Files in Unix

Files and Processes

Everything in Unix is either a file or a process. A process is an executing program identified by a unique PID (process identifier). A file is a collection of data. They are created by users using text editors, running compilers etc. Examples of files:

• a document (report, essay etc.)
• the text of a program written in some high-level programming language
• instructions comprehensible directly to the machine and incomprehensible to a casual user, for example, a collection of binary digits (an executable or binary file)
• a directory, containing information about its contents, which may be a mixture of other directories (subdirectories) and ordinary files

The Directory Structure

In Unix, folders are generally called directories. Directories are arranged in a hierarchical structure, like an inverted tree. The top of the hierarchy is traditionally called root.

Listing Files and Directories

ls (list)

When you first log in, your current working directory is your home directory. Your home directory has the same name as your username, for example, mst3k, and it is where your personal files and subdirectories are saved. To find out what is in your home directory, type

% ls

This is short for “list.” Most Unix commands are two to four letters, sometimes with nonintuitive meanings.

The ls command lists the contents of your current working directory. There may be no files visible in your home directory, in which case the prompt will be returned. Alternatively, there may already be some files or folders created when your account was set up.

In most Unix systems, ls does not report hidden files by default. Files or directories with names beginning with a dot (.) are hidden and usually contain important program configuration information. They are hidden because you should not change them unless you understand what you are doing. To list all files in your home directory including those whose names begin with a dot, type

% ls -a

The -a is an example of a command-line option. The options change the behavior of the command. There are online manual pages that tell you which options a particular command can take, and how each option modifies the behavior of the command.

Making Directories

mkdir (make directory)

We will now make a subdirectory in your home directory to hold the files you will be creating and using in the course of this tutorial. To make a subdirectory called unixstuff in your current working directory type

% mkdir unixstuff

To see the directory you have just created, type

% ls

Changing to a Different Directory

cd (change directory)

The command cd _directory_ means change the current working directory to “directory”. The current working directory may be thought of as the directory you are in, i.e. your current position in the directory tree. To change to the directory you have just made, type

% cd unixstuff

Type ls to see the directory’s contents (it should be empty).

Exercise 1A

Make another directory inside the unixstuff directory called backups.

The Directories . and ..

While still in the unixstuff directory, type

% ls -a

The unixstuff directory (and in all other directories) contains two special directories called (.) and (..). In Unix, (.) means the current directory, so typing

% cd .

means stay where you are (the unixstuff directory). This may not seem very useful at first, but using (.) as the name of the current directory will save a lot of typing, as we shall see later in the tutorial. (..) means the parent of the current directory, so typing

% cd ..

will take you one directory up the hierarchy. Try it now.

Note: there is a space between cd and the dot or double dot. Also note: typing cd with no argument always returns you to your home directory. This is very useful if you are lost in the file system.

Pathnames

pwd (print working directory)

Pathnames enable you to work out where you are in relation to the whole filesystem. For example, to find out the absolute pathname of your home directory, type cd to get back to your home directory and then type

% pwd

Most of the time you should see /home/mst3k. On a multiuser central system like Rivanna, /home may be a symbolic link, i.e. an alias, for something else. To see it, type

% pwd -P

The full pathname may look something like this: /sfs/qumulo/qhome/mst3k.

Exercise 1B

Use the commands ls, pwd and cd to explore the file system. (Remember, if you get lost, type cd with no argument to return to your home directory.)

More About Home Directories and Pathnames

Understanding Paths

First type cd to get back to your home directory, then type

% ls unixstuff

to list the contents of your unixstuff directory. Now type

% ls backups

You will get a message like this:

backups: No such file or directory

The reason is that “backups” is not in your current working directory. To use a command on a file (or directory) not in the current working directory (the directory you are currently in), you must either cd to the correct directory, or specify its full pathname. To list the contents of your backups directory, you must type

% ls unixstuff/backups

This path starts from the current location. A path may be absolute or relative. An absolute path starts from the root location. The absolute path is

ls /home/mst3k/unixstuff/backups

A relative path starts from the current working directory. The special symbols . and .. are often used for relative paths.

~ (your home directory)

Home directories can also be referenced by the tilde ~ character. It can be used to specify paths starting at your home directory. So typing

% ls ~/unixstuff

will list the contents of your unixstuff directory, no matter where you currently are in the file system. What do you think

% ls ~

would list? What do you think

% ls ~/..

would list?

Getting Help

Online Manuals

There are online manuals which give information about most commands. The manual pages tell you which options a particular command can take, and how each option modifies the behaviour of the command. Type man command to read the manual page for a particular command. For example, to find out more about the wc (word count) command, type

% man wc

In newer Linux systems, most commands have a --help option

% wc --help

However, man sends output through a pager, whereas –help prints directly to the console.

Another useful command,

% whatis wc

gives a one-line description of the command, but omits any information about options, etc.

Summary

Command	Operation
ls	list files and directories
ls -a	list all files and directories
mkdir	make a directory
cd directory	change to directory
cd	change to home directory
cd ~	change to home directory
cd ..	change to parent directory
pwd	display the path of the current directory
man	display the manual pages for the specified command
whatis	display a description of the specified command