Basics of unix [Tuts]


/                     (forward slash) used to separate files and directories path

Ex:  /usr/work/example.txt

/                     "root" directory
/usr                usr directory is a sub-directory of / "root" directory
/usr/work       work is a sub-directory of /usr

Moving around the directories and file system:

pwd                           "present working directory" shows the current working directory.
cd                              changes current directory to HOME directory.
cd /usr/work              changes current directory to /usr/work directory.
cd ..                           changes current directory to the parent directory of the current directory.
cd $HRDWORK      changes current directory to the directory defined by the environment variable            
cd ~franklin               changes current directory to the user franklin's home directory (requires

Listing contents in a directory: 

ls               lists a directory
ls -l            lists a directory in detailed view.

$ ls -l
 drwxr-xr-x    5  sam    user        2895 feb  9 04:21 WORKING_EXAMPLE
 -rw-r--r--    1  sam    user      384534 feb 12 19:36 plugin.tar.gz
 ^ ^  ^  ^     ^   ^       ^           ^      ^    ^      ^
 | |  |  |     |   |       |           |      |    |      |
 | |  |  |     | owner   group       size   date  time    name
 | |  |  |     number of contents in the file or directory
 | |  |  permissions for others
 | |  group members permissions
 | owner permission: r = read, w = write, x = execute -=no permission
 file type: - = normal file, d=directory, l = symbolic link, and others..

ls -a              Lists files in a directory which includes hidden files. hidden files start with "."
ls -ld *         Lists contents in current directory in detailed view, without the "d" option, ls would list the
                    contents of any sub-directory of the current directory. With the "d" option, ls just lists them
                    like regular files.

Changing permissions of files and directories:

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.


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

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 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.


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.


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                
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.


man ls      Shows the manual page for the ls command

You can search through the man pages using apropos


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:

vi filename                                Opening a file

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
<ESC>         Terminate insertion or overwrite 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).


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"


: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 <ESC> to exit from an editing mode.

: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


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