Basics of Unix/Linux - 2

 

Unix - Pipes and Filters

You can connect two commands together so that the output from one program becomes the input of the next program. Two or more commands connected in this way form a pipe.

To make a pipe,  put a vertical bar (|) on the command line between two commands.

When a program takes its input from another program, performs some operation on that input, and writes the result to the standard output, it is referred to as a filter.

The grep Command

The grep program searches a file or files for lines that have a certain pattern. The syntax is-

[oracle@localhost ~]$ ps -ef|grep "pmon"

oracle    7249     1  0 13:35 ?        00:00:01 ora_pmon_orcl

oracle    8869  1426  0 19:50 pts/1    00:00:00 grep pmon

Standard Unix Streams

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.

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.

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.

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.

Absolute/Relative Pathnames

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 /.

Ex: /u01/app/oracle

Relative Pathnames - A pathname can also be relative to your current working directory. Relative pathnames never begin with /.

Ex: home/app/oracle

Present Working Directory(pwd)

To determine where you are within the file system hierarchy at any time, enter the command pwd to print the current working directory.

Create Directory(mkdir)

It is used to create a directory as follows-

[oracle@localhost ~]$ mkdir dirname

In order to create more than one directory-

[oracle@localhost ~]$ mkdir dirname1 dirname2… dirnamen

You can create Parent Directory with the  -p  option to the  mkdir  command. It creates all the

necessary directories for you.

[oracle@localhost ~]$ mkdir –p /home/app/oracle/oradata/orcl

Removing Directories(rm)

Directories can be deleted (Single/Multiple Directories) using the rmdir command as follows-

[oracle@localhost ~]$ rmdir dirname

[oracle@localhost ~]$ rmdir dirname1 dirname2 dirname3

Changing Directories(cd)

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.

[oracle@localhost ~]$ cd dirname

[oracle@localhost ~]$ cd  /home/app/oracle/oradata/orcl

Renaming Directories(mv)

The mv (move) command can also be used to rename a directory.

[oracle@localhost ~]$ mv filename1 filename2

It changes from filename1 to filename2.

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.