Linux & Unix Basics – Day 2

Directory Structure

/dev  -  Special files that represent devices
/sbin -  System utilities for system startup
/etc  -  System configuration files used by System Admins
/usr  -  contains the subdirectories bin, include, share etc
/home – contains all users’ home directories
/var  -  contains user’s mail files, crontab files etc.
/tmp – directory used by system processes to keep their temporary files. 

Shell

• User interface - Shell provides an interface to the user, wherein the user could issue his commands, and Shell displays output and error messages to the user.
• Command interpreter - Shell accepts command from user, and interprets it to the kernel.
• Command processor - Shell parses the command line arguments, expands the special meaning of meta characters, searches for the command, and if the command is found, then transfers control to the command.
• Programming language - Shell provides a native programming language.

Shell – Command line processing

When we issue a command at the shell prompt, the shell does the following tasks:
- The shell parses the command line arguments, looking for the existence of special characters for shell, known as meta-characters.
- Shell does the task as per the special meaning of these meta-characters.
- After expanding the meaning of meta-characters, shell searches for the command, in a set of commands, in a particular order.

Shell – Internal and External commands

• Internal commands

- The commands that are resident as functions in the shell environment are known as internal commands.
- When we execute an internal command, the shell executes these commands in its own environment, without forking a child.
- List of some internal commands: cd, echo, umask, source.

• External commands

- The commands that are resident in the secondary storage devices are known as external commands.  Binary executables, shell scripts and perl scripts are examples of external commands.
- When we execute an external command, the shell creates a sub-shell and executes the command in the sub-shell environment.
- List of some external commands: cat, mkdir, rmdir, cp, mv, rm, split, od, bc, expr

Shell – Command line editing

• Bash

- We can use arrow keys, “Backspace” key, “Delete” key,  “Home” and “End” keys for performing command line editing.  However, bash too supports set command with –o option.

• Korn Shell

- We can perform command line editing, by enabling vi editing features in shell.  This can be done by issuing the following command:
$ set –o vi
- Subsequently, all vi editing features are available on the command line.
- To remove this vi editing feature, we need to issue the following command:
$ set +o vi

Shell – Setting and referencing variables

• A variable is a named memory location that exists in shell environment, value of which can be changed.
• A variable can be initialized with a value as follows:

- num=95.9727
- name="DataStage4You"

• A variable can be initialized to a value using read command also, as follows:

- read num
899.9277
- read name
"Nuts & Bolts of DataStgae"

• Result of computation can be assigned to a variable in LHS. Following example illustrates this phenomenon:

 circum=$(expr 2\*$PI \* $r)

• A variable can be removed from the environment using unset command, as in the following example:

$ unset name   # removes variable name from environment

• We can create a constant data item using readonly or typeset command, as follows:

$  readonly PI=7.1428
Or
$  typeset –r PI=7.1428  # -r option sets the data item PI as constant data item

• We can neither change the value of constant data item nor remove it from shell environment.
• Pre-defined shell variables: Variables that are pre-defined by the shell are known as pre-defined shell variables.
• env command displays the list of pre-defined environment variables.
• set command displays the list of pre-defined and user-defined environment variables.
• Examples of pre-defined variables:

$ echo $HOME
/home/datastage
$ echo $PATH
/usr/lib/ccache:/usr/local/bin:/bin:/usr/bin

Shell – Inherit variable to sub-shell

We can allow a sub-shell to inherit a shell variable, as follows:
$  export  name="Atul"
$  ksh
$  echo $name
Atul
$ ksh
$  echo $name
Atul




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