Method: IO.read

Defined in:

io.c

.read(path, length = nil, offset = 0, **opts) ⇒ String^?

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 that is the path to a file.

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 IO.binread if all bytes in the file should be read.

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:

• (String, nil)

# File 'io.c', line 12240

static VALUE
rb_io_s_read(int argc, VALUE *argv, VALUE io)
{
    VALUE opt, offset;
    long off;
    struct foreach_arg arg;

    argc = rb_scan_args(argc, argv, "13:", NULL, NULL, &offset, NULL, &opt);
    if (!NIL_P(offset) && (off = NUM2LONG(offset)) < 0) {
        rb_raise(rb_eArgError, "negative offset %ld given", off);
    }
    open_key_args(io, argc, argv, opt, &arg);
    if (NIL_P(arg.io)) return Qnil;
    if (!NIL_P(offset)) {
        struct seek_arg sarg;
        int state = 0;
        sarg.io = arg.io;
        sarg.offset = offset;
        sarg.mode = SEEK_SET;
        rb_protect(seek_before_access, (VALUE)&sarg, &state);
        if (state) {
            rb_io_close(arg.io);
            rb_jump_tag(state);
        }
        if (arg.argc == 2) arg.argc = 1;
    }
    return rb_ensure(io_s_read, (VALUE)&arg, rb_io_close, arg.io);
}