Basic File Permissions

Basic File Permissions

Permission Groups

Each file and directory has three user based permission groups:

  • owner - The Owner permissions apply only the owner of the file or directory, they will not impact the actions of other users.
  • group - The Group permissions apply only to the group that has been assigned to the file or directory, they will not effect the actions of other users.
  • all users - The All Users permissions apply to all other users on the system, this is the permission group that you want to watch the most.

Permission Types

Each file or directory has three basic permission types:

  • read - The Read permission refers to a user's capability to read the contents of the file.
  • write - The Write permissions refer to a user's capability to write or modify a file or directory.
  • execute - The Execute permission affects a user's capability to execute a file or view the contents of a directory.

Viewing the Permissions

You can view the permissions by checking the file or directory permissions in your favorite GUI File Manager (which I will not cover here) or by reviewing the output of the \"ls -l\" command while in the terminal and while working in the directory which contains the file or folder.

The permission in the command line is displayed as:
_rwxrwxrwx 1 owner:group

  1. User rights/Permissions
    1. The first character that I marked with an underscore is the special permission flag that can vary.
    2. The following set of three characters (rwx) is for the owner permissions.
    3. The second set of three characters (rwx) is for the Group permissions.
    4. The third set of three characters (rwx) is for the All Users permissions.
  2. Following that grouping since the integer/number displays the number of hardlinks to the file.
  3. The last piece is the Owner and Group assignment formatted as Owner:Group.

Modifying the Permissions

When in the command line, the permissions are edited by using the command chmod. You can assign the permissions explicitly or by using a binary reference as described below.

Explicitly Defining Permissions

To explicity define permissions you will need to reference the Permission Group and Permission Types.

The Permission Groups used are:

  • u - Owner
  • g - Group
  • o or a - All Users

The potential Assignment Operators are + (plus) and - (minus); these are used to tell the system whether to add or remove the specific permissions.

The Permission Types that are used are:

  • r - Read
  • w - Write
  • x - Execute

So for an example, lets say I have a file named file1 that currently has the permissions set to _rw_rw_rw, which means that the owner, group and all users have read and write permission. Now we want to remove the read and write permissions from the all users group.

To make this modification you would invoke the command:
chmod a-rw file1
To add the permissions above you would invoke the command:
chmod a+rw file1

As you can see, if you want to grant those permissions you would change the minus character to a plus to add those permissions.

Using Binary References to Set permissions

Now that you understand the permissions groups and types this one should feel natural. To set the permission using binary references you must first understand that the input is done by entering three integers/numbers.

A sample permission string would be
chmod 640 file1, which means that the owner has read and write permissions, the group has read permissions, and all other user have no rights to the file.

The first number represents the Owner permission; the second represents the Group permissions; and the last number represents the permissions for all other users. The numbers are a binary representation of the rwx string.

  • r = 4
  • w = 2
  • x = 1

You add the numbers to get the integer/number representing the permissions you wish to set. You will need to include the binary permissions for each of the three permission groups.

So to set a file to permissions on file1 to read
_rwxr_____, you would enter chmod 740 file1.

Owners and Groups

I have made several references to Owners and Groups above, but have not yet told you how to assign or change the Owner and Group assigned to a file or directory.
You use the chown command to change owner and group assignments, the syntax is simple
chown owner:group filename, so to change the owner of file1 to user1 and the group to family you would enter chown user1:family file1.
User, Group and World
so far I have treated permissions as either your permission or not your permission. The read, write and execute permissions are stored in three different places, called user (u), group (g) or world or other (o).
When you execute
chmod =r myfile
it changes the permissions in three places. When you list this file with "ls -l" you will see
-r--r--r-- 1 grymoire admin 0 Feb 1 19:30 myfile
Note that there are three "r"'s for the three different types of permissions.
All files have an owner and group associated with them. There are three sets of read/write/execute permissions: one set for the user of the file, one set for the group of the file, and one set for everyone else (other). These permissions are determined by 9 bits in the i-node information, and are represented by the characters "rwxrwxrwx." The first three characters specify the user, the middle three the group, and the last three the world. If the permission is not true, a dash is used to indicate lack of privilege. If you wanted to have a data file that you could read or write, but don't want any one else to see, the permission would be "rw-------."
Everyone belongs to at least one group in a Unix system. Some people belong to more than one group. If the computer is only used by one person, then groups aren't that useful except for set group-id programs, but that comes later.
Let's assume you have several people using a computer, and you want to allow people in a group to have access to a directory. Let's also say they belong to the same group as you. Assume the file is in a group directory, with the group "admin", and you wanted to allow them to read and You can create a directory that has read, write and execute permission for the group. But you want to prevent people outside of the group from reading or changing the file. You want the file to have the permission "rw-rw----" for user and group=read and write, and others have none. The chmod command can do this. You should remember that the command
chmod =rw myfile
will create the permission "rw-rw-rw-" which means user, group and other have read and write. So how can you change it to "rw-rw----"?
The chmod command has options, of course. using "=", "-" or "=" changes user (u), group (g)and other (o) permissons. You can explicitly specify u, g or o in the chmod command:
chmod u=rw myfile
chmod g=rw myfile
chmod ug=rw myfile
This is handy, but the three commands above do not change the "other" permission. They only change what is specified. To remove read and write permission for other, you can instead type
chmod o= myfile
chmod o-rw myfile
The first sets the permission to nothing, and the second removes the read and write permission.
If you want to change the group permission, use "g" instead of "o":
chmod g+r myfile
chmod g-w myfile
These comands will add read and remove write permission. You can combine these two commands
chmod g+r-w myfile
if you want to combine an operaiton on group, and other, you can put a comma between the permissions:
chmod g+r-w,o=rwx myfile
Besides "u", "g" or "o", you can use "a" to mean all three. The following commands do the same thing
chmod a=rw myfile
chmod =rw myfile
An easier way to specify these 9 bits is with 3 octal digits instead of 9 characters. The octal representative of the read, write and execute bits, "rwx" are
Read 4
Write 2
Execute 1
Octal representation is pure geek talk, and was the only form that worked in the early versions of Unix. The order is the same as the "rwx", so read/write permission, or "rw-" can be described the the octal number 6. However, we have to express the permission of all three parts, so the permission "rw-------" (read/write for the user, and group and world get nothing) is b00. The first number specifies the file owner's permission. The second number specifies the group permissions. The last number specifies permissions to everyone who is not the owner or not in the group of the file.
Let's review the different combinations. I will show the letter representaiton, the octal representaiton, and the meaning
| rwx | 7 | Read, write and execute |
| rw- | 6 | Read, write |
| r-x | 5 | Read, and execute |
| r-- | 4 | Read, |
| -wx | 3 | Write and execute |
| -w- | 2 | Write |
| --x | 1 | Execute |
| --- | 0 | no permissions |

You can use the octal notation, where the three digits correspond to the user, then group, then other.
Perhaps this might help
| Permission | Octal| Field |
| rwx------ | 700 | User |
| ---rwx--- | 070 | Group |
| ------rwx | 007 | Other |
let's put this all together. I will list some chmod commands in both character and octal representaion.
| chmod u=rwx,g=rwx,o=rx | chmod 775 | For world executables files
| chmod u=rwx,g=rx,o= | chmod 750 | For executables by group only
| chmod u=rw,g=r,o=r | chmod 644 | For world readable files
| chmod u=rw,g=r,o= | chmod 640 | For group readable files
| chmod u=rw,go= | chmod 600 | For private readable files
| chmod u=rwx,go= | chmod 700 | For private executables
Let's also review the same permissions for directories
| chmod u=rwx,g=rwx,o=rx | chmod 775 | For world readable directories
| | | Members of group can change files
| chmod u=rwx,g=rx,o= | chmod 750 | For group readable directories
| | | Members of group can change files
| chmod u=rwx,go= | chmod 700 | For private direcories
The importance of order in Unix Permissions
This last point is subtle. When testing for permissions, the system looks at the groups in order. When Unix checks permissions, the order is this

  • If the file is owned by the user, the user permissions determine the access.
  • If the group of the file is the same as the user's group, the group permisson determien the access.
  • If the user is not the file owner, and is not in the group, then the other permission is used.
If you are denied permission, Unix does not examine the next group. Consider the case of a file that is owned by user jo, is in the group guests, and has the permissions -----xrwx or 017 in octal. It would be listed as
------xrwx 1 jo guests 0 Feb 1 20:47 myfile
Let's assume the directory has the permission 775 (world readable and searchable). When considering this file, the exact permissions above mean:

  • jo cannot use the file.
  • Anyone in group guests can execute the program.
  • Everyone else besides jo and the members of the guests group can read, write, and execute the program.
This is not a very common set of permissions. But there are ways it can be used. However, to really understand it, we have to consider the permission of the directory. Remember, as I said, if jo of any one in the guest group has write permission of the directory, then they can rename or delete the file. Let's say the directory /testme is owned by the superuser, and has the permission 711. What does this mean? First of all, notice that the directory does not have group and world read or write - just search. This means that users cannot see what files are in this directory. It also means they have to know the name of the file to execute it. Any user can type /testme/myfile and because the directory is world searchable, the program can be executed. Now let's return to Jo and the Guests (Hmm. sounds like a 60's pop rock group). If the file "/testme/myfile" has the permission 017 then Jo cannot execute the program. Anyone in group Guests can, but only if the file is a compiled program (and not a shell script). ANd the rest of the world can execute the program. However, they have to know the name of the file. They cannot list the contents of the directory. people use a similar mechanism to deny one group of users from accessing or using a file. In the above case, jo cannot read or write the file she owns. She could use the chmod command to grant herself permission to read the file. However, if the file was in a directory owned by someone else (root), and the directory did not give Jo read or search permission, she would not be able to find the file to change its permission. Another example - using chmod 510 on a directory to provide group access Let's change the situation around a bit. Let's make the directory mode 510. Let's also make the file "myfile" and the directory "/testme" owned by Jo. Let's also assume the program "myfile" is a compiled program, and has the permission 711. Anyone in group "guests" can execute the program. However, if the administrator removed someone from the group, they can no longer execute the program. They do need to log off and log on again, and group permission is granted at logon time.

Typical Permissions Most of the time permissions fall into three cases:
  1. The information is personal. Many people have a directory or two they store information they do not wish to be public. Mail should probably be confidential, and all of your mailbox files should be in a directory with permission of 700, denying everyone but yourself and the system administrator read access to your letters.
  2. The information is not personal, yet no one should be able to modify the information. Most of my directories are set up this way, with the permission of 755.
  3. The files are managed by a team of people. This means group write permission, or directories with the mode 775.
You could just create a directory with the proper permission, and put the files inside the directory, hoping the permissions of the directory will "protect" the files in the directory.

Using Permissions in Work Groups This is not adequate. Suppose you had a directory with permission 755 and a file with permission 666 inside the directory. Anyone could change the contents of this file because the world has search access on the directory and write access to the file.

The umask command What is needed is a mechanism to prevent any new file from having world write access. This mechanism exists with the umask command. If you consider a new directory would get permissions of 777, and new files get permissions of 666, the umask command specifies permissions to take away from all new files. To "subtract" world write permission from a file, 666 must have 002 "subtracted" from the default value to get 664. To subtract group and world write, 666 must have 022 removed to leave 644 as the permission of the file. These two values of umask as so common it is useful to has some tcsh aliases defined: alias open umask 002 alias shut umask 022 With these two values of umask, new directories will have permissions of 775 or 755. Most people have a umask value of one of these two values. In a friendly work group, people tend to use the umask of 002, which allows others in your group make changes to your files. Someone who uses the mask of 022 will cause grief to others working on a project. Trying to compile a program is frustrating when someone else owns files that you must delete but can't. You can rename files if this is the case, or ask the system administrator for help. Members of a team who normally use a default umask of 022 should find a means to change the mask value when working on the project. (Or else risk flames from your fellow workers!) Besides the open alias above, some people have an alias that changes directories and sets the mask to group write permission: alias proj "cd /usr/projects/proj;umask 002" This isn't perfect, because people forget to use aliases. You could have a private shell file in each project directory called .dir that contains the line umask 002 If you had the following alias alias cd 'chdir !*; if ( -f .dir && -o .dir ) source .dir ' You would automatically set your mask value when to change to the project directory. Other people could have similar files in the project directory with a different name. Still another method is to run find three times a day and search for files owned by you in the project directory that have the wrong permission: find /usr/projects -user $USER ! -perm -020 -print | \ xargs chmod g+w You can use the command crontab -e to define when to run this command.

Which group is which? Since group write permission is so important in a team project, you might be wondering how the group of a new file determined? The answer depends on several factors. Before I cover these, you should note that Berkeley and AT&T based systems would use different mechanisms to determine the default group. These two variations were merged by Sun, and Linux has inherited the Sun approach. Originally Unix required you to specify a new group with the newgrp command. If there was a password for this group in the /etc/group file, and you were not listed as one of the members of the group, you had to type the password to change your group. Berkeley based versions of Unix would use the current directory to determine the group of the new file. That is, if the current directory has cad as the group of the directory, any file created in that directory would be in the same group. To change the default group, just change to a different directory. Both mechanisms had their good points and bad points. The Berkeley based mechanism made it convenient to change groups automatically. However, there is a fixed limit of groups one could belong to, which was 8 groups. SunOS 4 changed this to a limit of 16 groups. Sun supports both mechanisms for backwards compability. The entire disk can be mounted with either the AT&T or Berkeley mechanism. If it is necessary to control this on a directory by directory basis, a special bit in the file permissions are used. If a disk partition is mounted without the Berkeley group mechanism, then a directory with this special bit will make new files have the same group as the directory, Without the special bit, the group of all new files depends on the current group of the user
. The other three bits Besides the nine bits that specify read, write, and execute (or search) permissions for the owner, group and world, there are three other bits that have special characteristics. The most famous bit is the set uid or set user identification bit. Any person who executes a program with this bit set has their user identification changed to be the same as the owner of the file. With this simple ability, Unix allows users to gain special priviledges in a controlled, temporary fashion. Another similar bit is the set gid or set group identification bit, which changes the group instead of the user. This is the permission bit that can be applied to a directory to force it to follow the Berkeley group semantics. The last bit is called the sticky bit. It is used to reduce the start up time when executing a program. Earlier versions of Unix would keep a "sticky" program in the swapping area of the disk. The second time a sticky program is executed, the system doesn't have to search for the file in the file system. However, with diskless workstations, and kernels that cache recent files in memory, this isn't as much a benefit as before. In fact, SunOS used it to indicate special files used for diskless clients. When a directory is made sticky, it adds a special security feature. It prevents someone from deleting or renaming files in a directory unless they own the file. This is called the "append-only" mode for a directory. The /tmp directory is a good example where this is needed. The directory must be world writable to be useful to others. By adding the sticky bit to this directory, you prevent someone from replacing a file owned my another user.

Changing special permissions The octal value of the set uid bit is 4000. the set gid bit is 2000, and the sticky bit is 1000. These strange octal values aren't shown when you list them. Instead, the character representation is used. When using the chmod command chmod u+s myfile adds the setuid bit. The set group iD bit can be set using chmod g+s myfile To make a program set uid using the octal representaiton,the command would be chmod 4755 program Alternately, you could use the symbolic form: chmod u+s,g=rx,o=rx program To make a set gid program, use one of the following: chmod 2755 program chmod g+s program If the file is a directory, you must use the symbolic form. Sticky files and directories can be created using one of these two forms: chmod 1755 file chmod u+t file

Examining the permissions Besides using find to search for these permissions bits, ls displays the permissions when the -l flag is used. If a program is set uid, the "x" in the user area is displayed as a "s" A sticky file or directory is indicated with the last "x" displayed as a "t." If the corresponding execute bit is not set, the letter is capitalized. The capitalization of the letter is a flag that an unusal combination was chosen.

Conditional Changing Linux supports the +X option. For example, this mode: chmod a+X * gives all users permission to execute files (or search directories) if anyone could before.

Conclusion Here is a chart of the permissions displayed by ls and the corresponding octal values: +------------------+ |rwxrwxrwx 777 | all permissions granted |rwxr-xr-x 755 | Group and world readbale/eecutable |rwx------ 700 | Private |rwsr-xr-x 4755 | set UID |rwxr-sr-x 2755 | set GID |rwxr-xr-t 1755 | Sticky bit |rwSw-xr-x 4655 | setUID but not executable by user |rwxr-Sr-x 2745 | getGID, but not executable by group members |rwxr-xr-T 1754 | Sticky bit, but not world executable +------------------+

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