Homework #3

Due: May 11, 2026
Points: 100

This homework is designed to teach you about Linux file permissions. You are to write a program called “access” that will say what rights a user or group has over a file or directory.

Command

The command for your program is the following:

access[ -g ] name  file1 …

In this command, name is either a name or a non-negative integer. If the -g flag is given, name is interpreted as a group name or GID, and if that flag is not given, it is interpreted as a user name or UID. It is an error to give a non-existent user or group name, but it is not an error to give a UID or GID without an associated name.

Example Output

In these examples, “filex” is a file and “dirx” is a directory. Assume “filex” has owner bishop, group owner src, and protection mode 0654, and the directory “dirx” has the same owner and group and the protection mode 0751. Then the command

access bishop filex

prints

The user bishop (UID 917) can read and write the file filex

The command

access sshd filex

prints

The user sshd (UID 106) can read the file filex

and the command

access 40 filex

prints

The user with UID 40 can read the file filex

but the command

access -g 40 filex

prints

Members of the group src (GID 40) can read and execute the file filex

The difference is that, without the -g option, the 40 is interpreted as a UID, and there is no associated name. But with that option, the 40 is interpreted as a GID, and that has the associated name “users”.

If any of the named files are directories, you are to print “list the contents of”, “modify”, or “search” rather than “read”, “write”, and “execute”, respectively. So, the command

access bishop dirx

prints

The user bishop (UID 917) can list the contents of, modify, and search the directory dirx

(all on the same line), the command

access -g src dirx

prints

Members of the group src (GID 40) can list the contents of and search the directory dirx

and the command

access -g ntp dirx

prints

Members of the group ntp (GID 140) can search the directory dirx

If the named user has UID 0, all access is permitted regardless of the permission settings. This user is usually named root, but any user with a UID of 0 has such access.

Finally, sometimes the user or group will have no access to the named file or directory:

access bishop /root

prints

The user bishop (UID 917) can do nothing with the directory /root

on the standard output stdout (or file descriptor 1), and not the standard error stderr (or file descriptor 2).

Error Messages

If the file (or directory) does not exist, print the file (or directory) name followed by the message from perror(3) on the standard error stderr (or file descriptor 2):

access bishop filexy

prints

filexy: No such file or directory

If the user does not exist, print the user name followed by a colon and a space, then “no such user” on the standard error stderr (or file descriptor 2):

access bishopric filex

prints

bishopric: no such user

Similarly, if the group does not exist, print the group name followed by a colon and a space, then “no such group” on the standard error stderr (or file descriptor 2):

access -g unknown filex

prints

unknown: no such group

Note that if the user or group is given as an integer, your program should proceed as in the previous section and not print an error message, even if there is no name associated with that UID or GID.

Checking Your Output

A reference version of this program is available in the directory ~bishop/ecs153 on the CSIF; it is the executable file “access”. Your output, and especially your error output, is to match that of this program.

Submitting Your Program

Please turn in the source code, the Makefile (you must include one) and any related information (such as manual pages and README files). Put these in a directory called lastname-firstname/hw3 (where “lastname” and “firstname” are your last and first names, respectively), create a tar(1) file called hw3.tar, and submit that on Canvas.

We will grade your program on the CSIF, so be sure it runs there. To grade it, we will unpack the tar file, run make, and then run some tests. Your Makefile must create an executable called “access” as that is the command we will use to run the tests.

If you use an interpreted language such as python, be sure you make the script executable in such a way that it can be executed by invoking it as “access”.

Matt Bishop
Office: 2209 Watershed Sciences
Phone: +1 (530) 752-8060
Email: mabishop@ucdavis.edu

ECS 153A, Computer & Information Security & Privacy I
Version of April 29, 2026 at 10:18PM

You can also obtain a PDF version of this.