System Interface Interface that provides services from the OS (Higher than BIOS) Memory Scheduler File/Storage System Inter-process Communication and Network, etc program libc Operating System BIOS

File System A popular but also complex subsystem File descriptors (pseudo files, like stdin and stdout) Low level IO File Management Examples / usrbinhomefile file pointer EOF Tree-like Structure The top level is called root and is represented by / The representation of directory is represented as /aa/bb/cc A file in /aa/bb/cc can be represented as /aa/bb/cc/file

Several Important Directories / : root file system /bin Contains executable files /etc Configuration files Some executable shell files

Cont. /usr Contains many sub-directories /usr/inlcude and /usr/lib /usr/home- the home directories of users /usr/bin/ - contains more executables /usr/local- softwares from the third-party /sbin Contains executables which can only be executed by the super user (root)

Cont. /home Like /usr/home /dev or /devices Contains device files /var Contains files the content of which will be changed frequently. For example : mails, logs and so on.

Cont. /proc A special file system which contains runtime information of the system /sys A special file system which contains the information of the system and its kernel /mnt Can be used to mount other file sytems

The Home directory When you log onto the system, you will be assigned a home directory; Working directory – The directory which you are currently in; pwd- used to list your current working directory

Some Commands mkdir -- create a directory ls ---- list the files in a directory df ---- display the number of the blocks of a file system mount – mount a file system to a directory

A little about File Attributes File attributes (srwxrwxrwx) r:read, w:write and e:execute Each kind of right can be presented by 0 or 1 The combination of 3 kinds of rights is represented by a octet number (0 - 7), rwx, desribes the permission to a set of users Three sets of different users: owner, group and others Some unix commands: umask, chmod, etc to change the attributes

Cont. For example: gub]$ ls -al sommer.pdf -rw-r--r-- 1 gub cisgrad Aug 20 15:09 sommer.pdf The access right of the file is rw -> 110 ->6 r->100->4 The access right is 644 Change the access right of a file: chmod xxx filename

Cont. The following is required access right for a file (test1.txt): 1. Owner: rwx 2. Group: r— 3. Others: --- How to use chmod to change the access right of test1.txt?

The File Creating Mask Umask uu The required access for files is: Default access right – uu For example: Default : 555; umask 22 What is the required access right?

Other commands related to the access right of a file chown: change the owner of a file chgrp: change the group of a file chmod p + access right P can be u (user), g (group) ; o(other); a(all) Access right can be r, w x.

File Accesses Read and Write getc() and putc(); getchar() and putchar() int read(int fd, char *buf, int n); int write(int fd, char *buf, int n) #include int main () { char buf[1024]; int n; while ((n = read(0, buf, 1024)) > 0) write ( 1, buf, n); return 0; } #include int getchar () { char c; return (read(0, &c, 1) == 1 ) ? c : EOF; }

File Accesses -- continued Open, creat, close and unlink int open(char *name, int flags, int perms); Persm can be O_RONLY, O_RDWR, O_WRONLY and others int creat(char *name, int perms); /* within open, O_CREAT */ int close(int fd); int unlink(char *path); #include /* A copy program*/ int main () { char *f1 =“file1”, *f2=“file2”; int fd1, fd2, n; if (fd1 = open(f1, O_RDONLY, 0) == -1) usage(“uanble to opening source file %s\n”, f1); if (fd2 = creat(f2, 0666) == -1) usage(“unable to creating new file %s \n”, f2); while ((n = read(fd1, buf, 1024) > 0)) write ( fd2, buf, n); return 0; }