Programming Ruby

The Pragmatic Programmer's Guide
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class IO
Parent: Object
Version: 1.6

Index:

foreach new pipe popen readlines select << binmode clone close close_read close_write closed? each each_byte each_line eof eof? fcntl fileno flush getc gets ioctl isatty lineno lineno= pid pos pos= print printf putc puts read readchar readline readlines reopen rewind seek stat sync sync= sysread syswrite tell to_i to_io tty? ungetc write


Subclasses: File

Class IO is the basis for all input and output in Ruby. An I/O stream may be duplexed (that is, bidirectional), and so may use more than one native operating system stream.

Many of the examples in this section use class File, the only standard subclass of IO. The two classes are closely associated.

As used in this section, aPortname may take any of the following forms.

Ruby will convert pathnames between different operating system conventions if possible. For instance, on a Windows system the filename ``/gumby/ruby/test.rb'' will be opened as ``\gumby\ruby\test.rb''. When specifying a Windows-style filename in a Ruby string, remember to escape the backslashes: 

"c:\\gumby\\ruby\\test.rb"

Our examples here will use the Unix-style forward slashes; File::SEPARATOR can be used to get the platform-specific separator character.

I/O ports may be opened in any one of several different modes, which are shown in this section as aModeString. This mode string must be one of the values listed in Table 22.5 on page 326.

Mode strings
Mode Meaning
``r'' Read-only, starts at beginning of file (default mode).
``r+'' Read-write, starts at beginning of file.
``w'' Write-only, truncates existing file to zero length or creates a new file for writing.
``w+'' Read-write, truncates existing file to zero length or creates a new file for reading and writing.
``a'' Write-only, starts at end of file if file exists, otherwise creates a new file for writing.
``a+'' Read-write, starts at end of file if file exists, otherwise creates a new file for reading and writing.
``b'' (DOS/Windows only) Binary file mode (may appear with any of the key letters listed above).

mixins
Enumerable: collect, detect, each_with_index, entries, find, find_all, grep, include?, map, max, member?, min, reject, select, sort, to_a

class methods
foreach IO.foreach( aPortName, aSepString=$/ ) {| line | block } -> nil

Executes the block for every line in the named I/O port, where lines are separated by aSepString. 

IO.foreach("testfile") {|x| print "GOT ", x }
produces: 
GOT This is line one
GOT This is line two
GOT This is line three
GOT And so on...

new IO.new( anInteger, aModeString ) -> aFile

Returns a new File object (a stream) for the given integer file descriptor and mode string. See also IO#fileno . 

a = IO.new(2,"w")      # '2' is standard error
$stderr.puts "Hello"
a.puts "World"
produces: 
Hello
World

pipe IO.pipe -> anArray

Creates a pair of pipe endpoints (connected to each other) and returns them as a two-element array of IO objects: [ readFile, writeFile ]. 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>

popen IO.popen( aCmdString, aModeString="r" ) -> anIO
IO.popen( aCmdString, aModeString="r" ) {| anIO | block } -> nil

Runs the specified command string as a subprocess; the subprocess's standard input and output will be connected to the returned IO object. If aCmdString starts with a ``-'', then a new instance of Ruby is started as the subprocess. The default mode for the new file object is ``r'', but aModeString may be set to any of the modes in Table 22.5 on page 326.

If a block is given, Ruby will run the command as a child connected to Ruby with a pipe. Ruby's end of the pipe will be passed as a parameter to the block.

If a block is given with a aCmdString of ``-'', the block will be run in two separate processes: once in the parent, and once in a child. The parent process will be passed the pipe object as a parameter to the block, the child version of the block will be passed nil, and the child's standard in and standard out will be connected to the parent through the pipe. Not available on all platforms. 

f = IO.popen("uname")
p f.readlines
puts "Parent is #{Process.pid}"
IO.popen ("date") { |f| puts f.gets }
IO.popen("-") {|f| $stderr.puts "#{Process.pid} is here, f is #{f}"}
produces: 
["Linux\n"]
Parent is 561
Sun Jun  9 00:17:46 CDT 2002
564 is here, f is
561 is here, f is #<IO:0x401b54dc>

readlines IO.readlines( aPortName, aSepString=$/ ) -> anArray

Reads the entire file specified by aPortName as individual lines, and returns those lines in an array. Lines are separated by aSepString.

a = IO.readlines("testfile")
a[0] » "This is line one\n"

select IO.select( readArray [, writeArray [ errorArray [ timeout ] ] ] ) -> anArray
or nil

See Kernel#select on page 422.

instance methods
<< ios << anObject -> ios

String Output---Writes anObject to ios. anObject will be converted to a string using to_s. 

$stdout << "Hello " << "world!\n"
produces: 
Hello world!

binmode ios.binmode -> ios

Puts ios into binary mode. This is useful only in MS-DOS/Windows environments. Once a stream is in binary mode, it cannot be reset to nonbinary mode.

clone ios.clone -> anIO

Creates a new I/O stream, copying all the attributes of ios. The file position is shared as well, so reading from the clone will alter the file position of the original, and vice-versa.

close ios.close -> nil

Closes ios and flushes any pending writes to the operating system. The stream is unavailable for any further data operations; an IOError is raised if such an attempt is made. I/O streams are automatically closed when they are claimed by the garbage collector.

close_read ios.close_read -> nil

Closes the read end of a duplex I/O stream (i.e., one that contains both a read and a write stream, such as a pipe). Will raise an IOError if the stream is not duplexed. 

f = IO.popen("/bin/sh","r+")
f.close_read
f.readlines
produces: 
prog.rb:3:in `readlines': not opened for reading (IOError)
	from prog.rb:3

close_write ios.close_write -> nil

Closes the write end of a duplex I/O stream (i.e., one that contains both a read and a write stream, such as a pipe). Will raise an IOError if the stream is not duplexed. 

f = IO.popen("/bin/sh","r+")
f.close_write
f.print "nowhere"
produces: 
prog.rb:3:in `write': not opened for writing (IOError)
	from prog.rb:3:in `print'
	from prog.rb:3

closed? ios.closed? -> true or false

Returns true if ios is completely closed (for duplex streams, both reader and writer), false otherwise.

f = File.new("testfile")
f.close » nil
f.closed? » true
f = IO.popen("/bin/sh","r+")
f.close_write » nil
f.closed? » false
f.close_read » nil
f.closed? » true

each ios.each( aSepString=$/ ) {| line | block } -> ios

Executes the block for every line in ios, where lines are separated by aSepString. ios must be opened for reading or an IOerror will be raised. 

f = File.new("testfile")
f.each {|line| puts "#{f.lineno}: #{line}" }
produces: 
1: This is line one
2: This is line two
3: This is line three
4: And so on...

each_byte ios.each_byte {| byte | block }

-> nil

Calls the given block once for each byte (0..255) in ios, passing the byte as an argument. The stream must be opened for reading or an IOerror will be raised.

f = File.new("testfile")
checksum = 0
f.each_byte {|x| checksum ^= x } » #<File:0x401b5cac>
checksum » 12

each_line ios.each_line( aSepString=$/ ) {| line | block }

-> ios

Synonym for IO#each .

eof ios.eof -> true or false

Returns true if ios is at end of file. The stream must be opened for reading or an IOError will be raised.

f = File.new("testfile")
dummy = f.readlines
f.eof » true

eof? ios.eof? -> true or false

Synonym for IO#eof .

fcntl ios.fcntl( anIntegerCmd, anArg ) -> anInteger

Provides a mechanism for issuing low-level commands to control or query file-oriented I/O streams. Arguments and results are platform dependent. If anArg is a number, its value is passed directly. If it is a string, it is interpreted as a binary sequence of bytes. On Unix platforms, see fcntl(2) for details. Not implemented on all platforms.

fileno ios.fileno -> aFixnum

Returns an integer representing the numeric file descriptor for ios.

$stdin.fileno » 0
$stdout.fileno » 1

flush ios.flush -> ios

Flushes any buffered data within ios to the underlying operating system (note that this is Ruby internal buffering only; the OS may buffer the data as well). 

$stdout.print "no newline"
$stdout.flush
produces: 
no newline

getc ios.getc -> aFixnum or nil

Gets the next 8-bit byte (0..255) from ios. Returns nil if called at end of file.

f = File.new("testfile")
f.getc » 84
f.getc » 104

gets ios.gets( aSepString=$/ ) -> aString or nil

Reads the next ``line'' from the I/O stream; lines are separated by aSepString. A separator of nil reads the entire contents, and a zero-length separator reads the input a paragraph at a time (two successive newlines in the input separate paragraphs). The stream must be opened for reading or an IOerror will be raised. The line read in will be returned and also assigned to $_. Returns nil if called at end of file.

File.new("testfile").gets » "This is line one\n"
$_ » "This is line one\n"

ioctl ios.ioctl( anIntegerCmd, anArg ) -> anInteger

Provides a mechanism for issuing low-level commands to control or query I/O devices. Arguments and results are platform dependent. If anArg is a number, its value is passed directly. If it is a string, it is interpreted as a binary sequence of bytes. On Unix platforms, see ioctl(2) for details. Not implemented on all platforms.

isatty ios.isatty -> true or false
Returns true if ios is associated with a terminal device (tty), false otherwise.

File.new("testfile").isatty » false
File.new("/dev/tty").isatty » true

lineno ios.lineno -> anInteger

Returns the current line number in ios. The stream must be opened for reading. lineno counts the number of times gets is called, rather than the number of newlines encountered. The two values will differ if gets is called with a separator other than newline. See also the $. variable.

f = File.new("testfile")
f.lineno » 0
f.gets » "This is line one\n"
f.lineno » 1
f.gets » "This is line two\n"
f.lineno » 2

lineno= ios.lineno = anInteger -> anInteger

Manually sets the current line number to the given value. $. is updated only on the next read.

f = File.new("testfile")
f.gets » "This is line one\n"
$. » 1
f.lineno = 1000
f.lineno » 1000
$. # lineno of last read » 1
f.gets » "This is line two\n"
$. # lineno of last read » 1001

pid ios.pid -> aFixnum

Returns the process ID of a child process associated with ios. This will be set 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
produces: 
In parent, child pid is 600In child, pid is 600

pos ios.pos -> anInteger

Returns the current offset (in bytes) of ios.

f = File.new("testfile")
f.pos » 0
f.gets » "This is line one\n"
f.pos » 17

pos= ios.pos = anInteger -> 0

Seeks to the given position (in bytes) in ios.

f = File.new("testfile")
f.pos = 17
f.gets » "This is line two\n"

print ios.print( [ anObject=$_ ]* ) -> nil

Writes the given object(s) to ios. The stream must be opened for writing. If the output record separator ($\) is not nil, it will be appended to the output. If no arguments are given, prints $_. Objects that aren't strings will be converted by calling their to_s method. Returns nil. 

$stdout.print("This is ", 100, " percent.\n")
produces: 
This is 100 percent.

printf ios.printf( aFormatString [, anObject ]* ) -> nil

Formats and writes to ios, converting parameters under control of the format string. See Kernel#sprintf on page 423 for details.

putc ios.putc( anObject ) -> anObject

Writes the given character (taken from a String or a Fixnum) on ios. 

$stdout.putc "A"
$stdout.putc 65
produces: 
AA

puts ios.puts( [ anObject ]* ) -> nil

Writes the given objects to ios as with IO#print . Writes a record separator (typically a newline) after any that do not already end with a newline sequence. If called with an array argument, writes each element on a new line. If called without arguments, outputs a single record separator. 

$stdout.puts("this", "is", "a", "test")
produces: 
this
is
a
test

read ios.read( [ anInteger ] ) -> aString or nil

Reads at most anInteger bytes from the I/O stream, or to the end of file if anInteger is omitted. Returns nil if called at end of file.

f = File.new("testfile")
f.read(16) » "This is line one"

readchar ios.readchar -> aFixnum

Reads a character as with IO#getc , but raises an EOFError on end of file.

readline ios.readline( aSepString=$/ ) -> aString

Reads a line as with IO#gets , but raises an EOFError on end of file.

readlines ios.readlines( aSepString=$/ ) -> anArray

Reads all of the lines in ios, and returns them in anArray. Lines are separated by the optional aSepString. The stream must be opened for reading or an IOerror will be raised.

f = File.new("testfile")
f.readlines[0] » "This is line one\n"

reopen ios.reopen( anOtherIO ) -> ios
ios.reopen( aPath, aModeStr ) -> ios

Reassociates ios with the I/O stream given in anOtherIO or to a new stream opened on aPath. This may dynamically change the actual class of this stream.

f1 = File.new("testfile")
f2 = File.new("testfile")
f2.readlines[0] » "This is line one\n"
f2.reopen(f1) » #<File:0x401b5a54>
f2.readlines[0] » "This is line one\n"

rewind ios.rewind -> 0

Positions ios to the beginning of input, resetting lineno to zero.

f = File.new("testfile")
f.readline » "This is line one\n"
f.rewind » 0
f.lineno » 0
f.readline » "This is line one\n"

seek ios.seek( anInteger, whence=SEEK_SET ) -> 0

Seeks to a given offset anInteger in the stream according to the value of whence:

IO::SEEK_CUR Seeks to anInteger plus current position.
IO::SEEK_END Seeks to anInteger plus end of stream (you probably want a negative value for anInteger).
IO::SEEK_SET Seeks to the absolute location given by anInteger.

f = File.new("testfile")
f.seek(-13, IO::SEEK_END) » 0
f.readline » "And so on...\n"

stat ios.stat -> aStat

Returns status information for ios as an object of type File::Stat.

f = File.new("testfile")
s = f.stat
"%o" % s.mode » "100644"
s.blksize » 4096
s.atime » Sun Jun 09 00:17:48 CDT 2002

sync ios.sync -> true or false

Returns the current ``sync mode'' of ios. When sync mode is true, all output is immediately flushed to the underlying operating system and is not buffered by Ruby internally.

f = File.new("testfile")
f.sync » false

sync= ios.sync = aBoolean -> aBoolean

Sets the ``sync mode'' to true or false. When sync mode is true, all output is immediately flushed to the underlying operating system and is not buffered internally. Returns the new state. 

f = File.new("testfile")
f.sync = true

sysread ios.sysread( anInteger ) -> aString
Reads anInteger bytes from ios using a low-level read and returns them as a string. Do not mix with other methods that read from ios or you may get unpredictable results. Raises SystemCallError on error and EOFError at end of file.

f = File.new("testfile")
f.sysread(16) » "This is line one"

syswrite ios.syswrite( aString ) -> anInteger

Writes the given string to ios using a low-level write. Returns the number of bytes written. Do not mix with other methods that write to ios or you may get unpredictable results. Raises SystemCallError on error.

f = File.new("out", "w")
f.syswrite("ABCDEF") » 6

tell ios.tell -> anInteger

Synonym for IO#pos .

to_i ios.to_i -> anInteger

Synonym for IO#fileno .

to_io ios.to_io -> ios

Returns ios.

tty? ios.tty? -> true or false

Synonym for IO#isatty .

ungetc ios.ungetc( anInteger ) -> nil

Pushes back one character onto ios, such that a subsequent buffered read will return it. Only one character may be pushed back before a subsequent read operation (that is, you will be able to read only the last of several characters that have been pushed back). Has no effect with unbuffered reads (such as IO#sysread ).

f = File.new("testfile") » #<File:0x401b5b80>
c = f.getc » 84
f.ungetc(c) » nil
f.getc » 84

write ios.write( aString ) -> anInteger

Writes the given string to ios. The stream must be opened for writing. If the argument is not a string, it will be converted to a string using to_s. Returns the number of bytes written. 

count = $stdout.write( "This is a test\n" )
puts "That was #{count} bytes of data"
produces: 
This is a test
That was 15 bytes of data

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Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide"
Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)).

Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.

Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.