POiSON WORLD

In this section we will work with shell.

Shell is an interface between a user and the kernel. It is a command interpreter which interprets the commands entered by user and sends to kernel.

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Korn Shell Variables: It is referred to as temporary storage area in memory.It enables us to store value into the variable. These variables are of two types :
1. Variables that are exported to subprocesses.
2. Variables that are not exported to subprocesses.Lets check few commands to work with these variables:
To set a variable : 
#VAR=value
#export VAR
Note: There is no space on the either side of the ‘=’ sign.

To unset a variable: 
#unset VAR

To display all variables:  
We can use ‘set’ or ‘env’ or ‘export’ command.

To display value of a variable: 
echo $VAR or print $VAR
Note: When a shell variable follows $ sign, then the shell substitutes it by the value of the variable.

Default Korn Shell Variables :
EDITOR : The default editor for the shell.
FCEDIT : It defines the editor for the fc command.
HOME : Sets the directory to which cd command switches.
LOGNAME : Sets the login name of the user.
PATH : It specifies the paths where shell searches for a command to be executed.
PS1 :It specifies the primary korn shell ($)
PS2 : It specifies the secondary command prompt (>)
SHELL : It specifies the name of the shell.

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Using quoting characters:

Quoting is the process that instructs the shell to mask/ignore the special meaning of the metacharacters. Following are few use of the quoting characters:

Single quotation mark (”): It instructs the shell to ignore all enclosed metacharacters.
Example:
#echo $SHELL
/bin/ksh
#echo ‘$SHELL’
$SHELL
#
Double quotation mark (“”): It instructs the shell to ignore all enclosed shell metacharacters, except for following :
1. The single backward quotation(`) mark : This executes the solaris command inside the single quotation.Example: 
# echo “Your current working directory is `pwd`”
Your current working directory is /export/home/immam
In the above example the ‘`’ is used to execute the ‘pwd’ command inside the quotation mark.

2. The blackslash(\) in the front of a metacharacter : This ignores the meaning of the metacharacter.Example: 
#echo “$SHELL”
/bin/ksh
#echo “\$SHELL”
$SHELL
In the above example, the inclusion of ‘\’ ignores the meaning of metacharacter ‘$’

3. The ‘$’ sign followed by command inside parenthesis : This executes the command inside the parenthesis.Example: 
# echo “Your current working directory is $(pwd)”
Your current working directory is /export/home/immam

In the above example enclosing the pwd command inside parenthesis and $ sign before parenthesis, executes the pwd command.

Displaying the command history:
The shell keeps the history of all the commands entered. We can re-use this command in our ways. For a given user this list of command used is shared among all the korn shells.
Syntax: history option
The output will somewhat like following :
…
125 pwd
126 date
127 uname -a
128 cd
The numbers displayed on the left of the command are command numbers and can be used to re-execute the command corresponding to it.To view the history without command number -n option is used : #history -n

The ‘r’ command :
The r command is an alias in Korn Shell that enables us to repeat a command.
Example:
#pwd
/export/home/immam
#r
/export/home/immamThis can be used to re-execute the commands from history.
Example:
#history
…
126 pwd
127 cd
128 uname -a
#r 126
/export/home/immam
The ‘r’ command can also be used to re-execute a command beginning with a particular character, or string of characters. Example:
# r p
pwd
/export/home/immam
#
In the above example the ‘r’ command is used to re-run the most recent occurrence of the command starting with p.
#r ps
ps -ef
o/p of ps -ef command
In the above example the ‘r’ command is used to re-run the most recent command starting with ps.
We can also edit the previously run command according to our use. The following example shows that :
#r c
cd ~/dir1
#r dir1=dir
cd ~/dir
In this example the cd command has re-run but the argument passed to it has been changed to dir from dir1.

Note: The r command is alias for the command ” fc -e – “.

Editing the previously executed commands using vi-editor :
We can also edit the previously executed command under history using vi-editor. To do so, we need to enable shell history editing by using any one of the following commands :
#set -o vi
or
#export EDITOR=/bin/vi
or
#export VISUAL=/bin/viTo verify whether this feature is turned on, use the following command :
#set -o | grep -w vi
vi                on

Once it is on you can start editing the command history as follows :
1. Execute the history command: #history
2. Press Esc key and start using the vi editing options.
3. To run a modified command, press enter/return key.

File Name Completion : 
Suppose you are trying to list files under the directory “/directoryforlisting“. This is too big to type. There is a short method to list this directory.
Type ls d and then press Esc and then \ (backslash) key. The shell completes the file name and will display :
#ls directoryforlisting/We can also request to display all the file name beginning with ‘d’ by pressing Esc and = key sequentially.

Two points to be noted here :
1. The key sequence presented above works only in the vi mode of the command line editing.
2. The sequence in which the key is pressed is important.

Command Redirection:
There are two redirection commands:
1. The greater than (>) sign metacharacter
2. The less than (<) sign metacharacter
Both the above mentioned mentioned commands are implied by pipe (|) character.The File Descriptors:
Each process works with shell descriptor. The file descriptor determines where the input to command originates and where the output and error messages are sent.

All command that process file content read from the standard input and write to standard output.Redirecting the standard Input:
command < filename or command 0<filename
The above command the “command” takes the input from “filename” instead of keyboard.

Redirecting the standard Output:
command > filename or command 1>filename
#ls -l ~/dir1 > dirlist
The above command redirects the output to a file ‘dirlist’ instead of displaying it over the terminal.
command >> filename
#ls -l ~/dir1 >> dirlist
The above example appends the output to the file ‘dirlist’.

Redirecting the Standard Error:
command > filename 2> <filename that will save error>
command> filename 2>&1
The first example will redirect the error to the file name specified at the end.
The second example will redirect the error to the input file itself.

The Pipe character :
The pipe character is used to redirect the output of a command as input to the another command.
Syntax: command | command
Example:
# ps -ef | grep nfsd
In the above example the output of ps -ef command is send as input to grep command.
#who | wc -l

User Initialization Files Administration :
In this section we will see initialization files of Bourne, Korn and C shell.
Initialization files at Login
/bin/ksh

Bourne Shell Initialization file:
The .profile file in the user home directory is an initialization file which which shell executes when the user logs in. It can be used to a) customize the terminal settings & environment variables b)instruct system to initiate an application.

Korn Shell Initialization file: It has two initialization file :
1. The ~/.profile: The .profile file in the user home directory is an initialization file which which shell executes when the user logs in. It can be used to a) customize the terminal settings & environment variables b)instruct system to initiate an application.
2. The ~/.kshrc: It contains shell variables and aliases. The system executes it every time the user logs in and when a ksh sub-shell is started. It is used to define Korn shell specific settings. To use this file ENV variable must be defined in .profile file.Following settings can be configured in /.kshrc file :
Shell prompt definations (PS1 & PS2)
Alias Definitions
Shell functions
History Variables
Shell option ( set -o option)
The changes made in these files are applicable only when the user logs in again. To make the changes effective immediately, source the ~/.profile and ~/.kshrc file using the dot(.) command:
#. ~/.profile
#. ~/.kshrc
Note: The /etc/profile file is a separate system wide file that system administrator maintains to set up tasks for every user who logs in.

C Shell Initialization file: It has two initialization file :
1. The ~/.cshrc file : The . cshrc file in the user home directory is an initialization file which which shell executes when the user logs in. It can be used to a) customize the terminal settings & environment variables b)instruct system to initiate an application.

Following settings can be configured in .cshrc file :
Shell prompt definations (PS1 & PS2)
Alias Definitions
Shell functions
History Variables
Shell option ( set -o option)

2. The ~/.login file: It has same functionality as .cshrc file and has been retained for legacy reasons.
Note: The /etc/.login file is a separate system wide file that system administrator maintains to set up tasks for every user who logs in.

The changes made in these files are applicable only when the user logs in again. To make the changes effective immediately, source the ~/.cshrc and ~/.login file using the source command:
#source ~/.cshrc
#source ~/.login

The ~/.dtprofile file : It resides in the user home directory and determines generic and customized settings for the desktop environment.The variable setting in this file can overwrite the default desktop settings. This file is created when the user first time logs into the desktop environment.

Important: When a user logins to the desktop environment, the shell reads .dtprofile, .profile and .kshrsc file sequentially. If the DTSOURCEPROFILE variable under .dtprofle is not ture or does not exists, the .profile file is not read by the shell.
The shell reads .profile and .kshrsc file when user opens console window.
The shell reads .kshrsc file when user opens terminal window.Configuring the $HOME/.profile file:
It can be configured to instruct the login process to execute the initialization file referenced by ENV variable.
To configure that we need to add the following into the $HOME/.profile file:
ENV=$HOME/.kshrc
export ENV

Configuring the $HOME/.kshrc file :
This file contains korn shell specific setting.To configure PS1 variable, we need to add the following into the $HOME/.kshrc file:
PS1=””hostname’ $”
export PS1

In this module we will learn four important aspects of Korn shell.

Command	Value
jobs	List all jobs that are currently running or stopped in the background
bg %<jobID>	Runs the specified job in background
fg %<jobID>	Brings the  specified job in foreground
Ctrl+Z	Stops the foreground job and places it in the background as a stopped job
stop %<jobID>	Stops a job running in background

It is a text file that has series of command executed one by one. There are different shell available in Solaris. To ensure that the correct shell is used to run the script, it should begin with the characters #! followed immediately by the absolute pathname of the shell.

Example: 
#!/bin/sh

#!/bin/ksh

Comments: It provides information about the script files/commands. The text inside the comment is not executed. The comment starts with character ‘#’.

lets write our first shell script :

ls -ltr #This is used to list the files/directories

The shell executes the script line by line. It does not compile the script and keep it in binary form. So, In order to run a script, a user must have read and execute permission.
Example: 
#./MyFirstScript
The above example runs the script in sub-shell. If we want to run the script as if the commands in it were ran in same shell, the dot(.) command is used as follows:
#. ./MyFirstScriptPassing Value to the shell script:
We can pass value to the shell script using the pre-defined variables $1, $2 and so on. These variables are called Positional Parameters. When the user run the shell script, the first word after the script name is stored in $1, second in $2 and so on.
Example:
#cat welcome
#!/bin/sh
echo $1 $2
#welcome immam test
immam test

In the above example when we ran the script welcome, the two words after it ravi and ranjan was stored in $1 and $2 respectively.

Note: There is a limitation in Bourne shell. It accepts only a single number after $ sign. So if we are trying to access the 10th argument $10, it will result in the value of $1 followed by (0).
In order to overcome this problem, shift command is used.

Shift Command:
It enables to shift the value of positional parameter values back by one position i.e. the value of $2 parameter is assigned to $1, and $3 to $2, and so on.

Checking Exit status:
All commands under Solaris returns an exit status. The value ‘0’ indicates success and non-zero value ranging from 1-255 represents failure. The exit status of the last command run under foreground is held in ? special shell variable.

# ps -ef | grep nfsd
root  6525 22601   0 05:55:01 pts/11      0:00 grep nfsd
# echo ?
1
#
In the above example there is no nfsd process running, hence 1 is returned.

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Using the test Command:
It is used for testing conditions. It can be used to verify many conditions, including:
Variable contents
File Access permissions
File types
Syntax : #test expression or #[ expression ]

The test builtin command returns 0 (True) or 1 (False), depending on the evaluation of an expression, expr.
Syntax:test expr or [ expr ]

We can examine the return value by displaying $?;
We can use the return value with && and ||; or we can test it using the various conditional constructs.

We can compare arithmetic values using one of the following:

Option	Tests for Arithmetical Values
-eq	equal to
-ne	not equal to
-lt	less than
-le	less than or equal to
-gt	greater than
-ge	greater than or equal to

We can compare strings for equality, inequality etc. Following table lists the various options that can be used to compare strings:

Option	Tests for strings
=	equal to.  e.g #test “string1” = “string2”
!=	not equal to.   e.g #test “string1” = “string2”
<	less than.   e.g #test “ab” \< “cd”
>	greater than.   e.g #test  “ab” \> “cd” “
-z	for a null string.   e.g #test -z “string1”
-n	returns True if a string is not empty.  e.g. #test -n “string1”

Note: the < and > operators are also used by the shell for redirection, so we must escape them using \< or \>.

Example : 

Lets test that the value of variable $LOGNAME is ravi.
#echo $LOGNAME
immam
# test “LOGNAME” = “immam”
#echo $?
0

#[ “LOGNAME” = “immam” ]
#echo $?
0

Lets test if read permissions on the /immam
#ls -l /immam
-rw-r–r– 1 root sys 290 Jan 10 01:10 /immam
#test -r /immam
#echo $?
0

#[ -r /immam]
#echo $?
0

Lets test if /var is a directory
#test -d /var
#echo $?
0

#[ -d /var ]
#echo $?
0

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Executing Conditional Commands : 
In this section we will see the following three conditional commands:
1. Using If command: It checks for the exit status of the command and if exist status is (0), then the statement are run other wise statement under else is executed.
Syntax:
#if command1
>then
>execute command2
>else
>execute command3
>fi

The shell also provides two constructs that enable us to run the command based on the success or failure of the preceding command.
The constructs are &&(and) and ||(or).
Example:
#mkdir /immam && /syed
This command creates directory /immam only if /syed is created.

#mkdir /immam|| /syed
This command creates directory /immam even if /syed fails to create.

2. Using while command: It enables to repeat a command or group of command till the condition returns (0).
Syntax:
#while command1
>do
>command2
>done

3. Using case command: It compares a single value against other values and runs a command or commands when a match is found.
Syntax:
#case value in
>pat1)command
>command
>..
>command
>;;

>pat2)command
>command
>..
>command
>;;
…

>patn)command
>command
>..
>command

The special shell variable :

?	This contains the return value of the last command
#	This contains the number of arguments passed to the script
*	It contains the value of all command line argument