen, or nil if the stream was not created by IO#popen:

pipe = IO.popen("-")
if pipe
  $stderr.puts "In parent, child pid is #{pipe.pid}"
else
  $stderr.puts "In child, pid is #{$$}"
end

Output:

In child, pid is 26209
In parent, child pid is 26209

Behaves like #readpartial, except that it:

• Reads at the given offset (in bytes).

• Disregards, and does not modify, the stream’s position (see Position).

• Bypasses any user space buffering in the stream.

Because this method does not disturb the stream’s state (its position, in particular), pread allows multiple threads and processes to use the same IO object for reading at various offsets.

f = File.open('t.txt')
f.read # => "First line\nSecond line\n\nFourth line\nFifth line\n"
f.pos  # => 52
# Read 12 bytes at offset 0.
f.pread(12, 0) # => "First line\n"
# Read 9 bytes at offset 8.
f.pread(9, 8)  # => "ne\nSecon"
f.close

Not available on some platforms.

Writes the given objects to the stream; returns nil. Appends the output record separator $OUTPUT_RECORD_SEPARATOR ($\), if it is not nil. See Line IO.

With argument objects given, for each object:

• Converts via its method to_s if not a string.

• Writes to the stream.

• If not the last object, writes the output field separator $OUTPUT_FIELD_SEPARATOR ($,) if it is not nil.

With default separators:

f = File.open('t.tmp', 'w+')
objects = [0, 0.0, Rational(0, 1), Complex(0, 0), :zero, 'zero']
p $OUTPUT_RECORD_SEPARATOR
p $OUTPUT_FIELD_SEPARATOR
f.print(*objects)
f.rewind
p f.read
f.close

Output:

nil
nil
"00.00/10+0izerozero"

With specified separators:

$\ = "\n"
$, = ','
f.rewind
f.print(*objects)
f.rewind
p f.read

Output:

"0,0.0,0/1,0+0i,zero,zero\n"

With no argument given, writes the content of $_ (which is usually the most recent user input):

f = File.open('t.tmp', 'w+')
gets # Sets $_ to the most recent user input.
f.print
f.close

Formats and writes objects to the stream.

For details on format_string, see Format Specifications.

Writes a character to the stream. See Character IO.

If object is numeric, converts to integer if necessary, then writes the character whose code is the least significant byte; if object is a string, writes the first character:

$stdout.putc "A"
$stdout.putc 65

Output:

AA

Writes the given objects to the stream, which must be open for writing; returns nil.\ Writes a newline after each that does not already end with a newline sequence. If called without arguments, writes a newline. See Line IO.

Note that each added newline is the character "\n"<//tt>, not the output record separator (<tt>$\).

Treatment for each object:

• String: writes the string.

• Neither string nor array: writes object.to_s.

• Array: writes each element of the array; arrays may be nested.

To keep these examples brief, we define this helper method:

def show(*objects)
  # Puts objects to file.
  f = File.new('t.tmp', 'w+')
  f.puts(objects)
  # Return file content.
  f.rewind
  p f.read
  f.close
end

# Strings without newlines.
show('foo', 'bar', 'baz')     # => "foo\nbar\nbaz\n"
# Strings, some with newlines.
show("foo\n", 'bar', "baz\n") # => "foo\nbar\nbaz\n"

# Neither strings nor arrays:
show(0, 0.0, Rational(0, 1), Complex(9, 0), :zero)
# => "0\n0.0\n0/1\n9+0i\nzero\n"

# Array of strings.
show(['foo', "bar\n", 'baz']) # => "foo\nbar\nbaz\n"
# Nested arrays.
show([[[0, 1], 2, 3], 4, 5])  # => "0\n1\n2\n3\n4\n5\n"

Behaves like #write, except that it:

• Writes at the given offset (in bytes).

• Disregards, and does not modify, the stream’s position (see Position).

• Bypasses any user space buffering in the stream.

Because this method does not disturb the stream’s state (its position, in particular), pwrite allows multiple threads and processes to use the same IO object for writing at various offsets.

f = File.open('t.tmp', 'w+')
# Write 6 bytes at offset 3.
f.pwrite('ABCDEF', 3) # => 6
f.rewind
f.read # => "\u0000\u0000\u0000ABCDEF"
f.close

Not available on some platforms.

Reads bytes from the stream; the stream must be opened for reading (see Access Modes):

• If maxlen is nil, reads all bytes using the stream’s data mode.

• Otherwise reads up to maxlen bytes in binary mode.

Returns a string (either a new string or the given out_string) containing the bytes read. The encoding of the string depends on both maxLen and out_string:

• maxlen is nil: uses internal encoding of self (regardless of whether out_string was given).

• maxlen not nil:

  • out_string given: encoding of out_string not modified.

  • out_string not given: ASCII-8BIT is used.