Linux Shell Script

Introduction

1. You can use any editor text to write shell script.
    # vi script.sh

2. Write your shell script & save it.
    press ESC, then  :wq!.
    press ENTER.

3. Set execute permission to the script.
    # chmod +x [your_script_name]
    or
    # chmod 755 [your_script_name]

4. Then, you can execute your script.
    # ./[your_script_name]
    or
    # bash [your_script_name]
    # sh  [your_script_name]

 *****************************************************************************

How to Write a Shell Script? 

Important

There are 2 types of variable:

• System Variables (SV)                    - in capital letters
• Syntax :
• $variable_name
• User Defined Variables (UDV)      - in lower letters
• Syntax :
• variable_name=value
• UDV variable name must begin with Alphanumeric character or underscore character (_), followed by >= 1 Alphanumeric character.
• No space on either side of the equal sign.
• Variables are case-sensitive.
• Can define NULL variable.
• variable_name=
• variable name=""

 Quotes

"	Double Quotes	"Double Quotes" - Anything enclose in double quotes removed meaning of that characters (except \ and $).
'	Single quotes	'Single quotes' - Enclosed in single quotes remains unchanged.
`	Back quote	`Back quote` - To execute command

Syntax that holds number of arguments specified on command line.

• $#  

Syntax that refer to all arguments passed to script.

• $* or $@

Mathematical Operator in  Shell Script	Meaning	Normal Arithmetical/ Mathematical Statements	But in Shell
			For test statement with if command	For [ expr ] statement with if command
-eq	is equal to	5 == 6	if test 5 -eq 6	if [ 5 -eq 6 ]
-ne	is not equal to	5 != 6	if test 5 -ne 6	if [ 5 -ne 6 ]
-lt	is less than	5 < 6	if test 5 -lt 6	if [ 5 -lt 6 ]
-le	is less than or equal to	5 <= 6	if test 5 -le 6	if [ 5 -le 6 ]
-gt	is greater than	5 > 6	if test 5 -gt 6	if [ 5 -gt 6 ]
-ge	is greater than or equal to	5 >= 6	if test 5 -ge 6	if [ 5 -ge 6 ]

Operator	Meaning
string1 = string2	string1 is equal to string2
string1 != string2	string1 is NOT equal to string2
string1	string1 is NOT NULL or not defined
-n string1	string1 is NOT NULL and does exist
-z string1	string1 is NULL and does exist

Test	Meaning
-s file	Non empty file
-f file	Is File exist or normal file and not a directory
-d dir	Is Directory exist and not a file
-w file	Is writeable file
-r file	Is read-only file
-x file	Is file is executable

Operator	Meaning
! expression	Logical NOT
expression1  -a  expression2	Logical AND
expression1  -o  expression2	Logical OR

Standard File	File Descriptors number	Use	Example
stdin	0	as Standard input	Keyboard
stdout	1	as Standard output	Screen
stderr	2	as Standard error	Screen

2>&1 : is used to redirect both standard output and error to /dev/null
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Related Commands

To print or acees UDV.

• $variable_name

To print contains of variable "abc" type

• echo $abc

To display text or value of variable

• echo [options] [string, variables...]

Options

• -n Do not output the trailing new line.
• -e Enable interpretation of the following backslash escaped characters in the strings:
• \a alert (bell)
• \b backspace
• \c suppress trailing new line
• \n new line
• \r carriage return
• \t horizontal tab
• \\ backslash

To perform arithmetic operations.

• expr op1 math-operator op2

math-operator

• +
• -
• /
• %
• \*

To print output of an arithmetic operations.(Both are back quotes)

• echo `expr op1 math-operator op2`

To determine the exit status of command or shell script.

• $?

To get input (data from user) from keyboard and store (data) to variable.

• read variable1, variable2, ..., variableN

 To run 2 commands with 1 command line.

• command1;command2

To sort contents in a file & redirect the output to new file.
sort [file_name] [new_file_name]

To translate all lower case character to upper case letters.
tr "a-z" "A-Z" [file_name] [new_file_name]

[Remark: All  commands line you can use for Shell Script.]

**************************************************************

if condition
then
 command1 if condition is true or if exit status
 of condition is 0 (zero)
 ...
 ...
fi

************************************************************** 

if condition
then
 if condition
 then
  .....
  ..
  do this
 else
  ....
  ..
  do this
 fi
else
 ...
 .....
 do this
fi

************************************************************** 

if condition
then
        condition is zero (true - 0)
        execute all commands up to elif statement
elif condition1 

then
        condition1 is zero (true - 0)
        execute all commands up to elif statement  
elif condition2
then
        condition2 is zero (true - 0)
        execute all commands up to elif statement          
else
        None of the above condtion,condtion1,condtion2 are true (i.e. 
        all of the above nonzero or false)
        execute all commands up to fi
fi

************************************************************** 

for { variable name } in { list }
do
        execute one for each item in the list until the list is
        not finished (And repeat all statement between do and done)
done

**************************************************************  

for (( expr1; expr2; expr3 ))

do
        .....
 .....
        repeat all statements between do and 
        done until expr2 is TRUE
Done

************************************************************** 

while [ condition ]
do
       command1
       command2
       command3
       ..
       ....
done

************************************************************** 
case  $variable-name  in
      pattern1)   command
                  ...
                  ..
                  command;;
      pattern2)   command
                  ...
                  ..
                  command;;
      patternN)   command
                  ...
                  ..
                  command;;
      *)          command
                  ...
                  ..
                  command;;
esac


 
 
************************************************************** 
function-name ( )
{
    command1
    command2
    .....
    ...
    commandN
    return
} 
 
************************************************************** 
 
**************************************************************

Example 1:

#!/bin/bash

# ask user the enter a value
echo "Enter an order number: [Enter]"
read orderNumber

#postgresql command line to get the information for the entered order number
command=`psql -d [database_name] -U [user_name] -At << EOF
                   select * from [table_name] where [column_name]='$orderNumber';
                   EOF`
#print of the output
echo $commands
#if string $command is NOT NULL and does exit
if [ -n $command ]
then
#command1 if condition is true or if exit status of condition is 0 (zero)...
.....
#end of if condition
fi
#end the shell script
exit 0

**************************************************************

Example 2:

(a) aScript.sh
#!/bin/bash
#if there is no environment path for $APP_HOME
if [-z $APP_HOME]; then       #if two command line in a line, it need to be seperated by semicolon
     echo "ERROR MESSAGE: ....";
     exit 1;
fi

#remove an existed file
if [-e "[file_name]"]
then
   rm [file_name]
fi

#To remove the trailing slash
APP_HOME=${APP_HOME%/}  ;

(b) mainProgram.sh
#!/bin/bash

# run a script in background
./aScript.sh &

#set date format
NOW=`date + %d-%m-%y`
...
.....