Navigate the File System

Week 1 Index

Previous Section: Anatomy of a Command

Now that we have learned about the shell and the anatomy of a command, let's dive into different helpful programs that you typically encounter while working on UNIX systems.

pwd

pwd is a program that is occasionally helpful, but is a good way to get your feet wet in trying out different UNIX programs. All it does is print out what directory you are currently in. Usually, you will already know where you're at from the prompt, but it can be helpful to know the full path. pwd stands for print working directory:

$ pwd
/home/username

ls

ls is one of the most common UNIX programs that you may use while on UNIX systems. By default, it lists the contents of your current working directory:

$ ls
example-data    data.bak

There are many options that ls can use to modify its behavior. In addition, you can specify a directory as an argument to list contents of that directory.

Paths

All files in UNIX systems are organized into directories, and those directories may have subdirectories, creating a tree of files that span the entire operating system. Directory paths in UNIX systems can be absolute or relative. Absolute paths start at the root of the file system and thus always start with a slash (/). Relative paths are relative to your current working directory and do not start with a slash.

Absolute Path

$ ls /home/username/example-data
output_of_ls

Relative Path

$ ls example-data
output_of_ls

File Extensions

In UNIX systems, file extensions are technically meaningless, however, convention is important. Executables don't need to end in .exe and text files don't need to end in .txt. It is helpful if they do, so that people (including yourself) know what type of file it is.

cd

Another one of the most common programs to run on UNIX systems is cd, which stands for change directory, but should be thought of as change working directory. This will change which directory you are currently working in:

$ pwd
/home/username
$ cd example-data
$ pwd
/home/username/example-data

Info

Paths as the argument of the cd program can be absolute or relative.

Tip

Run cd - to navigate to the previous directory you were in. Run cd without any arguments to navigate back to your home directory.

Special Paths

There are three special paths that are commonly used in navigating UNIX file systems:

• ~
• .
• ..

The tilde (~) refers to your home directory. The single period (.) refers to the current working directory, which can sometimes be helpful, but it is more rare. Lastly, the double dot (..) refers to the parent directory of the directory you are in:

$ pwd
/home/username/example-data
$ cd ..
$ pwd
/home/username

Permissions

Since everything in UNIX systems is a file, file permissions are of supreme importance. To check the permissions of a file, run the command ls -l which runs the ls program with the l option which tells ls to print out more information about the files:

$ ls -a -l -h ~/.ssh
total 6.0K
drwxr-xr-x  2 username student    4 Jul 17 11:19 .
drwx------ 14 username student   28 Jul 16 22:26 ..
-rw-r--r--  1 username student 2.0K Oct 18  2021 authorized_keys
-rw-r--r--  1 username student    0 Jul 17 11:19 config

The three program options used here are: a, which displays all files/folders, even hidden ones; l which lists out more information about each listing; and h which shows the size of items in a human-readable format. In the first ten columns of the output are the permissions of that item, details of which will be discussed in the next paragraph.

The four items in the .ssh folder of this home directory are: the current directory (.), a link to the parent directory (..), and two files (authorized_keys and config).

Let's talk permissions. The first ten characters of each row represent the permissions for each item listed. The first column shows whether it's a directory (with a d) or not (with a dash). Then the next nine columns are broken into three groups of three. The first group of three is the permissions belonging to the user that owns (u) the file. The next group of three is the permissions for the group that owns the access to the file (g). And the last three are the permissions for other users (o). Each group of three is made up of the read (r), write (w), and execute (x) bits.

For directories, the permission bits mean slightly different things. The write bit is the same, it controls whether someone can modify (e.g., delete) is. The execute bit controls whether someone can see what's inside the directory. The read bit controls whether someone can go into that folder.

man

Another helpful program is man, which stands for manual. You may hear the term man page which is just short for manual page, or running the program man with the argument being the program you want more information about:

$ man ls

The man program pulls up a page that you can scroll up and down with your arrow keys or the j/k keys and quit out of by pressing the q key.

Quiz: who can find (using the ls man page) what are the options necessary to list items by reverse chronological order (older items listed first)?

Answer

The command would be $ ls -t -r

mkdir

The last program we'll go over in this section is the mkdir or make directory. This does what it sounds like and will create the directory noted in the argument if it doesn't already exist:

$ mkdir ~/example-data

You can also put multiple arguments and mkdir will create all of them. A helpful option to pass to the mkdir program is -p, which will create parent directories as needed:

$ mkdir -p ~/another_one/test1

Next Section: Editing files from the command line