Shell scripting tutorials

Doing an Arithmetic :

i=$((  i * 5 ))

Single Quotes:
Ignoring special Characters : echo ‘*’ [ anything inside single quotes is ignored ]

Double Quotes:
Same as single quotes but not restrictive.

The BackSlash:
Equivalent to single quotes around a single character. Backslash quotes the single character

$(…) Construct :
$(command) is same as command and is better option.

eg. echo Date is :$(date)

\ : use backslash \ to continue lines

**$# variable : **
$# gets set to the number of arguments that were typed:

$ variable :*
$* variable references all the arguments passed to the program.

${n} :
Arguments greater than 9 cannot be obtained by $10, so you need to get it by ${10}.

Shift Command :

Shift command allows to effectively left shift positional parameters. When you execute shift , whatever was assigned to $2 will be assigned to $1, $3 is assigned to $2 and the value of $1 will be lost.

eg. Usage :

echo $# $*<br></br>
shift<br></br>
echo $# $*<br></br>
shift```

**Exit Status:**  
 Exit status of a command is obtained by $? variable . 0 implies success and 1 implies failed.

**Test Command:**  
 Test command is used to test one or more conditions in an if command. Test evaluates expression and if result is true it returns an exit status of 0 and non zero if result is false.  
 syntax : test expression.

**String Operators:**  

string1 = string2 : string 1 is identical to string 2


string1 != string2 : string 1 is not identical to string2


string : string is not null


-n string : string is not null


-z string : string is null```

Integer Operators:

 x -eq y : x is equal to y<br></br>
x -ge y : x is greater than or equal to y<br></br>
x -gt y : x is greater than y<br></br>
x -le y : x is less than or equal to y<br></br>
x -lt y : x is less than y<br></br>
x -ne y : x is not equal to y```

**File Operators : **  

-d file : file is a directory


-e file : file exists


-f file : file is an ordinary file


-r file : file is readable by the process


-s file : file has a nonzero length


-w file : file is writable by the process


-x file : file is executable


-L file : file is a symbolic link```

Logical Operators :

Logical negation Operator : ! : can be placed in front of any other test expression.
Logical And Operator : -a
Parantheses: used to alter the order of evaluation.
Logical OR operator : -o

**IF Else : **

 if command<br></br>
then<br></br>
command<br></br>
else<br></br>
command<br></br>
fi```

ELIF:  

if command


then


command


command


elif command


command


command


fi```

Case Command:

case value in<br></br>
pattern1)<br></br>
commands..<br></br>
;;<br></br>
pattern2)<br></br>
commands..<br></br>
;;<br></br>
pattern3 | pattern4 )<br></br>
commands..<br></br>
;;<br></br>
esac```

**Exit Command :** Exit enables to immediately terminate execution of program:  
 Syntax: exit n  
 Where n is the exit status that is to be returned.

**  
 Tips and Tricks : **  
**  
 Checking if argument is passed : **  

if [ "$#" -ne 1 ]


then


echo "Usage: usage message"


fi```

..
**
Using -x for debugging:**
sh -x script.sh

**The Null Command : **
:
if expression
then
:
else
…
fi

**
The && and || Constructs:**

command1 && command2 : Execute command2 only if command1 is successful.
eg.
if [ -z "$var" ] && var = "value"

command1 || command2 : Command 2 is executed even if command1 fails.
eg.

 if test || test2<br></br>
then<br></br>
ok<br></br>
fi```

if test is successful ok is executed otherwise test2 is executed.

**Check if a variable is set :**

if [[ -z "$1" ]]