1. Chien-Chung Shen CIS, UD cshen@udel.edu
File Permissions
Chien-Chung Shen
CIS, UD

2. File Access Permissions
Stevens+Rago Chapter 4.2
Given pathname, stat() returns a structure of information about the named file
struct stat {
dev_t st_dev; /* ID of device containing file */
ino_t st_ino; /* inode number */
mode_t st_mode; /* protection */
nlink_t st_nlink; /* number of hard links */
uid_t st_uid; /* user ID of owner */
gid_t st_gid; /* group ID of owner */
dev_t st_rdev; /* device ID (if special file) */
off_t st_size; /* total size, in bytes */
blksize_t st_blksize; /* blocksize for file system I/O */
blkcnt_t st_blocks; /* number of 512B blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last status change */ };
$ stat <file name>

3. File Access Permissions
Octal values – rwx
User (owner), group, other (world)
rwxrwxrwx
400
200
100

4. Create New Files
the same
mode specifies the mode (permissions) to use in case a new file is created
the effective mode is modified by process’s umask, i.e., [mode & ~umask]

5. Experiments
umask touch a ls -al a // why rw-rw-r-- ? chmod 0666 a // force o+w ls -al a cp a b ls -al a b // why b does not have // the same permission?

6. File Mode Creation Mask
Most Unix users never deal with their umask value
it is usually set once, on login, by the shell’s start-up file, and never changed
When writing programs that create new files, to ensure that specific access permission bits are enabled, need to modify umask value while the process is running
All the shells have a built-in umask command to set or print the current file mode creation mask
umask –S
Changing the file mode creation mask of a process (a child) doesn’t affect the mask of its parent (often a shell)
Stevens+Rago Chapter 4.8

7. umask Permission Bits Octal values – rwx
User (owner), group, other (world)
rwxrwxrwx
Permissions can be denied by setting the corresponding bits
002 ( ): prevent others from writing your files
022 ( ): prevent group members and others from writing your files
027 ( ): prevent group members from writing your files and others from reading, writing, or executing your files