head和tail
他们看起来跟cat好像是近亲，做的事情差不多，都是读取文件内容，head从文件头读起，tai则从文件尾读取，默认读取10行并打印，这里两个命令都只实现了默认打印10行和指定打印行数参数-n。
实现方法
这里除了使用常用的文件操作open、close、read接口外，在tail实现里还是用了lseek，在线手册说明如下：
NAME
lseek - reposition read/write file offset
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
DESCRIPTION
The lseek() function repositions the offset of the open file associated with the
file descriptor fd to the argument offset according to the directive whence as
follows:
SEEK_SET
The offset is set to offset bytes.
SEEK_CUR
The offset is set to its current location plus offset bytes.
SEEK_END
The offset is set to the size of the file plus offset bytes.
The lseek() function allows the file offset to be set beyond the end of the file
(but this does not change the size of the file). If data is later written at
this point, subsequent reads of the data in the gap (a "hole") return null bytes
('\0') until data is actually written into the gap.
RETURN VALUE
Upon successful completion, lseek() returns the resulting offset location as
measured in bytes from the beginning of the file. Otherwise, a value of
(off_t) -1 is returned and errno is set to indicate the error.
ERRORS
EBADF fd is not an open file descriptor.
EINVAL whence is not one of SEEK_SET, SEEK_CUR, SEEK_END; or the resulting file
offset would be negative, or beyond the end of a seekable device.
EOVERFLOW
The resulting file offset cannot be represented in an off_t.
ESPIPE fd is associated with a pipe, socket, or FIFO.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
This document's use of whence is incorrect English, but maintained for histori‐
cal reasons.
Some devices are incapable of seeking and POSIX does not specify which devices
must support lseek().
On Linux, using lseek() on a tty device returns ESPIPE.
When converting old code, substitute values for whence with the following
macros:
old new
0 SEEK_SET
1 SEEK_CUR
2 SEEK_END
L_SET SEEK_SET
L_INCR SEEK_CUR
L_XTND SEEK_END
SVr1-3 returns long instead of off_t, BSD returns int.
Note that file descriptors created by dup(2) or fork(2) share the current file
position pointer, so seeking on such files may be subject to race conditions.
SEE ALSO
dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A description
of the project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2001-09-24 LSEEK(2)