Unix - Directory Management

A directory is a file whose sole job is to store file names and related information. All files, whether ordinary, special, or directory, are contained in directories.

UNIX uses a hierarchical structure for organizing files and directories. This structure is often referred to as a directory tree . The tree has a single root node, the slash character ( /), and all other directories are contained below it.

Home Directory

The directory in which you find yourself when you first login is called your home directory.

You will be doing much of your work in your home directory and subdirectories that you'll be creating to organize your files.

You can go in your home directory anytime using the following command −

$cd ~

$

Here ~ indicates home directory. If you want to go in any other user's home directory then use the following command −

$cd ~username

$

To go in your last directory you can use following command −

$cd -

$

Absolute/Relative Pathnames

Directories are arranged in a hierarchy with root (/) at the top. The position of any file within the hierarchy is described by its pathname.

Elements of a pathname are separated by a /. A pathname is absolute if it is described in relation to root, so absolute pathnames always begin with a /.

These are some example of absolute filenames.

/etc/passwd

/users/sjones/chem/notes

/dev/rdsk/Os3

A pathname can also be relative to your current working directory. Relative pathnames never begin with /. Relative to user amrood' home directory, some pathnames might look like this −

chem/notes

personal/res

To determine where you are within the filesystem hierarchy at any time, enter the commandpwd to print the current working directory −

$pwd

/user0/home/amrood

$

Listing Directories

To list the files in a directory you can use the following syntax −

$ls dirname

Following is the example to list all the files contained in /usr/local directory −

$ls /usr/local

X11       bin          gimp       jikes       sbin

ace       doc          include    lib         share

atalk     etc          info       man         ami

Creating Directories

Directories are created by the following command −

$mkdir dirname

Here, directory is the absolute or relative pathname of the directory you want to create. For example, the command −

$mkdir mydir

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Creates the directory mydir in the current directory. Here is another example −

$mkdir /tmp/test-dir

$

This command creates the directory test-dir in the /tmp directory. The mkdir command produces no output if it successfully creates the requested directory.

If you give more than one directory on the command line, mkdir creates each of the directories. For example −

$mkdir docs pub

$

Creates the directories docs and pub under the current directory.

Creating Parent Directories

Sometimes when you want to create a directory, its parent directory or directories might not exist. In this case, mkdir issues an error message as follows −

$mkdir /tmp/amrood/test

mkdir: Failed to make directory "/tmp/amrood/test";

No such file or directory

$

In such cases, you can specify the -p option to the mkdir command. It creates all the necessary directories for you. For example −

$mkdir -p /tmp/amrood/test

$

Above command creates all the required parent directories.

Removing Directories

Directories can be deleted using the rmdir command as follows −

$rmdir dirname

$

Note − To remove a directory make sure it is empty which means there should not be any file or sub-directory inside this directory.

You can create multiple directories at a time as follows −

$rmdir dirname1 dirname2 dirname3

$

Above command removes the directories dirname1, dirname2, and dirname2 if they are empty. The rmdir command produces no output if it is successful.

Changing Directories

You can use the cd command to do more than change to a home directory: You can use it to change to any directory by specifying a valid absolute or relative path. The syntax is as follows −

$cd dirname

$

Here, dirname is the name of the directory that you want to change to. For example, the command −

$cd /usr/local/bin

$

Changes to the directory /usr/local/bin. From this directory you can cd to the directory /usr/home/amrood using the following relative path −

$cd ../../home/amrood

$

Renaming Directories

The mv (move) command can also be used to rename a directory. The syntax is as follows −

$mv olddir newdir

$

You can rename a directory mydir to yourdir as follows −

$mv mydir yourdir

$

The directories . (dot) and .. (dot dot)

The filename . (dot) represents the current working directory; and the filename .. (dot dot) represent the directory one level above the current working directory, often referred to as the parent directory.

If we enter the command to show a listing of the current working directories files and use the -a option to list all the files and the -l option provides the long listing, this is the result.

$ls -la

drwxrwxr-x    4    teacher   class   2048  Jul 16 17.56 .

drwxr-xr-x    60   root              1536  Jul 13 14:18 ..

----------    1    teacher   class   4210  May 1 08:27 .profile

-rwxr-xr-x    1    teacher   class   1948  May 12 13:42 memo

Unix - File Management

All data in UNIX is organized into files. All files are organized into directories. These directories are organized into a tree-like structure called the filesystem.

When you work with UNIX, one way or another you spend most of your time working with files. This tutorial would teach you how to create and remove files, copy and rename them, create links to them etc.

In UNIX there are three basic types of files:

1. Ordinary Files: An ordinary file is a file on the system that contains data, text, or program instructions. In this tutorial, you look at working with ordinary files.
2. Directories: Directories store both special and ordinary files. For users familiar with Windows or Mac OS, UNIX directories are equivalent to folders.
3. Special Files: Some special files provide access to hardware such as hard drives, CD-ROM drives, modems, and Ethernet adapters. Other special files are similar to aliases or shortcuts and enable you to access a single file using different names.

Listing Files:

To list the files and directories stored in the current directory. Use the following command:

$ls

Here is the sample output of the above command:

$ls

bin        hosts  lib     res.03
ch07       hw1    pub     test_results
ch07.bak   hw2    res.01  users
docs       hw3    res.02  work

The command ls supports the -l option which would help you to get more information about the listed files:

$ls -l
total 1962188

drwxrwxr-x  2 amrood amrood      4096 Dec 25 09:59 uml
-rw-rw-r--  1 amrood amrood      5341 Dec 25 08:38 uml.jpg
drwxr-xr-x  2 amrood amrood      4096 Feb 15  2006 univ
drwxr-xr-x  2 root   root        4096 Dec  9  2007 urlspedia
-rw-r--r--  1 root   root      276480 Dec  9  2007 urlspedia.tar
drwxr-xr-x  8 root   root        4096 Nov 25  2007 usr
drwxr-xr-x  2    200    300      4096 Nov 25  2007 webthumb-1.01
-rwxr-xr-x  1 root   root        3192 Nov 25  2007 webthumb.php
-rw-rw-r--  1 amrood amrood     20480 Nov 25  2007 webthumb.tar
-rw-rw-r--  1 amrood amrood      5654 Aug  9  2007 yourfile.mid
-rw-rw-r--  1 amrood amrood    166255 Aug  9  2007 yourfile.swf
drwxr-xr-x 11 amrood amrood      4096 May 29  2007 zlib-1.2.3
$

Here is the information about all the listed columns:

1. First Column: represents file type and permission given on the file. Below is the description of all type of files.
2. Second Column: represents the number of memory blocks taken by the file or directory.
3. Third Column: represents owner of the file. This is the Unix user who created this file.
4. Fourth Column: represents group of the owner. Every Unix user would have an associated group.
5. Fifth Column: represents file size in bytes.
6. Sixth Column: represents date and time when this file was created or modified last time.
7. Seventh Column: represents file or directory name.

In the ls -l listing example, every file line began with a d, -, or l. These characters indicate the type of file that's listed.

Prefix	Description
-	Regular file, such as an ASCII text file, binary executable, or hard link.
b	Block special file. Block input/output device file such as a physical hard drive.
c	Character special file. Raw input/output device file such as a physical hard drive
d	Directory file that contains a listing of other files and directories.
l	Symbolic link file. Links on any regular file.
p	Named pipe. A mechanism for interprocess communications
s	Socket used for interprocess communication.

Meta Characters:

Meta characters have special meaning in Unix. For example * and ? are metacharacters. We use * to match 0 or more characters, a question mark ? matches with single character.

For Example:

$ls ch*.doc

Displays all the files whose name start with ch and ends with .doc:

ch01-1.doc   ch010.doc  ch02.doc    ch03-2.doc 
ch04-1.doc   ch040.doc  ch05.doc    ch06-2.doc
ch01-2.doc ch02-1.doc c

Here * works as meta character which matches with any character. If you want to display all the files ending with just .doc then you can use following command:

$ls *.doc

Hidden Files:

An invisible file is one whose first character is the dot or period character (.). UNIX programs (including the shell) use most of these files to store configuration information.

Some common examples of hidden files include the files:

• .profile: the Bourne shell ( sh) initialization script
• .kshrc: the Korn shell ( ksh) initialization script
• .cshrc: the C shell ( csh) initialization script
• .rhosts: the remote shell configuration file

To list invisible files, specify the -a option to ls:

$ ls -a

.         .profile       docs     lib     test_results
..        .rhosts        hosts    pub     users
.emacs    bin            hw1      res.01  work
.exrc     ch07           hw2      res.02
.kshrc    ch07.bak       hw3      res.03
$

• Single dot .: This represents current directory.
• Double dot ..: This represents parent directory.

Creating Files:

You can use vi editor to create ordinary files on any Unix system. You simply need to give following command:

$ vi filename

Above command would open a file with the given filename. You would need to press key i to come into edit mode. Once you are in edit mode you can start writing your content in the file as below:

This is unix file....I created it for the first time.....
I'm going to save this content in this file.

Once you are done, do the following steps:

• Press key esc to come out of edit mode.
• Press two keys Shift + ZZ together to come out of the file completely.

Now you would have a file created with filemame in the current directory.

$ vi filename
$

Editing Files:

You can edit an existing file using vi editor. We would cover this in detail in a separate tutorial. But in short, you can open existing file as follows:

$ vi filename

Once file is opened, you can come in edit mode by pressing key i and then you can edit file as you like. If you want to move here and there inside a file then first you need to come out of edit mode by pressing key esc and then you can use following keys to move inside a file:

• l key to move to the right side.
• h key to move to the left side.
• k key to move up side in the file.
• j key to move down side in the file.

So using above keys you can position your cursor where ever you want to edit. Once you are positioned then you can use i key to come in edit mode. Edit the file, once you are done press esc and finally two keys Shift + ZZ together to come out of the file completely.

Display Content of a File:

You can use cat command to see the content of a file. Following is the simple example to see the content of above created file:

$ cat filename
This is unix file....I created it for the first time.....
I'm going to save this content in this file.
$

You can display line numbers by using -b option along with cat command as follows:

$ cat -b filename
1   This is unix file....I created it for the first time.....
2   I'm going to save this content in this file.
$

Counting Words in a File:

You can use the wc command to get a count of the total number of lines, words, and characters contained in a file. Following is the simple example to see the information about above created file:

$ wc filename
2  19 103 filename
$

Here is the detail of all the four columns:

1. First Column: represents total number of lines in the file.
2. Second Column: represents total number of words in the file.
3. Third Column: represents total number of bytes in the file. This is actual size of the file.
4. Fourth Column: represents file name.

You can give multiple files at a time to get the information about those file. Here is simple syntax:

$ wc filename1 filename2 filename3

Copying Files:

To make a copy of a file use the cp command. The basic syntax of the command is:

$ cp source_file destination_file

Following is the example to create a copy of existing file filename.

$ cp filename copyfile
$

Now you would find one more file copyfile in your current directory. This file would be exactly same as original file filename.

Renaming Files:

To change the name of a file use the mv command. Its basic syntax is:

$ mv old_file new_file

Following is the example which would rename existing file filename to newfile:

$ mv filename newfile
$

The mv command would move existing file completely into new file. So in this case you would fine onlynewfile in your current directory.

Deleting Files:

To delete an existing file use the rm command. Its basic syntax is:

$ rm filename

Caution: It may be dangerous to delete a file because it may contain useful information. So be careful while using this command. It is recommended to use -i option along with rm command.

Following is the example which would completely remove existing file filename:

$ rm filename
$

You can remove multiple files at a tile as follows:

$ rm filename1 filename2 filename3
$

Standard Unix Streams:

Under normal circumstances every Unix program has three streams (files) opened for it when it starts up:

1. stdin : This is referred to as standard input and associated file descriptor is 0. This is also represented as STDIN. Unix program would read default input from STDIN.
2. stdout : This is referred to as standard output and associated file descriptor is 1. This is also represented as STDOUT. Unix program would write default output at STDOUT
3. stderr : This is referred to as standard error and associated file descriptor is 2. This is also represented as STDERR. Unix program would write all the error message at STDERR.