A file may have three types of permission: read ('r'), write ('w'), and execute ('x'). Each permission may be 'on' or 'off' for each of three categories of users: the file's owner; other people in the same group as the owner; and all others. To find out a file's mode, or permission settings, use the command 'ls -l filename'. The output will be of the form:

-rwxr-x--x 1 owner      2300 Jul 14 14:38 filename

The string of 10 characters at the left shows the mode. The initial '-' indicates that the file is a plain file; a 'd' would indicate a directory. Characters 2-4 are, respectively, 'r', 'w', or 'x' if the corresponding permission is turned on for the owner or '-' if the permission is turned off. Characters 5-7 similarly show the permissions for the group; characters 8-10 for all others.

To change the mode of a file, use the chmod command. The general form is:

chmod X@Y file1 file2 ...

where X is any combination of the letters 'u' (for owner), 'g' (for group), 'o' (for others), 'a' (for all; that is, for 'ugo'); @ is either '+' to add permissions, '-' to remove permissions, or '=' to assign permissions absolutely; and Y is any combination of 'r', 'w', 'x'. Examples:

# Give the owner rx permissions, but not w 
chmod u=rx file
 
# Deny rwx permission for group and others 
chmod go-rwx file       

# Give write permission to the group
chmod g+w file

# Give execute permission to everybody
chmod a+x file1 file2

# OK to combine like this with a comma
chmod g+rx,o+x file     

Regular accounts on SCF are generally assigned to the groups 'statfac' or 'statgrad' or 'statempl'. Each class on SCF forms its own group, eg, 's200a'. To find out what groups you are in, type the command 'groups'.

The same permission scheme applies to directories. For a directory, read permission gives the ability to list files in it via the ls command (and thus to discover what file names are); write permission gives the ability to create and delete files in it; execute permission gives the ability to access a file or subdirectory of known name (even without read permission). To find out the mode of a directory:

# Show permissions for the named directory(ies)
ls -dl dir ...

# Long list of all files in named directory(ies) (including those with names starting in '.')
ls -al dir ...

If no directories are specified, the listing is for all files in the current directory. The output will look something like:

drwx------  12  perito      592 Jul 11 13:46 .
drwxr-xr-x  24  stat       1424 Jul 10 13:07 ..

The initial 'd' in the 10-character mode string indicates that the file is a directory. The file name '.' always refers to the current directory; the file name '..' always refers to the parent of the current directory. Thus, this output shows the permissions for the current directory and its parent.

For more information, including octal specification of permissions, see 'man chmod', 'man ls', 'man umask'.