a person working on a UNIX or linux sytem issues commands, enters data, writes programs, changes directories and obtains inforamtion. all tehse activities are accomplished by accessinf iles that are utilized in three ways:

1. when we examine the contents of a file with utitlies such as more, cate, and vi, we read the files' contents. the files i snot changed, only read. when we us ls to list the contents of a directory, we are reading the directory.

2. when we have completed editing a file using vi and we type:w, we write the file, making changes. when we enter who >> file_name, we add the output of who to the file file_name. we write to it. when we add a file to a direcotyr or change a fliename, we write the name in the directory.

3. when we enter the name of a shell script, we start or execute a new chile jprocess that read the command file or shell script and then runs whatever command are in it.

if a user owns a file, that user may either allow or deny any of the three permission: read, wrtie, or execute. the owner can modify the permissions of a file of rth three classes of users: the owner or user, the members of the owner's group and all other users which are people not in your group

the first portionsof this post examines using letters which are also called mnemonics to modify the permissions for the three classes of users, followed by an in-depth look at using numbers to specify the permissions. the next sections investigate the affect of the file and directory permissions on users activities, follwed by an examination of how files and driectories are assgned their initial permissions. the last section guides you though midigy permssions foa ll files in the whole directory tree.

a great deal of information is packed into the permission fields. reexamine the permissions for all the regular fles in your home directory by entering:

ls -l

in the permissions field, ther are ten lsots for each file. for example:

-rwxr-x--x

every slot is occupied either by a minus sign or by a letter. if the first slot is a -, then the object is a file. if it is a d, then the object is a direcotry. in the last nine slots, a minus sgin indicates that the particular permission is denied. if a letter appears in the lsot, it indicates that a permission is alowed. the letter you see wil susually be r, w or x and for some files you might also see b,c,l,p,s,S,t, or T

for example, run this command

ls -ld /tmp

the result is usually:

-rwxrwxrwt /tmp

the t in the /tmp directory permissions is a special permission called the sticky bit