Unix Lab

Basic Commands

Passwords, File and Directory Commands


When you obtained a Unix account, you got a starting password that was common to the class. You should change that password to something that is not easily guessed. There are good reasons for that. There are persons "out there" who would rather tear down than build great new things. Some of these people go around trying to break into Unix systems so that they can show you how clever they are by deleting files or leaving messages telling you how stupid you are for letting them in. Usually these people get in because someone has given away their password or made it so easy that anyone could guess it (like your last name, or your last name backwards, etc.). Nevertheless, the password you were given doesn't have to be the one that you live with. The command to change it is to type


The Unix operating system will respond by asking you to type your old password, then the new password, then it will ask you to retype your new password (just to make sure you haven't mis-typed it). If you respond consistently, then Unix will change your password in its records.

Your userid and your passwords are valid for access to the Unix systems in MH 227 and DH 450 but not WSH 1. (You can get a separate Unix account for the computers in WSH 1).
These accounts are separate from any Windows accounts you may have in the department or on campus.

Guidelines for passwords

Don't use your name, your significant other's name, your license plate number, your student ID number, your pet's name, or any other password that can easily be guessed by someone who knows who you are.

Make it at least six characters.

DO use numbers and special characters mixed in with letters. Just because it has a special character (such as the ones along the top of the keyboard above the numbers) doesn't mean it has to be hard to remember.

Pick something that you don't have to write down to remember.

Unix commands in general

Unix commands are entered usually at a keyboard. If you are familiar with DOS, then you already know some Unix commands. Actually, Unix was developed many years earlier than DOS and DOS took some of the Unix commands and made them part of DOS.

Most Unix commands consist of a few letters. Since Unix was originally developed at a time when there were no graphical user interfaces, one of the goals was to make the commands short so there would be less to type. So you will see things like cp instead of "copy" or mkdir for "make directory". In addition to the command name itself, there are often options associated with the command. Usually these options are single letters or groups of letters preceeded by the '-' character (the hyphen symbol). These options are called "flag arguments" and alter the action of the command in some way or other depending on the command.

Following the command and its options there may be the name of a file or a directory. These names can be virtually any number of characters (unlike DOS). Get in the habit of using names that tell you what's in the file such as StackMaker.java or projectSpecifications. Click here for more information on Unix file names.

Here's the general form of a Unix command. Most Unix commands look like this but there is considerable variation. In particular, some commands don't have all the components.

commandName -options fileOrDirectoryNames

Unix directories

In Unix, as in most operating systems, files are located in a hierarchical structure of folders (usually referred to in Unix as directories). You can think of a directory as a place to store files or other folders. At the top of the structure, there is a directory called the "root". We denote the root directory by the / symbol. Inside that directory there are "sub-directories" containing files and other subdirectories. This continues down until we get to the directory that you first see when you login to the Unix system. This directory is called your "home" directory. Other users have their home directories in some other part of the hierarchy.

Your home directory is the directory you "are in" when you first open a terminal window. Your home directory may be in a directory called export which may be in the / (root) directory. Although there are similarities from system to system about where users home directories are located, there is no requirement that they must be in a particular location.

The ls command

ls is an abbreviation for "list" as in "give me a list of the files in my directory".

Try typing the following command in your home directory:


What do you see? Depending on what files you have in your home directory, you may see nothing.

Now type the following command in your home directory:

ls -a

The -a indicates an option to show all files in your home directory. You will notice some files that start with the '.' (dot) character. These are files that are required by various programs and utilities. Unless you use the -a option you will not see them listed. For this reason they are often referred to as "hidden files".

Now type the following command:

ls -F /

The / character is the directory name for the root directory. What you should see are the subdirectories and any files that are located in the root directory. The -F option will result in directory names having a / character following their name. You may notice other names with @ symbols or * symbols following their names. These are not actually part of the name but instead "markers" that identify the types of files. For example files with the * symbol are ones that are executable (somewhat like files with a .EXE suffix in DOS). Unless you use the -F option on the ls command, these markers will not be visible.

The cd command

In the section on the ls command, we can view the contents of the root directory, for example, but if you just logged into your Unix account, you are in your home directory.

To be in a directory means that you can access the files in that directory more easily than if you were in another directory. More easily just means that you can refer to a file by its local name instead of what is called its full path name.

Type the command:

ls -l /etc/hosts

You will see the long listing for the file called hosts which is located in the etc subdirectory of the root directory. The name /etc/hosts is the full path name for the hosts file.

Type the command:

cd /

followed by:

cd etc

followed by:

ls -l hosts

The cd command changes the directory in which you are working to the one named in the command. The first command moves you to the root directory (/). The second moves you to the subdirectory called etc. In this directory you can refer to the hosts file by its local name.

The full path name contains the sequence of subdirectories from the root directory down to the subdirectory in which the named file lives. The full path name for the hosts file, for example, is /etc/hosts.

Use the cd command to go to the etc subdirectory of the root directory again. Now type the command:

cd ..

followed by:

ls -l

In which directory are you located now?

Change directories to the root directory. Now enter the command:


followed by:

ls -l

In which directory are you located now? So what do you think the default directory is when you just type cd with no directory name?

The cp command

The cp command allows you to copy a file. Its basic form is given by:

cp fromFile toFile

A copy of the fromFile will be made and given the name toFile.

If the toFile name is a directory name, then a copy of the fromFile will be made, given the same name as the original, and placed in the named directory.

We saw in the cd command that the symbol .. (two dots) can be used as an alias for the directory just above the current directory in the directory hierarchy.

The symbol . (one dot) can be used as an alias for the current directory.

Place yourself in your home directory. How can you copy the /etc/hosts file into your home directory giving it the same name?

Place yourself in your home directory again. How can you copy the /etc/hosts file into your home directory and give the copy the name of hosts.copy?

The rm command

The rm command allows you to remove (delete) a file. Its basic form is given by:

rm -i fileToDelete

The -i option causes Unix to ask you for confirmation before deleting the named file. You should NEVER use the rm command without this option.

Using aliases, you will be able to customize your interface to Unix so that the -i option becomes the default and you won't have to keep remembering to include it. Aliases are covered in another part of this lab.

After you have done the exercises relating to the cp command, you will have some files in your home directory that you should delete. Delete them.

The mkdir command

The mkdir command allows you to create a subdirectory. Its basic form is given by:

mkdir directoryName

First of all, you must be located in the directory in which you want to create the subdirectory. Secondly, you must have permission to create a subdirectory in that directory. (Permissions are discussed in the context of the chmod command.)

You always have permission to create a subdirectory in your home directory or in any subdirectory under your home directory.

Create a subdirectory called temp.lab in your home directory. Now issue an ls command in your home directory (use the -F option). Change directory to your new subdirectory. Copy the file /etc/hosts to this subdirectory. Use the ls -l command to check to see if the copy was successful.

The rmdir command

The rmdir command allows you to remove (delete) a subdirectory. Its basic form is given by:

rmdir directoryName

You will find that at some point you no longer need a particular subdirectory and you wish to delete it. The rmdir command will do this. It will, however, only delete subdirectories that have been emptied of all their files and subdirectories. Consequently, if you want to delete a subdirectory, you must first go in and delete all files in that subdirectory. If the subdirectory has other subdirectories, then you must delete all the files in those subdirectories, too.

Of course, you must have permission to delete the named subdirectory. (Permissions are discussed in the context of the chmod command.)

You created the subdirectory called temp.lab while learning about the mkdir command. Delete that subdirectory now. First try issuing the rmdir command BEFORE deleting any files in the subdirectory. What happens? Now do what is necessary to delete the subdirectory and do it. How can you check to make sure that the subdirectory was deleted?

The pwd command

Sometimes, if you have been changing from one directory to another, you may not recall exactly where you are in the hierarchical structure. The pwd command allows you to determine this information. Its basic form is given by:


The output of this command gives you the path to your working directory, that is, the directory in which you are working.

In your home directory enter the pwd command. If the root directory is considered level 0 in the directory hierarchy, at what level is your home directory?

The mv command

You can move a file from one directory to another by using the mv ("move") command. Its basic form is given by:

mv fileName newFileOrDirectoryName

where fileName is the name of the file you wish to move and newFileOrDirectoryName is the name of the file in its new location. That is, you can change the name of the file as you move it. If newFileOrDirectoryName is the name of a directory, then the file is moved to that directory and keeps the same name.

Since the name of the file can be changed as you move it, you can use this command for renaming a file. For example, if you type:

mv oldFileName newFileName

then the file called oldFileName gets renamed to newFileName without moving out of the directory in which it is located.

You can even use this command for changing the names of directories. Simply replace oldFileName by oldDirectoryName and newFileName by newDirectoryName in the previous example.

Change the name of the temp.lab directory that you created when working with the mkdir command. Make its new name labFiles.

Go to your newly named subdirectory (labFiles) and move the file named hosts (which you copied from /etc/hosts) into your home directory.

The chmod command

In several of the Unix commands, you can only carry out the command if you have permission to do so. For example, try copying any file (e.g. the hosts file that you obtained earlier) into the root directory with the command:

cp hosts /

You should have obtained a Permission denied message. You don't (and shouldn't) have permission to place things in the root directory. Only the system administrator should be able to do this.

Suppose you have a file that you would like to keep private so that others would not be able to view it. Suppose you have another file that you would like to make available for others to copy.

In short, you would like to be able to set up permissions to restrict users from some files and to allow users to access other files. The Unix command to do this is called chmod (for "change mode").

Unix recognizes three classes of users when looking at permissions related to directories or files:

Each file has three kinds of permissions.

Go to your home directory and type the command:

ls -al

In the first column you will see something like




The first letter will be d if the named item is a directory and will be a - character if it is a file. After that the letters appear in clusters of three corresponding to the sequence:


The first cluster shows the permissions for the user/owner, the second cluster shows the permissions for the group, and the third cluster shows the permissions for other users. If the appropriate letter is present, then that permission is granted to that user or those users. In the file example just above (the one beginning with the - character), the owner has read, write, and execute permissions, the group has read and execute permissions, and other users have read and execute permissions.

If you have read permission then you can see and copy the file. If you have write permission, then you can change (write) the file. If you have execute permission, then you can execute the file (assuming it is an executable file). For directories, execute permission means that one can go to that directory.

Conversely, if you don't want someone to be able to copy a file, then they should not have read permission. If you don't want someone to be able to change a file, then they should not have write permission. If you don't want someone to be able to go to one of your subdirectories (or even your home directory) then they should not have execute permission.

The chmod command allows you to change the permissions of a file or directory by specifying the set of users you want to affect, the change (either granting or denying permission), and the permission category. For example to give all other users read permission to the file named sharedFile, enter the command:

chmod o+r sharedFile

The 'o' means "other users", the '+' means to grant permission, the 'r' means "read permission". These options are placed in this order with no space between characters.

As another example, to deny access to other users to the directory named privateFiles, enter the command:

chmod o-x privateFiles

The '-' means to deny permission.

Assuming you have a subdirectory named labFiles, find out what permissions other users have. Now change the permissions so that no other user can see inside that subdirectory.

How would you change the permissions in your home directory so that no one else could access your directory? (Recall that '.' can be used to represent the "current directory".)

How would you change permissions so that all other users can copy a file (called sharedFile) in a subdirectory of yours called labFiles? (In order for someone to copy a file, they have to be able to access all the directories including your home directory and any subdirectories leading to that file.)

Click on to go back to the main directory.

Click on to take the quiz for this module.

These pages were developed by John Avila SJSU CS Dept.