Shell Scripting Tutorial for Beginners 8 - Logical 'AND' Operator

#! /bin/bash

age=60

# for using And operator use &&
if [ "$age" -gt 18] && ["$age" -lt 30 ]
then
  echo "valid age"
  else
  echo "age not valid"
fi

#  The -a option provide
# an alternative compound condition test.
if [ "$age" -gt 18 -a "$age" -lt 30 ]
then
  echo "valid age"
  else
  echo "age not valid"
fi

# if [[ $condition1 && $condition2 ]]    # Also works.
if [[ "$age" -gt 18 && "$age" -lt 30 ]]
then
  echo "valid age"
  else
  echo "age not valid"
fi

Shell Scripting Tutorial for Beginners 7 - How to append output to the e...

#! /bin/bash

echo -e "Enter the name of the file : \c"
read file_name

if [ -f $file_name ]
then
      if [ -w $file_name ]
      then
         echo "Type some text data. To quit press ctrl+d."
         cat >>  $file_name
      else
          echo "The file do not have write permissions"
      fi
 else   
  echo "$file_name not exists"
fi

Shell Scripting Tutorial for Beginners 6 - File test operators

#! /bin/bash

echo -e "Enter the name of the file : \c"
read file_name

if [ -s $file_name ]
then
  echo "$file_name not empty"
 else   
  echo "$file_name empty"
fi

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Shell Scripting Tutorial for Beginners 5 - If Statement ( If then , If t...

Bash Shell

Conditional Statements 

• Conditionals let us decide whether to perform an action or not, this decision is taken by evaluating an expression. The most basic form is:

      if [ expression ];

      then

            statements

      elif [ expression ];

      then

            statements

      else

            statements

      fi

• the elif (else if) and else sections are optional

• Put spaces after [ and before ], and around the operators and operands. 

Bash Shell

Expressions 

• An expression can be: String comparison, Numeric comparison, File operators and Logical operators and it is represented by [expression]:

• String Comparisons: 

=  compare if two strings are equal

!=  compare if two strings are not equal

-n  evaluate if string length is greater than zero

-z  evaluate if string length is equal to zero

• Examples:

[ s1 = s2 ]  (true if s1 same as s2, else false)

[ s1 != s2 ]  (true if s1 not same as s2, else false)

[ s1 ]   (true if s1 is not empty, else false)

[ -n s1 ]   (true if s1 has a length greater then 0, else false)

[ -z s2 ]   (true if s2 has a length of 0, otherwise false)

Bash Shell

Expressions 

• Number Comparisons:

-eq compare if two numbers are equal

-ge         compare if one number is greater than or equal to a number

-le  compare if one number is less than or equal to a number

-ne  compare if two numbers are not equal

-gt  compare if one number is greater than another number

-lt  compare if one number is less than another number

• Examples:

[ n1 -eq n2 ]  (true if n1 same as n2, else false)

[ n1 -ge n2 ]  (true if n1greater then or equal to n2, else false)

[ n1 -le n2 ]  (true if n1 less then or equal to n2, else false)

[ n1 -ne n2 ]  (true if n1 is not same as n2, else false)

[ n1 -gt n2 ]  (true if n1 greater then n2, else false)

[ n1 -lt n2 ]  (true if n1 less then n2, else false)

Bash Shell

Examples 

$ cat user.sh

 #!/bin/bash

      echo -n “Enter your login name: "

      read name

      if [ “$name” = “$USER” ];

      then

            echo “Hello, $name. How are you today ?”

      else

            echo “You are not $USER, so who are you ?”

      fi

$ cat number.sh

#!/bin/bash

      echo -n “Enter a number 1 < x < 10: "

      read num

      if [ “$num” -lt 10 ]; then

            if [ “$num” -gt 1 ]; then

                  echo “$num*$num=$(($num*$num))”

            else

                  echo “Wrong insertion !”

            fi

      else

            echo “Wrong insertion !”

      fi

#! /bin/bash

word=a

if  [[ $word == "b" ]]
then
  echo "condition b is true"
elif [[ $word == "a" ]]
then 
  echo "condition a is true" 
else
  echo "condition is false"    
fi

Shell Scripting Tutorial for Beginners 4 - Pass Arguments to a Bash-Script

#! /bin/bash

# $* Returns a single string (``$1, $2 ... $n'') 
# comprising all of the positional parameters
# separated by the internal field separator character 
#(defined by the IFS environment variable).
# $0 Refers to the name of the script itself
echo $0 $1 $2 $3  ' > echo $1 $2 $3'

# $@ Returns a sequence of strings 
# (``$1'', ``$2'', ... ``$n'')
# wherein each positional parameter 
# remains separate from the others.
args=("$@")

echo ${args[0]} ${args[1]} ${args[2]} 

echo $@

# $# Refers to the number of arguments 
# specified on a command line.
echo $#

output:

test@test$ ./hello.sh Mark Tom John
./hello.sh Mark Tom John  > echo $1 $2 $3
Mark Tom John
Mark Tom John
3

Shell Scripting Tutorial for Beginners 3 - Read User Input

#! /bin/bash
 
echo "Enter name : "
read name
echo "Enterd name : $name"
 
# Multiple inputs
echo "Enter names : "
read name1 name2 name3
echo "Names : $name1 , $name2, $name3"
 
# Two commonly used options however are 
# -p which allows you to specify a prompt
# -s which makes the input silent.
read -p 'username : ' user_var
read -sp 'password : ' pass_var
echo
echo "username : $user_var"
echo "password : $pass_var"
 
# -a makes read command to read into an array
echo "Enter name : "
read -a names
echo "Names : ${names[0]}, ${names[1]}"
 
# read command will now store the reply into the default build-in variable $REPLY
echo "Enter name : "
read 
echo "Name : $REPLY"