Unix/Linux文件类型及访问权限

在Linux系统中，有7种文件类型。

1. 普通文件 (regular file)
2. 目录文件 (directory)
3. 链接文件 (symbolic link)
4. 管道文件 (FIFO)
5. 套接字文件 (socket)
6. 字符设备文件 (character device)
7. 块设备文件    (block device)

在Solaris上，还有一种文件类型， 叫做door文件。

而一个文件的Unix访问权限，包括12位，通常用4个8进制位表示，

位标志	八进制值	含义
S_ISUID	04000	set user ID on execution  [1]
S_ISGID	02000	set group ID on execution [2]
S_ISVTX	01000	sticky bit                [3]
S_IRUSR	00400	owner has read permission
S_IWUSR	00200	owner has write permission
S_IXUSR	00100	owner has execute permission
S_IRGRP	00040	group has read permission
S_IWGRP	00020	group has write permission
S_IXGRP	00010	group has execute permission
S_IROTH	00004	others have read permission
S_IWOTH	00002	others have write permission
S_IXOTH	00001	others have execute permission

通常我们使用"chmod 777 <file>", 本质上是"chmod 0777 <file>"。

SUID, SGID, SVTX不常见，这里重点解释一下。

[1] SUID bit: Set User ID 位

When you execute a program that has the SUID bit enabled, you inherit the
permissions of that program's owner. Programs that do not have the SUID bit
set are run with the permissions of the user who started the program.

[2] SGID bit: Set Group ID位

The set-group-ID bit (S_ISGID) has several special uses.  For a directory it
indicates that BSD semantics is to be used for that directory: files created
there inherit their group ID from the directory, not from the effective group ID
of the creating process, and directories created there will also get the S_ISGID
bit set. For a file that does not have the group execution bit (S_IXGRP) set,
the set-group-ID bit indicates mandatory file/record locking.

[3] SVTX bit: Sticky(粘滞)位

// From: https://en.wikipedia.org/wiki/Sticky_bit
When a directory's sticky bit is set, the filesystem treats the files in such
directories in a special way so only the file's owner, the directory's owner,
or root user can rename or delete the file. Without the sticky bit set, any user
with write and execute permissions for the directory can rename or delete
contained files, regardless of the file's owner. Typically this is set on the
/tmp directory to prevent ordinary users from deleting or moving other users' files.

e.g. （目录/tmp的sticky位已经设置，虽然文件/tmp/dorax的权限是0777, 但是它不能被用户veli删除，因为其ower是用户fanw。）

 $ id -un
 veli
 $ stat /tmp | egrep 'Access:.*Uid'
 Access: (/drwxrwxrwt)  Uid: (    /    root)   Gid: (    /    root)
 $ ls -l /tmp/dorax
 -rwxrwxrwx  fanw fanw  Feb  : /tmp/dorax
 $ rm -f /tmp/dorax
 rm: cannot remove ‘/tmp/dorax’: Operation not permitted

扩展阅读：

1. Unix File Permissions

2. Unix - File Permission / Access Modes

最后，给出一个文件类型及权限位识别的C代码实现。

 /*
  * foo.c - get Unix/Linux file type and its access,
  *         mostly simliar to what GNU stat does
  */

 #include <stdio.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <errno.h>

 int
 main(int argc, char *argv[])
 {
     char        *filepath = NULL;
     char        *filetype = NULL;
     char        s[] = "----------";
     int        rc;
     struct stat    sb;

     if (argc != ) {
         (void) fprintf(stderr, "Usage %s <file>\n", argv[]);
         return (-);
     }

     filepath = argv[];

     (void) memset(&sb, , sizeof (struct stat));

     errno = ;
     /* NOTE: don't use stat() as we will get symbolic link file type */
     if ((rc = lstat(filepath, &sb)) != ) {
         (void) fprintf(stderr, "%s: cannot stat `%s': %s\n",
             argv[], filepath, strerror(errno));
         return (-);
     }

     /* get file type */
     if (S_ISREG(sb.st_mode)) {
         s[] = '-';
         filetype = "regular file";
     }
     if (S_ISDIR(sb.st_mode)) {
         s[] = 'd';
         filetype = "directory";
     }
     if (S_ISCHR(sb.st_mode)) {
         s[] = 'c';
         filetype = "character special file";
     }
     if (S_ISBLK(sb.st_mode)) {
         s[] = 'b';
         filetype = "block special file";
     }
     if (S_ISFIFO(sb.st_mode)) {
         s[] = 'p';
         filetype = "fifo";
     }
     if (S_ISLNK(sb.st_mode)) {
         s[] = 'l';
         filetype = "symbolic link";
     }
     if (S_ISSOCK(sb.st_mode)) {
         s[] = 's';
         filetype = "socket";
     }
 #ifdef __sunos
     if (S_ISDOOR(sb.st_mode)) {
         s[] = 'D';
         filetype = "door";
     }
 #endif

     /* parse set UID, set-group-ID, sticky bit */
     if (sb.st_mode & S_ISUID)
         s[] = 'S';
     if (sb.st_mode & S_ISGID)
         s[] = 'S';
     if (sb.st_mode & S_ISVTX)
         s[] = 'T';

     /* parse owner rwx bit */
     if (sb.st_mode & S_IRUSR)
         s[] ='r';
     if (sb.st_mode & S_IWUSR)
         s[] ='w';
     if (sb.st_mode & S_IXUSR)
         s[] = (s[] == 'S') ? 's' : 'x';

     /* parse group rwx bit */
     if (sb.st_mode & S_IRGRP)
         s[] = 'r';
     if (sb.st_mode & S_IWGRP)
         s[] = 'w';
     if (sb.st_mode & S_IXGRP)
         s[] = (s[] == 'S') ? 's' : 'x';

     /* parse others rwx bit */
     if (sb.st_mode & S_IROTH)
         s[] = 'r';
     if (sb.st_mode & S_IWOTH)
         s[] = 'w';
     if (sb.st_mode & S_IXOTH)
         s[] = (s[] == 'T') ? 't' : 'x';

     (void) printf("  File: %s\n", filepath);
     (void) printf("Access: (%04o/%s)\n", (int)(sb.st_mode & ~S_IFMT), s);
     (void) printf("  Type: %s\n", filetype);

     return ();
 }

o 在Linux上编译并测试

$ gcc -g -Wall -m32 -o foo foo.c

$ ./foo /usr/bin/passwd
  File: /usr/bin/passwd
Access: (/-rwsr-xr-x)
  Type: regular file

$ ./foo /var/tmp
  File: /var/tmp
Access: (/drwxrwxrwt)
  Type: directory

$ ./foo /bin/sh
  File: /bin/sh
Access: (/lrwxrwxrwx)
  Type: symbolic link

$ mkfifo /tmp/fifo
$ ./foo /tmp/fifo
  File: /tmp/fifo
Access: (/prw-rw-r--)
  Type: fifo

$ ./foo /dev/null
  File: /dev/null
Access: (/crw-rw-rw-)
  Type: character special file

$ ./foo /dev/sda
  File: /dev/sda
Access: (/brw-rw----)
  Type: block special file

$ ./foo /var/tmp/.sshmux
  File: /var/tmp/.sshmux
Access: (/srw-------)
  Type: socket

o 在Solaris上编译并测试

$ gcc -D__sunos -g -Wall -m64 -o foo foo.c

$ ./foo /usr/bin/passwd
  File: /usr/bin/passwd
Access: (/-r-sr-sr-x)
  Type: regular file

$ ./foo /var/tmp
  File: /var/tmp
Access: (/drwxrwxrwt)
  Type: directory

$ ./foo /dev/null
  File: /dev/null
Access: (/lrwxrwxrwx)
  Type: symbolic link

$ mkfifo /tmp/fifo && ./foo /tmp/fifo
  File: /tmp/fifo
Access: (/prw-r--r--)
  Type: fifo

$ ./foo /var/tmp/.sshmux
  File: /var/tmp/.sshmux
Access: (/srw-------)
  Type: socket

$ ./foo /devices/pseudo/mm@:null
  File: /devices/pseudo/mm@:null
Access: (/crw-rw-rw-)
  Type: character special file

$ ./foo /devices/pci@,/pci8086,@/pci1000,@/sd@,:wd
  File: /devices/pci@,/pci8086,@/pci1000,@/sd@,:wd
Access: (/brw-r-----)
  Type: block special file

$ ./foo /var/run/zonestat_door
  File: /var/run/zonestat_door
Access: (/Drw-r--r--)
  Type: door