1.)
#programme to merge two files or search for a pattern in a file
read -p "Enter 1 to merge two files and 2 to search for a pattern " choice
case $choice in
  1) echo "Two files will be merged"
  read -p "Enter the first file's name " name1
  read -p "Enter the second file's name " name2
  read -p "Enter the merged file's name " name3
  cat $name1 $name2 > $name3
  echo "the first file is "
  cat $name1
  echo "the second file is "
  cat $name2
  echo "the merged file is "
  cat $name3;;
  2) echo -e "Search for a pattern in a file \n"
  read -p "Enter the name of the file " filename
  read -p "Enter the pattern to be searched for " pattern
  grep -i -n $pattern $filename;;
  *) echo "Wrong Input";;
esac
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2.) 
#programme to check for palindrome
read -p "Enter a word " word
len=`echo $word | wc -c`
len=`expr $len - 1`
i=1
j=`expr $len / 2`
while test $i -le $j
do
  k=`echo $word | cut -c $i`
  l=`echo $word | cut -c $len`
  if test $k != $l
  then
    echo "String is not palindrome"
    exit
  fi
i=`expr $i + 1`
len=`expr $len - 1`
done
echo "String is palindrome"
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3.) 
read -p "Enter the file name " filename
read -p "Enter the starting line number " start
read -p "Enter the number of lines to be displayed " num
head -`expr $start + $num - 1` $filename | tail -$num
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4.)
echo -e "Enter 1 --> check active users\n2-->Display some lines from some file\n3-->update access time of some file\n"
read -p "Enter your choice_ " choice
case $choice in
  1) w;;
  2) read -p "Enter the file name_ " filename
  read -p "Enter the no. of lines to be displayed from top_ " number
  head -$number $filename;;
  3) read -p "Enter the file name_ " filename
  touch -a $filename;;
  *) echo "Invalid choice entered"
esac
-------------------------------------------------------------------------------------------------------------------

5.) 
read -p "Enter the number of elements to be sorted_ " n
echo "Enter the numbers i.e. array elements"
i=0
for ((i=0; i<n; i++))
do
  read arr[$i]
done
echo "The eneterd array is:"
for ((i=0; i<n; i++))
do
  echo ${arr[$i]}
done
for ((i=0; i<n-1; i++))#outer loop of selection sort
do
  max=`expr ${arr[$i]}`
  pos=`expr $i`
  for ((j=i+1; j<n; j++))#inner loop of selection sort
  do
    if test ${arr[$j]} -ge $max
    then
      max=`expr ${arr[$j]}`
      pos=`expr $j`
    fi
  done
  arr[$pos]=`expr ${arr[$i]}`
  arr[$i]=`expr $max`
done #sorting completed  
echo "The sorted array is:"
for ((i=0; i<n; i++))
do
  echo ${arr[$i]}
done
-------------------------------------------------------------------------------------------------------------------

6.) 
read -p "Enter the number of elements_ " n
echo "Enter the numbers i.e. array elements"
i=0
for ((i=0; i<n; i++))
do
  read arr[$i]
done
echo "The eneterd array is:"
for ((i=0; i<n; i++))
do
  echo ${arr[$i]}
done
max=`expr ${arr[0]}`
min=`expr ${arr[0]}`
for ((i=0; i<n; i++))#outer loop of selection sort
do
  if test ${arr[$i]} -ge $max #to update max value
    then
      max=`expr ${arr[$i]}`
  fi
  if test ${arr[$i]} -le $min #to update min value
    then
      min=`expr ${arr[$i]}`
  fi
done #min max found 
echo "The min value is: $min"
echo "The max value is: $max"
-------------------------------------------------------------------------------------------------------------------

7.) 
( bash -c ' echo $PPID ' > /tmp/myprocess.pid 
while true ;
do
  true;
  sleep 3
done ) &
#c|kill $(</tmp/myprocess.pid)
-------------------------------------------------------------------------------------------------------------------

8.) 
read -p "Enter the number of elements to be sorted_ " n
echo "Enter the numbers i.e. array elements"
i=0
for ((i=0; i<n; i++))
do
  read arr[$i]
done
echo "The eneterd array is:"
echo ${arr[*]}
for ((i=0; i<n-1; i++))#outer loop of selection sort
do
  for ((j=0; j<n-$i-1; j++))#inner loop of selection sort
  do
    if test ${arr[$j]} -ge ${arr[$j+1]}
    then
      temp=`expr ${arr[$j]}`
      arr[$j]=`expr ${arr[$j+1]}`
      arr[$j+1]=`expr $temp`
      
    fi
  done
done #sorting completed  
echo "The sorted array is:"
echo ${arr[*]}
-------------------------------------------------------------------------------------------------------------------

9.) 
read -p "Enter the number of elements to be stored_ " n
echo "Enter the numbers i.e. array elements"
i=0
for ((i=0; i<n; i++))
do
  read arr[$i]
done
echo "The array is ${arr[*]}"
read -p "Enter the position of element to be removed_ " i
i=`expr $i - 1`
unset 'arr[i]'
echo "After removal the array is ${arr[*]}"
-------------------------------------------------------------------------------------------------------------------

10.) 
echo -e "Enter 1 --> Display File\n2-->Display a particular entry\n3-->Delete a specific entry\n4-->Add a new entry\n5-->Update a specific entry\n_______________________________________\n"
read -p "Enter your choice_ " choice
filename=roll_name_city.txt
case $choice in
  1) echo -e "___________________________\nThe whole $filename is\n\n"
  cat $filename;;
  2) echo "A specific entry will be displayed"
  read -p "Enter the roll no. of the entry to be displayed_ " roll
  entry=`grep -e ^$roll $filename`
  name=`echo $entry | cut -d" " -f2`
  city=`echo $entry | cut -d" " -f3`
  echo -e "\n______________________\nRoll: $roll \nName: $name \nCity: $city \n_____________________\n";;
  3) echo "An entry will be deleted"
  read -p "Enter the roll no. of the entry to be deleted_ " roll
  grep -v ^$roll $filename > temp.txt
  cat temp.txt > $filename
  rm temp.txt
  echo "Entry for roll= $roll has been deleted";;
  4) echo "Add a new entry"
  read -p "Enter the <roll><space><name><space><city>" entry
  echo $entry >> $filename
  echo "Entry added";;
  5) echo "An entry will be updated"
  read -p "Enter the roll no. of the entry to be updated_ " roll
  line_of_roll=`grep -n ^$roll $filename | cut -d":" -f 1`
  no_of_lines=`wc -l $filename | cut -d " " -f 1`
  head -`expr $line_of_roll - 1` $filename >temp.txt
  echo "Enter the updated entry in following format"
  read -p "Enter the <roll><space><name><space><city>" entry
  echo $entry >>temp.txt
  tail -`expr $no_of_lines - $line_of_roll ` $filename >>temp.txt
  cat temp.txt > $filename
  rm temp.txt;;
  *) echo "Invalid entry";;
esac
-------------------------------------------------------------------------------------------------------------------

11.) 
read -p "Enter a number_ " n
i=0
sum=0
length=`expr $(echo $n | wc -c) - 1`
while [ $i -lt $length ]
do
  digit=`echo $n | cut -c \`expr $length - $i \``
  # echo "The digit is $digit"
  let var=$digit**$length
  sum=`expr $var + $sum`
  i=`expr $i + 1`
  # echo "The sum is $sum"
done
if [ $sum -eq $n ]
then
  echo "$n is an armstrong no."
else
  echo "$n is not an armstrong no."
fi
-------------------------------------------------------------------------------------------------------------------

12.) 
read -p "Enter a three digit no._  " n
echo "the permutations are:"

for ((i=1; i<4; i++))
do
  a=`echo $n | cut -c $i`
  # echo  "i=$i"
  for ((j=1; j<4; j++))
  do
    b=`echo $n | cut -c $j`
    # echo  "j=$j"
    for ((k=1; k<4; k++))
    do
      c=`echo $n | cut -c $k`
      if [ $i != $j ] && [ $i != $k ] && [ $j != $k ]
      then
        echo "$a$b$c"
      fi
    done
  done
done
-------------------------------------------------------------------------------------------------------------------

13.) 
read -p "Enter a word/sentence_  " str
read -p "Enter the substring to be searched for_  " sub
str_len=`expr $(echo $str | wc -c) - 1`
sub_len=`expr $(echo $sub | wc -c) - 1`
last=`expr $str_len - $sub_len + 1`
# echo "$str_len $sub_len $last"
for i in `seq 1 $last`
do
  word=$(echo $str | cut -c $i-$(expr $sub_len + $i - 1) )
  echo "for i= $i substring extracted =  $word"
  if [ $word == $sub ]
  then
    echo -e "\nmatch found. position = $i\n"
  fi
done
-------------------------------------------------------------------------------------------------------------------

14.) 
read -p "Enter a number_  " n1
read -p "Enter another number_  " n2
gcd()
{
  if [ $1 -eq 0 -o $1 -eq $2 ]
  then
    return $2
  elif [ $2 -eq 0 ]
  then
    return $1
  else
    gcd $2 `expr $1 % $2`
  fi
}
gcd $n1 $n2
echo $?
-------------------------------------------------------------------------------------------------------------------

15.) 
read -p "Enter a sentence_  " str
words=$(echo "$str" | wc -w)
chars=`echo "$str" | wc -c`
white=`echo $str | tr -c -d " \t" | wc -c`
special=`echo $str | tr -c -d "./\{}[]()@!#%^&*-+=" |wc -c`
echo -e "No. of \nwords = $words\ncharacters = $chars\nwhite spaces = $white\nspecial characters = $special"
-------------------------------------------------------------------------------------------------------------------

16.) 
echo -n "Enter a number_  "
read n
fact=1
for i in $(seq 2 $n)
do
  fact=$((fact * i)) 
done
echo "the factorial of $n = $fact"
-------------------------------------------------------------------------------------------------------------------

17.) 
echo -n "Enter the number of terms to be shown_  "
read n
a=0
b=1
for i in $(seq 1 $n)
do
  echo "$a"
  temp=$a
  a=$b
  b=$(($temp + $b))
done
-------------------------------------------------------------------------------------------------------------------

18.) 
read -p "Enter the number of elements to be sorted_ " n
echo "Enter the numbers i.e. array elements"
i=0
for ((i=0; i<n; i++))
do
  read arr[$i]
done
echo "The entered array is:"
echo ${arr[*]}
for ((i=0; i<n-1; i++))#outer loop of bubble sort
do
  for ((j=0; j<n-$i-1; j++))#inner loop of bubble sort
  do
    if test ${arr[$j]} -le ${arr[$j+1]}
    then
      temp=`expr ${arr[$j]}`
      arr[$j]=`expr ${arr[$j+1]}`
      arr[$j+1]=`expr $temp`
    fi
  done
done #sorting completed  
echo "The sorted array is:"
echo ${arr[*]}
-------------------------------------------------------------------------------------------------------------------