Directories:
Moving around the file system:
Listing directory contents:
Changing file permissions and attributes
Moving, renaming, and copying files:
Viewing and editing files:
Shells
Environment variables
Interactive History
Filename Completion
Bash is the way cool shell.
Redirection:
Pipes:
Command Substitution
Searching for strings in files: The grep command
Searching for files : The find command
Reading and writing tapes, backups, and archives: The tar command
File compression: compress, gzip, and bzip2
Looking for help: The man and apropos commands
Basics of the vi editor
FAQs
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Basic UNIX Command Line (shell) navigation : Last revised May 17 2001
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Directories:
File and directory paths in UNIX use the forward slash "/"
to separate directory names in a path.
examples:
/ "root" directory
/usr directory usr (sub-directory of / "root" directory)
/usr/STRIM100 STRIM100 is a subdirectory of /usr
Moving around the file system:
pwd Show the "present working directory", or current directory.
cd Change current directory to your HOME directory.
cd /usr/STRIM100 Change current directory to /usr/STRIM100.
cd INIT Change current directory to INIT which is a sub-directory of the current
directory.
cd .. Change current directory to the parent directory of the current directory.
cd $STRMWORK Change current directory to the directory defined by the environment
variable 'STRMWORK'.
cd ~bob Change the current directory to the user bob's home directory (if you have permission).
Listing directory contents:
ls list a directory
ls -l list a directory in long ( detailed ) format
for example:
$ ls -l
drwxr-xr-x 4 cliff user 1024 Jun 18 09:40 WAITRON_EARNINGS
-rw-r--r-- 1 cliff user 767392 Jun 6 14:28 scanlib.tar.gz
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
owner group size date time name
number of links to file or directory contents
permissions for world
permissions for members of group
permissions for owner of file: r = read, w = write, x = execute -=no permission
type of file: - = normal file, d=directory, l = symbolic link, and others...
ls -a List the current directory including hidden files. Hidden files start
with "."
ls -ld * List all the file and directory names in the current directory using
long format. Without the "d" option, ls would list the contents
of any sub-directory of the current. With the "d" option, ls
just lists them like regular files.
Changing file permissions and attributes
chmod 755 file Changes the permissions of file to be rwx for the owner, and rx for
the group and the world. (7 = rwx = 111 binary. 5 = r-x = 101 binary)
chgrp user file Makes file belong to the group user.
chown cliff file Makes cliff the owner of file.
chown -R cliff dir Makes cliff the owner of dir and everything in its directory tree.
You must be the owner of the file/directory or be root before you can do any of these things.
Moving, renaming, and copying files:
cp file1 file2 copy a file
mv file1 newname move or rename a file
mv file1 ~/AAA/ move file1 into sub-directory AAA in your home directory.
rm file1 [file2 ...] remove or delete a file
rm -r dir1 [dir2...] recursivly remove a directory and its contents BE CAREFUL!
mkdir dir1 [dir2...] create directories
mkdir -p dirpath create the directory dirpath, including all implied directories in the path.
rmdir dir1 [dir2...] remove an empty directory
Viewing and editing files:
cat filename Dump a file to the screen in ascii.
more filename Progressively dump a file to the screen: ENTER = one line down
SPACEBAR = page down q=quit
less filename Like more, but you can use Page-Up too. Not on all systems.
vi filename Edit a file using the vi editor. All UNIX systems will have vi in some form.
emacs filename Edit a file using the emacs editor. Not all systems will have emacs.
head filename Show the first few lines of a file.
head -n filename Show the first n lines of a file.
tail filename Show the last few lines of a file.
tail -n filename Show the last n lines of a file.
Shells
The behavior of the command line interface will differ slightly depending
on the shell program that is being used.
Depending on the shell used, some extra behaviors can be quite nifty.
You can find out what shell you are using by the command:
echo $SHELL
Of course you can create a file with a list of shell commands and execute it like
a program to perform a task. This is called a shell script. This is in fact the
primary purpose of most shells, not the interactive command line behavior.
Environment variables
You can teach your shell to remember things for later using environment variables.
For example under the bash shell:
export CASROOT=/usr/local/CAS3.0 Defines the variable CASROOT with the value
/usr/local/CAS3.0.
export LD_LIBRARY_PATH=$CASROOT/Linux/lib Defines the variable LD_LIBRARY_PATH with
the value of CASROOT with /Linux/lib appended,
or /usr/local/CAS3.0/Linux/lib
By prefixing $ to the variable name, you can evaluate it in any command:
cd $CASROOT Changes your present working directory to the value of CASROOT
echo $CASROOT Prints out the value of CASROOT, or /usr/local/CAS3.0
printenv CASROOT Does the same thing in bash and some other shells.
Interactive History
A feature of bash and tcsh (and sometimes others) you can use
the up-arrow keys to access your previous commands, edit
them, and re-execute them.
Filename Completion
A feature of bash and tcsh (and possibly others) you can use the
TAB key to complete a partially typed filename. For example if you
have a file called constantine-monks-and-willy-wonka.txt in your
directory and want to edit it you can type 'vi const', hit the TAB key,
and the shell will fill in the rest of the name for you (provided the
completion is unique).
Bash is the way cool shell.
Bash will even complete the name of commands and environment variables.
And if there are multiple completions, if you hit TAB twice bash will show
you all the completions. Bash is the default user shell for most Linux systems.
Redirection:
grep string filename > newfile Redirects the output of the above grep
command to a file 'newfile'.
grep string filename >> existfile Appends the output of the grep command
to the end of 'existfile'.
The redirection directives, > and >> can be used on the output of most commands
to direct their output to a file.
Pipes:
The pipe symbol "" is used to direct the output of one command to the input
of another.
For example:
ls -l more This commands takes the output of the long format directory list command
"ls -l" and pipes it through the more command (also known as a filter).
In this case a very long list of files can be viewed a page at a time.
du -sc * sort -n tail
The command "du -sc" lists the sizes of all files and directories in the
current working directory. That is piped through "sort -n" which orders the
output from smallest to largest size. Finally, that output is piped through "tail"
which displays only the last few (which just happen to be the largest) results.
Command Substitution
You can use the output of one command as an input to another command in another way
called command substitution. Command substitution is invoked when by enclosing the
substituted command in backwards single quotes. For example:
cat `find . -name aaa.txt`
which will cat ( dump to the screen ) all the files named aaa.txt that exist in the current
directory or in any subdirectory tree.
Searching for strings in files: The grep command
grep string filename prints all the lines in a file that contain the string
Searching for files : The find command
find search_path -name filename
find . -name aaa.txt Finds all the files named aaa.txt in the current directory or
any subdirectory tree.
find / -name vimrc Find all the files named 'vimrc' anywhere on the system.
find /usr/local/games -name "*xpilot*"
Find all files whose names contain the string 'xpilot' which
exist within the '/usr/local/games' directory tree.
Reading and writing tapes, backups, and archives: The tar command
The tar command stands for "tape archive". It is the "standard" way to read
and write archives (collections of files and whole directory trees).
Often you will find archives of stuff with names like stuff.tar, or stuff.tar.gz. This
is stuff in a tar archive, and stuff in a tar archive which has been compressed using the
gzip compression program respectivly.
Chances are that if someone gives you a tape written on a UNIX system, it will be in tar format,
and you will use tar (and your tape drive) to read it.
Likewise, if you want to write a tape to give to someone else, you should probably use
tar as well.
Tar examples:
tar xv Extracts (x) files from the default tape drive while listing (v = verbose)
the file names to the screen.
tar tv Lists the files from the default tape device without extracting them.
tar cv file1 file2
Write files 'file1' and 'file2' to the default tape device.
tar cvf archive.tar file1 [file2...]
Create a tar archive as a file "archive.tar" containing file1,
file2...etc.
tar xvf archive.tar extract from the archive file
tar cvfz archive.tar.gz dname
Create a gzip compressed tar archive containing everything in the directory
'dname'. This does not work with all versions of tar.
tar xvfz archive.tar.gz
Extract a gzip compressed tar archive. Does not work with all versions of tar.
tar cvfI archive.tar.bz2 dname
Create a bz2 compressed tar archive. Does not work with all versions of tar
File compression: compress, gzip, and bzip2
The standard UNIX compression commands are compress and uncompress. Compressed files have
a suffix .Z added to their name. For example:
compress part.igs Creates a compressed file part.igs.Z
uncompress part.igs Uncompresseis part.igs from the compressed file part.igs.Z.
Note the .Z is not required.
Another common compression utility is gzip (and gunzip). These are the GNU compress and
uncompress utilities. gzip usually gives better compression than standard compress,
but may not be installed on all systems. The suffix for gzipped files is .gz
gzip part.igs Creates a compressed file part.igs.gz
gunzip part.igs Extracts the original file from part.igs.gz
The bzip2 utility has (in general) even better compression than gzip, but at the cost of longer
times to compress and uncompress the files. It is not as common a utility as gzip, but is
becoming more generally available.
bzip2 part.igs Create a compressed Iges file part.igs.bz2
bunzip2 part.igs.bz2 Uncompress the compressed iges file.
Looking for help: The man and apropos commands
Most of the commands have a manual page which give sometimes useful, often more or less
detailed, sometimes cryptic and unfathomable discriptions of their usage. Some say they
are called man pages because they are only for real men.
Example:
man ls Shows the manual page for the ls command
You can search through the man pages using apropos
Example:
apropos build Shows a list of all the man pages whose discriptions contain the word "build"
Do a man apropos for detailed help on apropos.
Basics of the vi editor
Opening a file
vi filename
Creating text
Edit modes: These keys enter editing modes and type in the text
of your document.
i Insert before current cursor position
I Insert at beginning of current line
a Insert (append) after current cursor position
A Append to end of line
r Replace 1 character
R Replace mode
Deletion of text
x Delete single character
dd Delete current line and put in buffer
ndd Delete n lines (n is a number) and put them in buffer
J Attaches the next line to the end of the current line (deletes carriage return).
Oops
u Undo last command
cut and paste
yy Yank current line into buffer
nyy Yank n lines into buffer
p Put the contents of the buffer after the current line
P Put the contents of the buffer before the current line
cursor positioning
^d Page down
^u Page up
:n Position cursor at line n
:$ Position cursor at end of file
^g Display current line number
h,j,k,l Left,Down,Up, and Right respectivly. Your arrow keys should also work if
if your keyboard mappings are anywhere near sane.
string substitution
:n1,n2:s/string1/string2/[g] Substitute string2 for string1 on lines
n1 to n2. If g is included (meaning global),
all instances of string1 on each line
are substituted. If g is not included,
only the first instance per matching line is
substituted.
^ matches start of line
. matches any single character
$ matches end of line
These and other "special characters" (like the forward slash) can be "escaped" with \
i.e to match the string "/usr/STRIM100/SOFT" say "\/usr\/STRIM100\/SOFT"
Examples:
:1,$:s/dog/cat/g Substitute 'cat' for 'dog', every instance
for the entire file - lines 1 to $ (end of file)
:23,25:/frog/bird/ Substitute 'bird' for 'frog' on lines
23 through 25. Only the first instance
on each line is substituted.
Saving and quitting and other "ex" commands
These commands are all prefixed by pressing colon (:) and then entered in the lower
left corner of the window. They are called "ex" commands because they are commands
of the ex text editor - the precursor line editor to the screen editor
vi. You cannot enter an "ex" command when you are in an edit mode (typing text onto the screen)
Press
:w Write the current file.
:w new.file Write the file to the name 'new.file'.
:w! existing.file Overwrite an existing file with the file currently being edited.
:wq Write the file and quit.
:q Quit.
:q! Quit with no changes.
:e filename Open the file 'filename' for editing.
:set number Turns on line numbering
:set nonumber Turns off line numbering
Permissions
The Unix operating system (and likewise, Linux) differs from other computing environments in that it is not only a multitasking system but it is also a multi-user system as well.
What exactly does this mean? It means that more than one user can be operating the computer at the same time. While your computer will only have one keyboard and monitor, it can still be used by more than one user. For example, if your computer is attached to a network, or the Internet, remote users can log in via telnet or ssh (secure shell) and operate the computer. In fact, remote users can execute X applications and have the graphical output displayed on a remote computer. The X Windows system supports this.
The multi-user capability of Unix is not a recent "innovation," but rather a feature that is deeply ingrained into the design of the operating system. If you remember the environment in which Unix was created, this makes perfect sense. Years ago before computers were "personal," they were large, expensive, and centralized. A typical university computer system consisted of a large mainframe computer located in some building on campus and terminals were located throughout the campus, each connected to the large central computer. The computer would support many users at the same time.
In order to make this practical, a method had to be devised to protect the users from each other. After all, you could not allow the actions of one user to crash the computer, nor could you allow one user to interfere with the files belonging to another user.
This lesson will cover the following commands:
chmod - modify file access rights
su - temporarily become the superuser
chown - change file ownership
chgrp - change a file's group ownership
File permissions
Linux uses the same permissions scheme as Unix. Each file and directory on your system is assigned access rights for the owner of the file, the members of a group of related users, and everybody else. Rights can be assigned to read a file, to write a file, and to execute a file (i.e., run the file as a program).
To see the permission settings for a file, we can use the ls command as follows:
[me@linuxbox me]$ ls -l some_file
-rw-rw-r-- 1 me me 1097374 Sep 26 18:48 some_file
We can determine a lot from examining the results of this command:
The file "some_file" is owned by user "me"
User "me" has the right to read and write this file
The file is owned by the group "me"
Members of the group "me" can also read and write this file
Everybody else can read this file
Let's try another example. We will look at the bash program which is located in the /bin directory:
[me@linuxbox me]$ ls -l /bin/bash
-rwxr-xr-x 1 root root 316848 Feb 27 2000 /bin/bash
Here we can see:
The file "/bin/bash" is owned by user "root"
The superuser has the right to read, write, and execute this file
The file is owned by the group "root"
Members of the group "root" can also read and execute this file
Everybody else can read and execute this file
In the diagram below, we see how the first portion of the listing is interpreted. It consists of a character indicating the file type, followed by three sets of three characters that convey the reading, writing and execution permission for the owner, group, and everybody else.
chmod
The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify. There are two ways to specify the permissions, but I am only going to teach one way.
It is easy to think of the permission settings as a series of bits (which is how the computer thinks about them). Here's how it works:rwx rwx rwx = 111 111 111
rw- rw- rw- = 110 110 110
rwx --- --- = 111 000 000
and so on...
rwx = 111 in binary = 7
rw- = 110 in binary = 6
r-x = 101 in binary = 5
r-- = 100 in binary = 4
Now, if you represent each of the three sets of permissions (owner, group, and other) as a single digit, you have a pretty convenient way of expressing the possible permissions settings. For example, if we wanted to set some_file to have read and write permission for the owner, but wanted to keep the file private from others, we would:
[me@linuxbox me]$ chmod 600 some_file
Here is a table of numbers that covers all the common settings. The ones beginning with "7" are used with programs (since they enable execution) and the rest are for other kinds of files.
Value
Meaning
777
(rwxrwxrwx) No restrictions on permissions. Anybody may do anything. Generally not a desirable setting.
755
(rwxr-xr-x) The file's owner may read, write, and execute the file. All others may read and execute the file. This setting is common for programs that are used by all users.
700
(rwx------) The file's owner may read, write, and execute the file. Nobody else has any rights. This setting is useful for programs that only the owner may use and must be kept private from others.
666
(rw-rw-rw-) All users may read and write the file.
644
(rw-r--r--) The owner may read and write a file, while all others may only read the file. A common setting for data files that everybody may read, but only the owner may change.
600
(rw-------) The owner may read and write a file. All others have no rights. A common setting for data files that the owner wants to keep private.
Directory permissions
The chmod command can also be used to control the access permissions for directories. In most ways, the permissions scheme for directories works the same way as they do with files. However, the execution permission is used in a different way. It provides control for access to file listing and other things. Here are some useful settings for directories:
Value
Meaning
777
(rwxrwxrwx) No restrictions on permissions. Anybody may list files, create new files in the directory and delete files in the directory. Generally not a good setting.
755
(rwxr-xr-x) The directory owner has full access. All others may list the directory, but cannot create files nor delete them. This setting is common for directories that you wish to share with other users.
700
(rwx------) The directory owner has full access. Nobody else has any rights. This setting is useful for directories that only the owner may use and must be kept private from others.
Becoming the superuser for a short while
It is often useful to become the superuser to perform important system administration tasks, but as you have been warned (and not just by me!), you should not stay logged on as the superuser. In most distributions, there is a program that can give you temporary access to the superuser's privileges. This program is called su (short for substitute user) and can be used in those cases when you need to be the superuser for a small number of tasks. To become the superuser, simply type the su command. You will be prompted for the superuser's password:
[me@linuxbox me]$ suPassword:[root@linuxbox me]#
After executing the su command, you have a new shell session as the superuser. To exit the superuser session, type exit and you will return to your previous session.
In some distributions, most notably Ubuntu, an alternate method is used. Rather than using su, these systems employ the sudo command instead. With sudo, one or more users are granted superuser privileges on an as needed basis. To execute a command as the superuser, the desired command is simply preceeded with the sudo command. After the command is entered, the user is prompted for the user's password rather than the superuser's:
[me@linuxbox me]$ sudo some_commandPassword:[me@linuxbox me]$
Changing file ownership
You can change the owner of a file by using the chown command. Here's an example: Suppose I wanted to change the owner of some_file from "me" to "you". I could:
[me@linuxbox me]$ suPassword:[root@linuxbox me]# chown you some_file[root@linuxbox me]# exit[me@linuxbox me]$
Notice that in order to change the owner of a file, you must be the superuser. To do this, our example employed the su command, then we executed chown, and finally we typed exit to return to our previous session.
chown works the same way on directories as it does on files.
Changing group ownership
The group ownership of a file or directory may be changed with chgrp. This command is used like this:
[me@linuxbox me]$ chgrp new_group some_file
In the example above, we changed the group ownership of some_file from its previous group to "new_group". You must be the owner of the file or directory to perform a chgrp.
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