硬链接：硬链接与目录项是同义词，而且建立硬链接有限制，不能给分区建立，不能给目录建立。
如下 bigfile 文件 Inode: 12849444 Links: 1。
ln bigfile bigfile_link 硬链接之后
bigfile ： Inode: 12849444 Links: 2
bigfile_link ： Inode: 12849444 Links: 2
Links加一，所以硬链接就是 目录项的同义词，所以用ln 给一个文件做了硬链接，本质就是在当前目录下的目录文件当中多写了一行目录项，该目录项 用 bigfile_link 文件名 关联 Inode: 12849444 。即两个文件名对应同一个 inode。
mhr@ubuntu:~/work/linux/wenjianxitong/20$ stat bigfile
File: 'bigfile'
Size: 5368709120 Blocks: 8 IO Block: 4096 regular file
Device: 801h/2049d Inode: 12849444 Links: 1
Access: (0600/-rw-------) Uid: ( 1000/ mhr) Gid: ( 1000/ mhr)
Access: 2020-05-04 09:07:52.395872798 -0700
Modify: 2020-05-04 09:04:27.883813275 -0700
Change: 2020-05-04 09:04:27.883813275 -0700
Birth: -
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$ ln bigfile bigfile_link
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$ stat bigfile
File: 'bigfile'
Size: 5368709120 Blocks: 8 IO Block: 4096 regular file
Device: 801h/2049d Inode: 12849444 Links: 2
Access: (0600/-rw-------) Uid: ( 1000/ mhr) Gid: ( 1000/ mhr)
Access: 2020-05-04 09:07:52.395872798 -0700
Modify: 2020-05-04 09:04:27.883813275 -0700
Change: 2020-05-05 01:46:59.918698615 -0700
Birth: -
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$ stat bigfile_link
File: 'bigfile_link'
Size: 5368709120 Blocks: 8 IO Block: 4096 regular file
Device: 801h/2049d Inode: 12849444 Links: 2
Access: (0600/-rw-------) Uid: ( 1000/ mhr) Gid: ( 1000/ mhr)
Access: 2020-05-04 09:07:52.395872798 -0700
Modify: 2020-05-04 09:04:27.883813275 -0700
Change: 2020-05-05 01:46:59.918698615 -0700
Birth: -
mhr@ubuntu:~/work/linux/wenjianxitong/20$
符号链接：符号链接的优点是可以跨分区，可以给目录建立。
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$ ln -s bigfile_link bigfile_s
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$ stat bigfile_link
File: 'bigfile_link'
Size: 5368709120 Blocks: 8 IO Block: 4096 regular file
Device: 801h/2049d Inode: 12849444 Links: 2
Access: (0600/-rw-------) Uid: ( 1000/ mhr) Gid: ( 1000/ mhr)
Access: 2020-05-04 09:07:52.395872798 -0700
Modify: 2020-05-04 09:04:27.883813275 -0700
Change: 2020-05-05 01:46:59.918698615 -0700
Birth: -
mhr@ubuntu:~/work/linux/wenjianxitong/20$
mhr@ubuntu:~/work/linux/wenjianxitong/20$ stat bigfile_s
File: 'bigfile_s' -> 'bigfile_link'
Size: 12 Blocks: 0 IO Block: 4096 symbolic link
Device: 801h/2049d Inode: 12847417 Links: 1
Access: (0777/lrwxrwxrwx) Uid: ( 1000/ mhr) Gid: ( 1000/ mhr)
Access: 2020-05-05 01:58:18.333394043 -0700
Modify: 2020-05-05 01:58:18.201401500 -0700
Change: 2020-05-05 01:58:18.201401500 -0700
Birth: -
mhr@ubuntu:~/work/linux/wenjianxitong/20$
符号链接文件的 Size: 12 就是 ‘bigfile_link’ 名的大小，所占用磁盘空间是0
如果删除符号链接文件指向的文件，源文件失效，再使用该符号链接文件，会出错。
link
命令 ln 是由系统调用 link()封装来的
NAME
link, linkat - make a new name for a file
SYNOPSIS
#include <unistd.h>
int link(const char *oldpath, const char *newpath);
utime：
NAME
utime, utimes - change file last access and modification times
修改文件最后读和最后修改时间
SYNOPSIS
#include <sys/types.h>
#include <utime.h>
int utime(const char *filename, const struct utimbuf *times);
目录的创建和销毁
NAME
mkdir, mkdirat - create a directory
SYNOPSIS
#include <sys/stat.h>
#include <sys/types.h>
int mkdir(const char *pathname, mode_t mode);
NAME
rmdir - delete a directory
SYNOPSIS
#include <unistd.h>
int rmdir(const char *pathname);
DESCRIPTION
rmdir() deletes a directory, which must be empty. 只能删除一个空目录
更改当前工作路径：cd
NAME
chdir, fchdir - change working directory
SYNOPSIS
#include <unistd.h>
int chdir(const char *path);
int fchdir(int fd);
获取当前工作路径 ：pwd
NAME
getcwd, getwd, get_current_dir_name - get current working directory
获取当前工作路径
SYNOPSIS
#include <unistd.h>
char *getcwd(char *buf, size_t size);