Description
Linux file system structure
Unlike a simple standalone personal computer, the file system has components that may exist on many different
machines, but the operating system makes it look like the file system exists on the computer you are using. The file
system on a Linux system is conceptually hierarchical (has a tree structure). The “root” of the tree is shown at the
top of the figure below and is designated by the slash character (‘/’). Every item in the file system is either a file or
a directory (known as a “folder” in Windows-talk). The concept is essentially the same as in DOS or Windows: a
directory can contain other directories and files. A file can contain data, but cannot contain other files or
directories.
At any time while using the system, you can think of yourself as being “located” somewhere in the file system. Your
current location in the file system is called your current working directory or pwd.
To describe a specific location in the file system hierarchy, you must specify a “path.” The path to a location can be
defined as an absolute path from the root anchor point, or as a relative path, starting from your pwd. When
specifying a path, you simply trace a route through the file system tree, listing the sequence of directories you pass
through as you go from one point to another. Each directory listed in the sequence is separated by a slash (a
forward slash /, not a DOS-type backslash \). Linux also provides the shorthand notation of “.” to refer to the
current location, and “..” to refer to the parent directory.
For example, the absolute path to the bin directory that is in the usr directory would be /usr/bin; we start at the
root (/), walk to the usr directory, then walk to the bin directory. Notice that there is another directory named
“bin”. The absolute path to this one is /bin, so it’s not the same as /usr/bin.
TASK 1:
1. Write down the absolute path to the directory named include on the far left of the figure.
Relative paths use the same notation, but they do not start at the root of the tree. For example, if your cwd is
/usr/bin, the relative path to the /usr/include directory would be ../include. The “..” takes you one step up the
tree, putting you in the /usr directory. From there you go down the tree to the /usr/include directory.
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TASK 2:
1. Write down the relative path from the directory /dev to the file /bin/ls.
File and directory permissions
Remember what the meanings of r, w, and x are for directories and for files:
Permission Effect on files Effect on directories
Read (r) May view file contents. May list directory contents.
Write (w) May modify file contents. May modify directory contents (e.g.,
delete a file).
Execute (x) May run the file, if it’s an executable program
or script. May cd into the directory.
Setting permissions
Linux allows you to set the permissions on files that you own. The command to change the file permission mode is
chmod (some people pronounce it “shh-mod”). The chmod requires you to specify the new permissions you want,
and specify the file or directory you want the changes applied to.
The easy way to set file permissions, is by use of the “rwx” notation to specify the type of permissions, and the
“ugo” (u = user, the owner; g = group; o = others) notation to specify those the permissions apply to.
To define the kind of change you want to make to the permissions, use the plus sign (+) to add a permission, the
minus sign (-) to remove a permission, and the equal sign (=) to set a permission directly.
For example, to give members of the group permission to write (modify) a file named foo, you would invoke the
command chmod g+w foo
TASK 3:
1. Create a file ~/foo (the ~ notation is shorthand for your home directory). Do this by invoking the
command touch ~/foo.
2. Write out your answers to the following questions:
a. What are the permissions on the file?
b. Who owns the file?
c. What group is associated with the file?
d. Are you in the group? (Invoke groups to find out).
e. Write down the names of all the groups you are in.
3. Change the permissions on ~/foo so owner has only execute permission, group has only write permission,
and all others have both read and execute permission. Write out the command(s) you used to do this.
4. Attempt to delete the file. Write down the command you used and what happened.
5. Change permissions on ~/foo so you can delete it, then do so.
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Linux Documentation
Linux provides on-line documentation for just about every system command or function. There are basically three
ways to access the documentation (not counting documentation available on the web): man (manual) pages, the
info system, and apropos.
man pages
Manual pages are displayed by the man command. Man pages give in-depth information on commands and
functions. The system is divided into sections. The sections are not necessarily the same on every system, but
typically they are as follows:
Section Contents
1 User Commands
2 System Calls
3 Library Calls
4 Special Files
5 File Formats and Conversions
6 Games
7
Macro Packages and
Conventions
8
System Management
Commands
9 Kernel Routines
For example, to view the manual page for the system call lseek, you would invoke man 2 lseek Actually, the “2” is
optional in this case since there is only one lseek. The section number is only needed when there are multiple
entities with the same name, such as time which is the name of both a user command and a system call.
TASK 4:
1. Invoke the command man ls to view the manual page for the user command ls (you could also try
man 1 ls to see that the section number is optional in this case). You should see something that starts like
the following:
LS(1) User Commands LS(1)
NAME ls – list directory contents
SYNOPSIS ls [OPTION]… [FILE]…
DESCRIPTION List information about the FILEs (the current directory by
default). Sort entries alphabetically if none of -cftuSUX nor
–sort.
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Every man page has this structure. The first line gives the command and the section. The name of the
command is given along with a short description. The synopsis is often the most important information. It
shows the forms the command might take with options and arguments. Options or arguments in square
brackets, [], are optional. The description section gives a more detailed description of the command. In
this example, all options and files are optional, so the command ls is enough to list the contents of the
current directory.
2. There are many options available for ls but the most important ones are a,l, and R. Write down the
meanings of each of these options.
3. Move to your home directory and invoke ls with no options. Now invoke ls -l. Now invoke ls -a. Write
down how the output of ls differs from the output of ls -a.
4. Different entities may have the same name. For example time as a user command differs from time as a
system call. Write down the NAME information for both of these time entities.
5. Of course, man itself is a user command, so it should have a man page. Write down the NAME
information for the man page on man.
info
The info system gives much the same information as man, but in a different, more searchable format.
apropos
The apropos command reports on any uses of the given word in the man pages. It can be useful for finding man
pages when you don’t remember the exact name of a command. For example, the command apropos pwd lists all
the man pages that have the word pwd in them. Try it out. Sometimes you get more than you want. For example,
try apropos time.
Changing Directories
When you log in, you are automatically placed in your “home directory.” To see where you are, type the command
pwd which stands for “print working directory.”
To change your location in the file system hierarchy, use the cd (change directory) command, followed by an
argument defining where you want to go. The argument can be either an absolute path to the destination, or a
relative path. (If you type the cd command without an argument, the shell will place you in your home directory.)
TASK 5:
1. Check that you are in your home directory. If not, get there.
2. Write down the result of invoking the command pwd.
3. Walk one level up the file system hierarchy using the cd command. Write down the exact command you
invoked.
4. Write down the result of invoking the command pwd.
5. Write down the permissions on your home directory.
6. Get back to your home directory.
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Viewing the type and contents of files
You can determine the type of an unknown file by using the file command. This command can be useful to check
that a file is a text file before attempting to view its contents. Only text files should be directly viewed since other
types of files (such as executables) probably contain non-printing characters. The file also works with directories.
TASK 6:
1. Write down the file type of /bin/bash.
2. Create an empty file named foo. Write down the file type of this file.
3. Write down the file type of the file ~/.bash_history (yes, there is a dot before the file name. It indicates
that this is a “hidden” file, not displayed with ls by default.)
4. Write down the file type of /usr/local.
TASK 7: In this exercise, you will be introduced to a few commands for displaying the contents
of a text file.
1. The cat command catenates files and sends them to standard output (namely, the screen). cat does not
format the text in any way, and long output may scroll off the screen before you can read it. Use cat to
display the contents of ~/.bash_history. Nothing to write down.
2. The less and more commands display a text file, one screenful at a time. You can scroll forward a line at a
time by pressing the return key, or a screenful at a time by pressing the spacebar. You can quit at any time
by pressing the q key. Use less to display the contents of ~/.bash_history. Nothing to write down.
3. Now, use more to do the same thing. Nothing to write down.
4. The head command allows you to see the top part of a file. You may specify the number of lines you want,
or default to ten lines. Use head to display the first 5 lines of ~/.bash_history. Write down the exact
command you used.
5. The tail command works like head, except that it shows the last lines of a file. Use tail to display the last
10 lines of ~/.bash_history. Write down the exact command you used.
Copying files and directories
The Linux command to copy a file or directory is cp. The basic cp command syntax is cp source destination
(meaning copy the “source” file to the “destination” file).
TASK 8:
1. Copy the file ~/.bash_history to the file ~/dotbashhistory. Write down the exact command you used.
2. Write down the file and directory permissions necessary to successfully copy a file in general.
3. Invoke the exact command again in an attempt to copy the “source” to the now-existing “destination.”
Write down how the system handled this.
4. Compare the permissions and date on the original file and the copy. Write down if and how they differ.
5. Write down the cp command you would use to insure that the copy has the same permissions and date as
the original.
Don’t delete your ~/dotbashhistory, you wil
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Moving and renaming files
The Linux mv command moves files and directories. You can move a file to a different location in the filesystem, or
change the name by moving the file within the current location.
TASK 9:
1. Rename your ~/dotbashhistory as ~/dotbashhistory.old. Write down the exact command you used.
Removing files
The rm command is used for removing files and directories. The syntax of the basic rm command is rm filename.
TASK 10:
1. Remove ~/dotbashhistory.old. Write down the exact command you used.
Creating a directory
The Linux mkdir command is used to make directories. The basic syntax is mkdir directoryname. If you do not
specify the full path to the directory created, the new directory will be placed within the current working directory.
TASK 11:
1. Create the directory ~/Foo. Write down the exact command you used.
2. Write down the permissions of this new directory.
3. Create a directory named Bar, within the Foo directory. Write down the exact command you used.
Removing a directory
The Linux rmdir command removes a directory from the filesystem. By default, the directory to be removed must
be empty.
TASK 12:
1. Write down what happens when you invoke rmdir ~/Foo.
2. Write down a sequence of rmdir commands that would allow removal of the ~/Foo directory and its
subdirectory Bar. Don’t invoke the commands, just write down what you would do.
3. Invoke a single rm command to remove the ~/Foo directory and all its contents (check the man page).
Write down the exact command you used. Hint: it involves two flags, one to force the operation, the
other to descend the directory recursively.
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What to Hand In
Please organize your materials in the order given below. Clearly identify your materials by task. Write your name
on every page. Staple the pages neatly.
Task 1:
• The absolute path to the directory named include on the far left of the figure.
Task 2:
• The relative path from the directory /dev to the file /bin/ls.
Task 3:
• Answers to the four questions
• Names of all the groups you are in.
• Command(s) used to change the permissions.
Task 4:
• Meanings of the three options.
• How the output of ls differs from ls -a.
• The NAME information for time as user command and as system call.
• The NAME information for man.
Task 5:
• Result of invoking the command pwd.
• Command to go up one level in file system.
• Result of invoking the command pwd again.
• Permissions on your home directory.
Task 6:
• File type of /bin/bash.
• File type of foo.
• File type of ~/.bash_profile
• File type of /usr/local.
Task 7:
• Command used to display first 5 lines of file
• Command used to display last 10 lines of file
Task 8:
• Command to copy ~/.bash_history
• File and directory permissions necessary to make the copy.
• Result of trying the copy again.
• Differences in permissions and date of copy and original
• cp command to insure copy has same permissions and date as original.
Task 9:
• Command to rename .bash_history
Task 10:
• Command to remove ~/dotbashhistory.old.
Task 11:
• Command to create directory ~/Foo.
• Permissions of this new directory.
• Command to create directory Bar, within Foo
Task 12:
• Result of invoking rmdir ~/Foo.
• Sequence of rmdir commands that would be used.
• Single rm command to remove ~/Foo and all its contents.
