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2. The Basics
Contents:
2.1 Files and Directories
2.1.1 Getting Started: Navigating Your Directories
2.1.2 Some Common Unix Commands
2.2 Basic Commands
2.2.1 Command Arguments
2.2.2 Special Characters
2.2.3 One Pattern, Many Instances
2.2.4 Quoting
2.3 Modifying and Combining Commands
2.3.1 Redirection
2.3.2 Pipes
2.3.3 xargs
2.3.4 find
2.4 Protection
2.4.1 Protection and Permissions
2.4.2 chmod
2.4.3 Beware the umask!
2.4.4 Planning for Protection
2.1. Files and Directories
Contents:
2.1.1 Getting Started: Navigating Your Directories
2.1.2 Some Common Unix Commands
2.1.2.1 Session Control
2.1.2.2 File and Directory Manipulation
2.1.2.3 Text File Manipuation
2.1.2.4 Printing Files

Files in Unix are organized by listing them in directories. Directories are themselves files, and so may appear within other directories. The result is a tree-like hierarchy. At the root of this tree is a directory known simply as “/” .6

It may be more precise to say that this directory's name is the empty string “” .
This directory lists various others:

The bin directory contains many of the programs for performing common Unix commands. The usr directory contains many of the data files that are required by those and other commands. Of particular interest, however, is the home directory, which contains all of the files associated with individual users like you and me. Each individual user gets a directory within home bearing their own login name. My login name is zeil.

We can expand our view of the Unix files then as:

cd and ls are two common Unix commands, as will be explained later.

Within my own home directory, I have a directory also named “bin” , containing my own personal programs. Two of these are called “clpr” and “psnup” . So these files are arranged as:

The full name of any file is given by listing the entire path from the root of the directory tree down to the file itself, with “/” characters separating each directory from what follows. For example, the full names of the four programs in the above diagram are

/bin/cd /bin/ls /home/zeil/bin/clpr /home/zeil/bin/psnup

There are some common abbreviations that can be used to shorten file names.

  • You can refer to the home directory of someone with login name name as ~name.
  • You can refer to your own home directory simply as ~.

    So you could refer to the file containing my clpr program as either /home/zeil/bin/clpr or ~zeil/bin/clpr.

    When I myself am logged in, I can refer to this program by either of those two names, or simply as ~/bin/clpr.

  • At all times when entering Unix commands, you have a “working” directory. If the file you want is within that directory (or within other directories contained in the working directory), the name of the working directory may be omitted from the start of the filename. When you first log in, your home directory is your working directory. For example, when I have just logged in, I could refer to my program simply as bin/clpr, dropping the leading /home/zeil/ because that would be my working directory at that time.
  • The working directory itself can be referred to as simply “.” .
  • The “parent” of the working directory (i.e., the directory containing the working directory) can be referred to as “..” .

Unix filenames can be almost any length and may contain almost any characters. As a practical matter, however, you should avoid using punctuation characters other than the hyphen, the underscore, and the period. Also, avoid blanks, and non-printable characters within file names. All of these have special meanings when you are typing commands and so would be very hard to enter within a filename.

Some things to keep in mind about Unix file names that may be different from other file systems you have used:

  • Unix file names are often very long so that they describe their contents.7
    As we will see, one almost never needs to type an entire filename in a Unix command, so long file names are no harder to work with than short ones.
    The rather perverse exception to this rule is that program/command names are, by tradition, very short, often confusingly so.
  • Upper and lower case letters are distinct in Unix filenames. “MyFile” and “myfile” are different names.
  • Periods ( “.” ) are not treated by Unix as a special character. “This.Is.a.legal.name” is perfectly acceptable as a Unix filename. Many programs, however, expect names of their data files to end in a period followed by a short “standard” extension indicating the type of data in that file. Thus data files with names like “arglebargle.txt” for text files or “nonsense.cpp” for C++ source code are common.

    By convention, files containing executable programs generally do not receive such an extension.

  • Keep in mind that directories are separated by “/” in file names, not by “\” as is common in some other operating systems.

2.1.1. Getting Started: Navigating Your Directories

If you have not yet done so, log in now so that you can work though the following commands.

Throughout this course you will encounter sections labelled "Try This:". That means that I really want you to log in, try the commands or procedures I describe, and observe and think about the results.

If you don't understand the output you receive, please use the symbol at the top of the page to post a question in the Forum.

Some of the “Try This:” exercises will build on the earlier ones, so skipping these early examples may lead to problems later on.

Try This:

Upon logging in, your working directory should be your home directory. The command pwd will print the working directory. Give the command

pwd
You should see something like
/home/yourname

Now, let's make a place to play in. mkdir will make a new directory. Enter the command

mkdir playing
to create a directory named “playing” .

The command ls lists the contents of the working directory. More generally, ls directoryname will list the contents of any directory.8

Well, not really any directory. People can protect their own directories from the prying eyes of others, in which case ls will fail.

Give the command

ls
and you should see playing listed. In fact, it may be the only thing listed.

The command cd is used to change the working directory. Give the command sequence

pwd cd playing pwd cd .. pwd cd ./playing pwd
to see this in action.

The cp command copies one or more files. You can give this command as cp file1 file2 to make a copy of file file1, the copy being named file2. Alternatively, you can copy one or more files into a directory by giving the command as

cp file1 file2…filen directory

Try This:

Now try the following:

ls /usr/include
You should see a large number of files, including many ending in “.h” . Copy two of these files and check to see that the copy was successful, as follows:
cp /usr/include/math.h /usr/include/stdio.h ~/playing ls ~/playing

8. Well, not really any directory. People can protect their own directories from the prying eyes of others, in which case ls will fail. (back)
2.1.2. Some Common Unix Commands
Contents:
2.1.2.1 Session Control
2.1.2.2 File and Directory Manipulation
2.1.2.3 Text File Manipuation
2.1.2.4 Printing Files

Here are some common Unix commands. Some of these will be discussed in more detail later, but if this list at least makes you aware of the existence of a command that does something you want, then you can always get the details on the command by reading its on-line manual page via the man command:

man command-name

You might try experimenting with these in your ~/playing directory.

2.1.2.1. Session Control

exit
Shut down the current shell. If this shell is the one you got at log-in, this command logs you out.
rlogin machine
Logs you in to another machine on the network. Use this if the machine you are on seems to be running slowly and the who command indicates that there are lots of others on the same machine.

who
Lists everyone logged into the same machine that you are using.

2.1.2.2. File and Directory Manipulation

cd directory
Changes your current working directory to the indeicated directory, which may be absolute or relative.
find directory instructions
Searches the indicated directory and any subdorectories within it for files. The instructions may serve to limit the files found (e.g., search for files with a given name or that have been modified after a given date) or may indicate commands to run on those files.
ls path
Lists all the files matching the indicated path. If that path is a directory, lists all the files within that directory. You can provide multiple paths in the same command, or omit the path entirely (in which case the contents of your current working directory will be listed). ls can be modified with a number of “flags” . Some of the more common are:
ls -a
By default, filenames beginning with “.” are considered “hidden” and not shown by the ls command. The -a (for “all” ) option causes these files to be shown as well.
ls -l
This is the “long” form of ls. It displays additional information about each file, such as the size and date on which the file was last modified.

Note that options can be combined. For example, you can say ls -la to get extra information including normally hidden files.

ls -F
Adds a little bit of extra information to each file name. If the file is an executable program, its name is marked with an “*” . If the file is a directory, its name is marked with “/” .
mkdir directory
Creates a directory.
mv file1 file2
Renames file1 as file2. file2 may be in a different directory.
mv file1 directory
Moves file1 to the given directory.
pwd
Prints your current working directory.
rm file1…filen
Deletes the listed files. Be very careful using wildcards with this command. rm * will delete everything in the current working directory!
rm -i file1…filen
Deletes the listed files, but firsts asks permission to delete each one.
rm -r file1…filen
Deletes the listed files. If any of these files is a directory, it deletes that directory and everything in it as well.
rmdir directory
Deletes a directory (if empty).

2.1.2.3. Text File Manipuation

cat file1…filen
Lists the contents of each of the listed files on your screen.
grep text-pattern file1…filen
Searches the files for lines matching the indicated text pattern and prints the matching lines.
more file1…filen
Lists files one screen at a time, pausing after each screen-full. Hit the space bar to advance to the next screen.

A related program is less, which also allows you to move backwards through the files by hitting “b” .

sed editing-instructions file
Applies the editing instructions to each line of the file, writing out the resulting changed version of the file.

2.1.2.4. Printing Files

cancel request
Remove a file from the printer queue, so that it won't get printed. The request identifier is found using the lpstat command.
lp file
Send file to printer for printing. Most sites have multiple printers, each having its own name. One of these will be the “default” printer used by the lp command. For the others, you must give the printer name as part of the printer command:
lp -d printer file
For example, at the Norfolk ODU campus, lp by itself prints to a fast printer in the public workstation lab for text only. lp -dcookie prints to “cookie” , a printer in the same room that offers the extra capability of printing Postscript graphics.

You will need to consult your local staff to see what printers are available at other sites.

lpstat
Shows the list of files you have “queued up” awaiting their turn on printers. Entered by itself,
lpstat
it lists only your own print jobs, giving a unique identifier for each one.

To see the entire list of print jobs on some printer, enter

lpstat -o printer

As you explore Unix, you are bound to have questions. Some ways to get answers include:

  • The entire Unix manual is on-line.
    man command
    displays the manual page for the given command.

    man -k keyword
    looks up the given keyword in an index and lists the commands that may be relevant.

  • The CS Department systems staff has collected a variety of additional help documents. You can find them by going to the Dept home page (http://www.cs.odu.edu/) and selecting “Frequently Asked Questions” under the “Systems Group” heading.
  • A staff member is generally on duty or on call in the public CS lab in Hughes Hall (on the Norfolk campus) whenever that room is open.
  • If none of the above help, then send e-mail to “root@cs.odu.edu” . This is also how you report bugs, machine failures, etc.

6. It may be more precise to say that this directory's name is the empty string “” . (back)
7. As we will see, one almost never needs to type an entire filename in a Unix command, so long file names are no harder to work with than short ones. (back)

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