Web Technology Experts Notes

Monday, 28 August 2017

Unix Shell Scripting Tutorial - page 3

Question: What are special variable in unix?
Special variable are those variable which are reserved by unix shell. For Example $$ is special variable and it return the current process id.

Question: How to use array variable in for loop?
Yes, we can define.

NUMS="1 2 3 4 5 6 7 8 9"
for NUM in $NUMS
do
   Q=`expr $NUM % 2`
   if [ $Q -eq 0 ]
   then
      echo "Number is an even number!!"
      continue
   fi
   echo "Found odd number"
done

Question: Give example of while with break?

a=2
while [ $a -lt 10 ]
do
   echo $a
   if [ $a -eq 5 ]
   then
      break
   fi
   a=`expr $a + 1`
done

Once $a will reach to 5, then it will break the statement.
output

Question: How to print the special character as string?
Use the Backslash before the special character, It will print. For Example:

echo \$;

Question: What is Output Redirection?
We use > notation to Output Redirection.

who > users

If a command has its output redirected to a file and the file already contains some data, that data will be lost.

We can use >> to append in file, it will not lost the data.

who >> users

Question: What are the Redirection Commands?

pgm > file: Output of pgm is redirected to file.
pgm < file: Program pgm reads its input from file.
pgm >> file: Output of pgm is appended to file.
n > file: Output from stream with descriptor n redirected to file
n >> file: Output from stream with descriptor n appended to file
n >& m: Merges output from stream n with stream m.

Question: How to create a custom function?

# Create a new function with name of Hello
Hello () {
   echo "Hello World! How are you?"
}

# Call the function
Hello

Question: How to create a custom function with parameter?

# Define your function here
Hello () {
   echo "Hello World! How are you?"
   echo "First parameter: $1";
   echo "Second parameter: $2";    
}

# Invoke your function
Hello first second

Output

Hello World! How are you?
First parameter: first
Second parameter: second

Thursday, 24 August 2017

Unix Shell Scripting Tutorial - page 2

$$: Give the current process ID.
$0: File name of current script.
$n: $1 will give first argument, $2 will give 2nd argument.
$#: The number of arguments supplied to a script.
$*: Return all the arguments.
$?: The exit status of the last command executed.
$!: Process number of last command

Question: Can we define array variables?
Yes, we can define.

NAME01="Name 1"
NAME02="Name 2"
NAME03="Name 3"
NAME04="Name 4"
NAME05="Name 5"

Question: How to define array variable and use them?
Define Array variable

NAME01="Name 1"
NAME02="Name 2"
NAME03="Name 3"
NAME04="Name 4"
NAME05="Name 5"

Use single variable

echo $NAME[2];

Print each and every variable

for i in NAME01 NAME02 NAME03 NAME04 NAME05
do
  echo $i;
done

Question: How to print array in single line?

echo  ${NAME[*]};

Question: How to use Relational Operators?

-eq: Equal
-ne: Not Equal
-gt: Greater than
-lt: Less than
-ge: Greater than OR Equal to
-le: Less than OR Equal to

Question: How to use Boolean Operators?

!: Not
-o: OR
-a: And

Question: Give an example of while?

a=1
while [ $a -lt 10 ]
do
   echo $a
   if [ $a -eq 5 ]
   then
      break
   fi
   a=`expr $a + 1`
done

Question: What is Substitution?
The shell performs substitution when it encounters an expression that contains one or more special characters.

Question: Give few example of Variable Substitution?
Variable substitution enables the shell programmer to manipulate the value of a variable based on its state.
Examples

${var}

${var:-word}

If var is null, word is substituted for var.

${var:=word}

If var is null, var is set to the value of word.

${var:?message}

If var is null, message is printed to standard error.

${var:+word}

If var is set, word is substituted for var.

Question: What are the Metacharacters?
metacharacters have special meaning while using them in any Shell Script and causes termination of a word unless quoted.

Following are the meta characters.