NWE Help: Others: Shell: Files: chmod

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The cryptic looking string at the far left of a long directory listing are actually quite important, and chmod is there to help you manage them.

Note: If you don't understand the basics of permissions, please read our page for changing permissions with the file manager before continuing with chmod. Using chmod improperly can be dangerous!

Under Unix, every file or directory has three sets of attributes known as permissions. Chmod can apply changes to either of these three sets, or all of them:

set (and abbreviation)	class
user (u)	the user who owns the file or directory -- also known as the owner
group (g)	a group of users (defined by the system administrators)
others (o)	all the other users on the system (everyone not in the file or directory's group)
all (a)	the user, group, and others combined

Using the chmod command, the owner of a given file or directory can set three types of permissions for each class of user. These types are often called bits because they are either on or off. Permission bits have slightly different meanins for files and directories:

type (and abbreviation)	for files	for directories
read (r)	Allows file to be read, viewed, and copied	Allows directory contents to be listed, and files in the directory to be copied
write (w)	Allows file to be modified	Allows new files to be created in the directory
execute (x)	If file is a program, allows that program to be executed	Allows contents of directory to be accessed

Here is the entire left side of the directory listing for a sample directory, dissected to illustrate the various permissions and classes of users. (You get this sort of list by typing ls -l.)

This graphic is color coded. Data that is not relevant to permissions is grayed out.

+--- the first slot displays special information
|    "-" if the item is a file
|    "d" if the item is a directory
|    "l" if the item is a link
|
|    +--- these three slots show the permissions for
|    |    the users in the item's group
|    |
|    |                    +--- the item's group
v   vvv                   v
-rw-------   1 mjm      spinners   46580 Feb 10 18:18 nwefixthis
-rw-r--r--   1 dilger   spinners  483648 Feb 12 12:25 pcyc.tar
drwxr-xr-x  16 dilger   instr       1024 Feb 09 20:22 perl/
 ^^^   ^^^       ^                                      ^
  |     |        +---- the user who owns                +--- the item
  |     |              the item                              name
  |     |
  |     +----- these three slots show the permissons
  |            for other users
  |
  +--- these three slots show the permissions for the user

Most times you use chmod you will want to specify the permissions for all three classes of users: u for user (yourself), g for group, o for other users), or a for all users.

The basic command line is constructed in this fashion:

chmod [u g o and/or a][+ - or =][r w and/or x] itemname

Here are some examples and explanations of what they do:

example command	explanation	example of use
`chmod u=rw,g=r,o=r index.html`	Make the file `archive.html` read and write for yourself, and read only for other users.	Most Web pages.
`chmod u=rw,g=,o= mygrades.sdw`	Make the file `mygrades.sdw` read and write for yourself only.	Any file which needs to be private.
`chmod u=rw,g=rw,o=r shared.html`	Make the file `shared.html` readable and writeable for you and your group, but read only for others.	A file which others in your class (or other group) should edit, like a group assignment.
`chmod a=r archive.html`	Make the file `archive.html` read-only for all users, including yourself.	Files you don't want to edit or delete.
`chmod u=rwx,g=x,o=x mydir/`	Set the directory `mydir` so you can list its contents, create new files in it, and access files in it. Users in your group, and all other users, can access files in `mydir/` but can't read its directory listing or make new files there.	Most of your directories.
`chmod u=rwx,g=,o= grades/`	Set the directory `grades` so you can list its contents, create new files in it, and access files in it. No other permissions are granted.	Any directory which needs to be private.
`chmod u=rwx,g=rx,o=rx texts/`	Set the directory `texts` so you can list its contents, create new files in it, and access files in it. Allow group and other users to read the directory listing and access files in the directory	Any directory which contains shareable files.

Using the chmod command with plus or minus (+/-) will add or remove permissions for one class without affecting others. For example, if you type chmod g+w index.html group write permission is added to whatever permissions already exist. However, it's easier to just specify the permissions for each class of users.

Note that the only way that any user other than the file's owner can write to a file is if the permissions are set correctly. For collaborative work, that means that the file must be group-writeable, which is why it is imperative that collaborating students issue the following command for every file they create:

chmod g+w filename

Ask our system administrators for a quick permissions audit if you're unsure that you're doing it correctly. They can also set up groups for you which enable student, teacher, and other sorts of collaboration with the appropriate level of access.

Finally, there are numeric ways of dealing with permissions, and other useful things like sticky bit that we have not covered here. The chmod man page can help you learn about those items.