Approximate time: 40 minutes

Learning Objectives

Permissions

Unix controls who can read, modify, and run files by conferring permissions to files and directories. As you can imagine, on a shared system it is important to protect each user’s data. This is done by use of permissions.

In this lesson, we are going to learn more about how those permissions are set and how they can be modified.

To start, every file and directory on a Unix computer belongs to one owner (referred to as “user”) and one group. Along with each file’s content, the operating system stores the information about the user and group that own it, i.e. the “metadata” for a given file.

Users of a multi-user Unix system (i.e. the FASRC cluster) can belong to any number of groups.

Let’s see what groups we all belong to. Type groups into the command prompt.

$ groups

Depending on our affiliation, we all belong to at least a couple of groups. You will likely see groups like the following:

The user-and-group model means that for each file/directory every user on the system falls into one of three categories:

For each of these three categories, the computer keeps track of whether people in that category can read the file (r), write to the file (w), or execute the file (i.e., run the program written in it) (x). More about this aspect of permissions is coming up later in this lesson.

Let’s look at this model in action by running the command ls -l /n/holylfs05/LABS/hsph_bioinfo/, to list the files in that directory:

$ ls -l /n/holylfs05/LABS/hsph_bioinfo/
total 12
drwxrwsr-x  3 root hsph_bioinfo 4096 Sep 20 15:13 Everyone
drwxrws---  5 root hsph_bioinfo 4096 Aug  9 08:56 Lab
drwxr-s--- 11 root hsph_bioinfo 4096 Jul  9 09:03 Users

As we have learned, the -l flag tells ls to give us a long-form listing. Let’s take another look at the columns in this output, starting from the right side moving left.

  1. File/directory names
  2. Times and dates last modified. Backup systems and other tools use this information in a variety of ways, but you can use it to tell when you (or anyone else with permission) last changed a file.
  3. File size in bytes.
  4. Name of the group that owns the file.
  5. User name of the file’s owner.
  6. File’s number of hard links (not important for this class).
  7. Permissions to the file

NOTE: When listing the contents of a directory using the ls -l command, the file size reported for directories do not reflect the size of the data inside it. The number actually represents the size of space on the disk that is used to store the metadata for the directory.

The command you’ll want to use to get the size of a directory’s contents is du -sh. du is short for “disk usage”.

Let’s list the contents of the unix_lesson directory:

ls -l ~/unix_lesson/

drwxrwxr-x 2 rkhetani hsph_bioinfo  78 Oct  6 10:57 genomics_data
drwxrwxr-x 2 rkhetani hsph_bioinfo  73 Oct  6 10:57 other
drwxrwxr-x 5 rkhetani hsph_bioinfo 302 Oct  6 11:53 raw_fastq
-rw-rw-r-- 1 rkhetani hsph_bioinfo 377 Oct  6 10:57 README.txt
drwxrwxr-x 2 rkhetani hsph_bioinfo  62 Oct  6 10:57 reference_data

Who is the owner of the files in this directory? Which group do the files belong to?

The FASRC cluster has you (your account ID) listed as the owner and your lab group as the group, and this is usually the assignment for the files and folders in your personal directory.

Interpreting the permissions string

Let’s have a closer look at one of those permission strings in the first column for the README.txt file:

-rw-r--r--

In our case, it is - which means README.txt is a regular file.

r = read permission

w = write/edit permission

x = execute permission (run a script/program or traverse a directory).

The first triplet is the permissions for the file’s owner (u). Here, the owner can read and write the file: rw-. If the permission is turned off, we see a dash, so rw- means “read and write, but not execute”.

rw-

The second triplet shows us the group’s permissions (g). Here, the group can read the file.

r--

The final triplet shows us what everyone else (o) can do. In this case, it’s r--, so everyone else on the system can only read the file’s contents.

“Everyone” else refers to other users on the system who are not the file’s owner, or in the group that the file’s owner belongs to.

r--

The execute permissions

We don’t see the execute permission set here since we are not working with executable files. To see an example of a file that is actually executable, try ls -l /bin/ls.

Sometimes the x is replaced by another character, but it is beyond the scope of today’s class. You can get more information here, if you are interested.

Is the permissions string interpreted in the same way for directories?

If we take a look at the permissions for directories (e.g. drwxrwsr-x): the x for the permissions here indicates that “execute” is turned on. What does that mean, given that a directory isn’t a program or an executable file, we can’t “execute” it?

Well, x means something different for directories. It gives someone the right to traverse the directory, but not to look at (or list) its contents. This is beyond the scope of today’s class, but note that you can give someone access to a file that’s deep inside a directory structure without allowing them to see what other files exist in the sub-directories which are part of the path.

Changing permissions

To change permissions, we use the chmod command (whose name stands for “change mode”). The arguments we provide chmod include:

Let’s make our README.txt file inaccessible to all users other than you and the group the file belong to. Currently, everyone else is able to read the file.

$ ls -l ~/unix_lesson/README.txt

-rw-r--r-- 1 rkhetani hsph_bioinfo 377 Oct  6 10:57 ~/unix_lesson/README.txt

$ chmod o-r ~/unix_lesson/README.txt         # the "-" after o denotes removing that permission

$ ls -l ~/unix_lesson/README.txt

-rw-r----- 1 rkhetani hsph_bioinfo 377 Oct  6 10:57 ~/unix_lesson/README.txt

The o signals that we’re changing the privileges of “others” which also represents “everyone else” as we have referred to throughout this lesson.

Let’s change it back to allow it to be readable by others:

$ chmod o+r ~/unix_lesson/README.txt         # the "+" after o denotes adding/giving that permission

$ ls -l ~/unix_lesson/README.txt

-rw-r--r-- 1 rkhetani hsph_bioinfo 377 Oct  6 10:57 /home/rsk27/unix_lesson/README.txt

If we wanted to make this an executable file for ourselves (the file’s owners) we would say chmod u+x, where the u signals that we are changing permission for the file’s owner. To change permissions for the “group”, you’d use the letter g, e.g. remove write permissions for the group with chmod g-w.

The fact that something is marked as executable doesn’t actually mean it contains or is a program of some kind. We could easily mark the ~/unix_lesson/raw_fastq/Irrel_kd_1.subset.fq file as executable using chmod. Depending on the operating system we’re using, trying to “run” it will fail (because it doesn’t contain instructions the computer recognizes, i.e. it is not a script of some type).

Exercise

If ls -l myfile.php returns the following details:

-rwxr-xr-- 1 caro zoo  2312  2014-10-25 18:30 myfile.php

Which of the following statements is true?

  1. members of caro (a group) can read, write, and execute myfile.php
  2. members of zoo (a group) cannot execute myfile.php

  3. caro (the owner) can read, write, and execute myfile.php

    Click here for the answer The third statement is true.

Environment Variables

Environment variables are, in short, variables that describe the environment in which programs run, and they are predefined for a given computer or cluster that you are on. You can reset them to customize the environment.

Let’s see the full list of environment variables on the FASRC cluster:

$ env

It’s a pretty long list! In the context of the shell the environment variables are usually all in upper case.

In this lesson, we are going to focus on two most commonly encountered environment variables: $HOME and $PATH.

Environment variables, in most systems, are called or denoted with a “$” before the variable name, just like a regular variable. Let’s use the echo command to see what is stored in $HOME:

$ echo $HOME

You should see the path to your home directory. $HOME can be used instead of the ~ (if you want to type 4 more characters).

$HOME is pretty straightforward, how about we take a look at what is stored in the $PATH variable:

$ echo $PATH

/usr/local/bin:/usr/lib64/qt-3.3/bin:/bin:/usr/bin:/opt/dell/srvadmin/bin:/n/home13/rkhetani/.local/bin:/n/home13/rkhetani/bin

This output is a lot more complex! Let’s break it down. When you look closely at the output of echo $PATH, you should a list of full paths separated from each other by a “:”.

Here is the list of paths in a more readable format:

Each of these paths are referring to a directory, in this case all of them are named bin.

What are all these paths? And what do they represent?

These are the directories that the shell will look through (in the same order as they are listed) for any given command or executable file that you type on the command prompt.

For example, we have been using the ls command to list contents in a directory. When we type ls at the command prompt, the shell searches through each path in $PATH until it finds an executable file called ls. So which of those paths contain that executable file?

For any command you execute on the command prompt, you can find out where the executable file is located using the which command.

$ which ls

What path was returned to you? Does it match with any of the paths stored in $PATH?

Try it on a few of the basic commands we have learned so far:

$ which <your favorite command>
$ which <your favorite command>

Check the path /usr/bin/ and see what other executable files you recognize. (Note that executable files will be listed as green text or have the * after their name).

$ ls -lF /usr/bin/

The path /usr/bin is usually where executables for commonly used commands are stored.

As pointed out earlier, a lot of the folders listed in the $PATH variable are called bin. This is because of a convention in Unix to call directories that contain all the commands (in binary format) bin.

Exercise

Are the directories listed by the which command within $PATH?

Click here for the answer It should be. For example, if you would like to check the directory of command pwd - the output for which pwd is /usr/bin/pwd, and /usr/bin is within $PATH.

Modifying Environment Variables

You can modify the contents of the $PATH environment variable with the export command.

The export command:

This command is often used to add paths to a directory with commands you commonly want to use.

Closer look at the inner workings of the shell, in the context of $PATH

Each time you log in to a cluster, or start a new interactive session on a compute node, 2 special shell scripts are run automatically in the background. You have the ability to modify these scripts, so if you want to customize your environment you can add the customizing commands to these shell scripts. One common use of this is to add an export command that adds paths to the pre-existing contents of the $PATH environment variable.

So, what are these files and where are they located? These are called .bashrc and .bash_profile, and they are located in your home directory. You can create them if they don’t exist, and Shell will use them!

Check what hidden files (files starting with .) exist in our home directory using the -a flag with the ls command:

$ ls -al ~/

You can use vim to modify these files and/or create them.

In closing, permissions and environment variables, especially $PATH, are very useful and important concepts to understand in the context of UNIX and HPC.

This lesson has been developed by members of the teaching team at the Harvard Chan Bioinformatics Core (HBC). These are open access materials distributed under the terms of the Creative Commons Attribution license (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.