#!/bin/bash
echo "type file name to show"
read fname
cat $fname
echo " choose the line in the file u need to replace word with
read tname
(suggest me from here so that i can achive my desired result)
 

#!/bin/bash
 
get_file()
{
  inp_file=""
  while ! test -f ${inp_file} || ! test "${inp_file}"
  do
    echo -n "File to read: [file.txt]: "
    read inp_file
    if [ "${inp_file}" = "" ]; then
      inp_file="./file.txt"
    fi
  done
}
 
get_line()
{
  inp_line=""
  while ! [ ${inp_line} -eq ${inp_line} >> /dev/null 2>&1 ] || ! test ${inp_line}
  do
    echo -n "Line Number to replace: "
    read inp_line
  done
}
 
test_line_nbr()
{
  local cnt=`wc -l $1 | awk '{ print $1 }'`
  if [ ${2} -le ${cnt} ] ; then
    valid_linenbr=1
  else
    echo ""
    echo "Invalid line number"
    echo ""
  fi
}
 
get_search_word()
{
  search_word=""
  while ! test ${search_word}
  do
    echo -n "Word to search for: "
    read search_word
  done
}
 
get_replace_word()
{
  replace_word=""
  while ! test ${replace_word}
  do
    echo -n "Replacement value: "
    read replace_word
  done
}
 
valid_linenbr=0
get_file
 
until [ ${valid_linenbr} -eq 1 ]
do
  get_line
  test_line_nbr ${inp_file} ${inp_line}
done
 
get_search_word
get_replace_word
 
#use sed -i to replace the file in place
#use sed "s//g" >> newfile to save the output
sed "${inp_line} s/${search_word}/${replace_word}/g" ${inp_file}

sk@sk:/tmp/d$ ls
file.txt  script.sh
sk@sk:/tmp/d$ wc -l file.txt
11 file.txt
sk@sk:/tmp/d$ ./script.sh
File to read: [file.txt]:
Line Number to replace: 999
 
Invalid line number
 
Line Number to replace: 5
Word to search for: a
Replacement value: xxxxxx
a
a
a
a
xxxxxx
a
a
a
a
a
 
sk@sk:/tmp/d$

BASH=/bin/bash
BASH_ARGC=()
BASH_ARGV=()
BASH_LINENO=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="3" [1]="00" [2]="15" [3]="1" [4]="release" [5]="i686-redhat-linux-gnu")
BASH_VERSION='3.00.15(1)-release'
COLORS=/etc/DIR_COLORS.xterm
COLUMNS=80
DIRSTACK=()
EUID=0
GROUPS=()
G_BROKEN_FILENAMES=1
HISTFILE=/root/.bash_history
HISTFILESIZE=1000
HISTSIZE=1000
HOME=/root
HOSTNAME=LAB-A
HOSTTYPE=i686
IFS=$' \t\n'
INPUTRC=/etc/inputrc
KDEDIR=/usr
LANG=en_US.UTF-8
LESSOPEN='|/usr/bin/lesspipe.sh %s'
LINES=24
LOGNAME=root
LS_COLORS='no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lzh=00;31:*.zip=00;31:*.z=00;31:*.Z=00;31:*.gz=00;31:*.bz2=00;31:*.bz=00;31:*.tz=00;31:*.rpm=00;31:*.cpio=00;31:*.jpg=00;35:*.gif=00;35:*.bmp=00;35:*.xbm=00;35:*.xpm=00;35:*.png=00;35:*.tif=00;35:'
MACHTYPE=i686-redhat-linux-gnu
MAIL=/var/spool/mail/root
MAILCHECK=60
OPTERR=1
OPTIND=1
OSTYPE=linux-gnu
PATH=/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/root/bin
PIPESTATUS=([0]="0")
PPID=21897
PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME%%.*}:${PWD/#$HOME/~}\007"'
PS1='[\u@\h \W]\$ '
PS2='> '
PS4='+ '
PWD=/root
QTDIR=/usr/lib/qt-3.3
SHELL=/bin/bash
SHELLOPTS=braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor
SHLVL=1
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
SSH_CLIENT='::ffff:10.180.18.199 1109 22'
SSH_CONNECTION='::ffff:10.180.18.199 1109 ::ffff:10.180.18.210 22'
SSH_TTY=/dev/pts/1
SUPPORTED=en_US.UTF-8:en_US:en
TERM=xterm
UID=0
USER=root
_=USERNAME
mpi_selection=
mpi_selector_dir=/var/lib/mpi-selector/data
mpi_selector_homefile=/root/.mpi-selector
mpi_selector_sysfile=/etc/sysconfig/mpi-selector

get_line()
 
{
  inp_line=""
#   while ! [ ${inp_line} -eq ${inp_line} >> /dev/null 2>&1 || ! test ${inp_line}
   # do
     echo -n "Line Number to replace: "
      read inp_line
      #done
}