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Use the below Syntax to use BC in shell SCRIPT.

Variable=`ECHO “OPTIONS; expression” | bc`.

Use the below Syntax to use bc in shell script.

Variable=`echo “options; expression” | bc`.

A function is SIMPLY a block of of CODE with a name. When we GIVE a name to a block of code, we can then call that name in our script, and that block will be executed.

Example is SHOWN below:$ disk usage () { df -h ; }

A function is simply a block of of code with a name. When we give a name to a block of code, we can then call that name in our script, and that block will be executed.

Example is shown below:$ disk usage () { df -h ; }

The do-while statement is similar to the while statement but PERFORMS the statements before CHECKING the CONDITION statement.

The FOLLOWING is the format for the do-while statement:

do

{

statements

} while (condition)

The do-while statement is similar to the while statement but performs the statements before checking the condition statement.

The following is the format for the do-while statement:

do

{

statements

} while (condition)

There are two WAYS to perform arithmetic operations:

1. Using expr command (# expr 5 + 2).
2. Using a dollar sign and square brackets ( $[ operation ] ) EXAMPLE : TEST=$[16 + 4] ; test=$[16 + 4].

There are two ways to perform arithmetic operations:

‘unset’ command is USED to de-assign or unset a VARIABLE.

Syntax is shown below:

# unset <Name_of_Variable>

‘unset’ command is used to de-assign or unset a variable.

Syntax is shown below:

# unset <Name_of_Variable>

‘read’ COMMAND READS in data from the terminal (using KEYBOARD). The read command takes in whatever the user types and places the TEXT into the variable you name.

Example is shown below:

# VI /tmp/test.sh

#!/bin/bash

echo ‘Please enter your name’

read name

echo “My Name is $name”

# ./test.sh

Please enter your name

LinuxTechi

My Name is LinuxTechi

‘read’ command reads in data from the terminal (using keyboard). The read command takes in whatever the user types and places the text into the variable you name.

Example is shown below:

# vi /tmp/test.sh

#!/bin/bash

echo ‘Please enter your name’

read name

echo “My Name is $name”

# ./test.sh

Please enter your name

LinuxTechi

My Name is LinuxTechi

Comments are the MESSAGES to yourself and for other users that describe what a script is SUPPOSED to do and how its WORKS. To PUT comments in your script, start each comment line with a hash SIGN (#).

Example is shown below:

#!/bin/bash

# This is a command

echo “I am logged in as $USER”

Comments are the messages to yourself and for other users that describe what a script is supposed to do and how its works. To put comments in your script, start each comment line with a hash sign (#).

Example is shown below:

#!/bin/bash

# This is a command

echo “I am logged in as $USER”

TEST command is used to perform different test on the files. Basic test are LISTED below:

Test: Usage

• -d file_name: RETURNS true if the file exists and is a directory.
• -e file_name: Returns true if the file exists.
• -f file_name: Returns true if the file exists and is a regular file.
• -r file_name: Returns true if the file exists and have read PERMISSIONS.
• -s file_name: Returns true if the file exists and is not empty.
• -w file_name: Returns true if the file exists and have write permissions.
• -x file_name: Returns true if the file exists and have execute permissions.

Test command is used to perform different test on the files. Basic test are listed below:

Test: Usage

The following table lists the special VARIABLES set by the Bourne shell for COMMAND line ARGUMENTS.

Special Variables: Holds

• $0: Name of the Script from the command line
• $1: FIRST Command-line argument
• $2: Second Command-line argument
• …..
• …….
• $9: Ninth Command line argument
• $#: Number of Command line arguments
• $*: All Command-line arguments, separated with spaces

The following table lists the special variables set by the Bourne shell for command line arguments.

Special Variables: Holds

TEST command is USED to COMPARE the text strings. The test command compares text strings by COMPARING each CHARACTER in each string.

Test command is used to compare the text strings. The test command compares text strings by comparing each character in each string.

A shell script can be DEBUG if we EXECUTE the script with ‘-X’ option ( sh -x myscript.sh). Another WAY to debug a shell script is by using ‘-NV’ option (sh -nv myscript.sh).

A shell script can be debug if we execute the script with ‘-x’ option ( sh -x myscript.sh). Another way to debug a shell script is by using ‘-nv’ option (sh -nv myscript.sh).

BASIC SYNTAX of for LOOP is GIVEN below:

for VARIABLES in list_of_items

do

command1

command2

….

last_command

done

Basic Syntax of for loop is given below:

for variables in list_of_items

do

command1

command2

….

last_command

done

#!/bin/bash is the first of a shell script , KNOWN as SHEBANG , where # SYMBOL is called HASH and ‘!’ is called as BANG. It shows that command to be executed via /bin/bash.

#!/bin/bash is the first of a shell script , known as shebang , where # symbol is called hash and ‘!’ is called as bang. It shows that command to be executed via /bin/bash.

USING the CHMOD command we can make a SHELL SCRIPT executable.

Example is shown below :

# chmod a+x myscript.sh

Using the chmod command we can make a shell script executable.

Example is shown below :

# chmod a+x myscript.sh

LIKE the for LOOP, the while loop repeats its block of commands a number of times. UNLIKE the for loop, however, the while loop iterates until its while CONDITION is no longer true.

The basic syntax is :

while [ test_condition ]

do

commands…

done

Like the for loop, the while loop repeats its block of commands a number of times. Unlike the for loop, however, the while loop iterates until its while condition is no longer true.

The basic syntax is :

while [ test_condition ]

do

commands…

done

The basic SYNTAX is SHOWN below:

case word in

VALUE1)

command1

command2

…..

last_command

!!

VALUE2)

command1

command2

……

last_command

;;

esac

The basic syntax is shown below:

case word in

value1)

command1

command2

…..

last_command

!!

value2)

command1

command2

……

last_command

;;

esac

The continue command is identical to break command except it causes the present iteration of the loop to exit, instead of the ENTIRE loop. Continue command is useful in some scenarios where ERROR has occurred but we STILL WANT to execute the next COMMANDS of the loop.

The continue command is identical to break command except it causes the present iteration of the loop to exit, instead of the entire loop. Continue command is useful in some scenarios where error has occurred but we still want to execute the next commands of the loop.

The break COMMAND is a simple way to escape out of a loop in progress. We can USE the break command to exit out from any loop, INCLUDING while and until loops.

The break command is a simple way to escape out of a loop in progress. We can use the break command to exit out from any loop, including while and until loops.

Test COMMAND is used to compare numbers in if-then statement.

Example is SHOWN below:

#! /bin/bash

X=10

y=20

if [ $x -gt $y ]

then

echo “x is GREATER than y”

else

echo “y is greater than x”

fi

Test command is used to compare numbers in if-then statement.

Example is shown below:

#! /bin/bash

x=10

y=20

if [ $x -gt $y ]

then

echo “x is greater than y”

else

echo “y is greater than x”

fi

While writing a shell script, if you want to check whether PREVIOUS command is executed successfully or not, then we can use “$?” with if statement to check the EXIT status of previous command.

Basic example is SHOWN below:

root@localhost:~# ls /usr/bin/shar

/usr/bin/shar

root@localhost:~# echo $

0

If exit status is 0 , then command is executed successfully

root@localhost:~# ls /usr/bin/share

ls: cannot ACCESS /usr/bin/share: No such file or directory

root@localhost:~# echo $

2

If the exit status is other than 0, then we can say command is not executed successfully.

While writing a shell script, if you want to check whether previous command is executed successfully or not, then we can use “$?” with if statement to check the exit status of previous command.

Basic example is shown below:

root@localhost:~# ls /usr/bin/shar

/usr/bin/shar

root@localhost:~# echo $

0

If exit status is 0 , then command is executed successfully

root@localhost:~# ls /usr/bin/share

ls: cannot access /usr/bin/share: No such file or directory

root@localhost:~# echo $

2

If the exit status is other than 0, then we can say command is not executed successfully.

Basic Syntax is shown below:

if [ CONDITION ]

then

command1

command2

…..

else

if [ condition ]

then

command1

command2

….

else

command1

command2

…..

FI

fi

Basic Syntax is shown below:

if [ Condition ]

then

command1

command2

…..

else

if [ condition ]

then

command1

command2

….

else

command1

command2

…..

fi

fi

There two METHODS to REDIRECT STD output and std error to the same location:

Method:1 2>&1 (# ls /usr/share/doc > out.txt 2>&1 )

Method:2 &> (# ls /usr/share/doc &> out.txt )

There two methods to redirect std output and std error to the same location:

Method:1 2>&1 (# ls /usr/share/doc > out.txt 2>&1 )

Method:2 &> (# ls /usr/share/doc &> out.txt )

In a SHELL script we can use TWO types of variables:

• System defined variables
• User defined variables

System defined variables are defined or created by Operating System (Linux) itself. These variables are generally defined in Capital Letters and can be viewed by “set” command.

User defined variables are created or defined by system USERS and the values of variables can be viewed by using the command “echo $<Name_of_Variable>”.

In a shell script we can use two types of variables:

System defined variables are defined or created by Operating System (Linux) itself. These variables are generally defined in Capital Letters and can be viewed by “set” command.

User defined variables are created or defined by system users and the values of variables can be viewed by using the command “echo $<Name_of_Variable>”.

In LINUX like Operating system “/bin/bash” is the default login shell which is assigned while user creation. We can change default shell using the “chsh” COMMAND.

Example is shown below:

# chsh &LT;username&GT; -s <new_default_shell>

# chsh linuxtechi -s /bin/sh

In Linux like Operating system “/bin/bash” is the default login shell which is assigned while user creation. We can change default shell using the “chsh” command.

Example is shown below:

# chsh <username> -s <new_default_shell>

# chsh linuxtechi -s /bin/sh

A SHELL Script is a text file that CONTAINS one or more commands. As a system administrator we often need to issue number of commands to accomplish the task, we can ADD these all commands together in a text file (Shell Script) to complete DAILY ROUTINE task.

A Shell Script is a text file that contains one or more commands. As a system administrator we often need to issue number of commands to accomplish the task, we can add these all commands together in a text file (Shell Script) to complete daily routine task.