Replacing a string with a given text using Linux shell script

Question

I have a file named testfile.It contains some information like

methun:x:500:500:comment:/home/methun:bin/bash
salahuddin:x:501:500:comment:/home/methun:bin/bash

Now implemented a following shell program:

 echo "Enter a Name:"
   read username
   users='cat /mypractice/myfiles/testfile | awk -F ':' '{print $1}''
   for user in $users
    do
      if [ "$user" == "$username" ]; then
        echo "Name found and Enter a new name to change."
        read newUsername
          #need code to change text on my file --->testfile
      fi
    done

Now suppose I need to change methun to Moin. Comment to newcomment. I used

sed -i 's/"$user"/"$newuser"/g' /mypractice/myfiles/testfile

But it not working here. I test with it in my testfile singly it change and replace all.But i need to change only that position i want .

I also tried with usermod but it will not works here..

Can anyone give me the solution or correct my code...Thanks

jaypal singh · Accepted Answer · 2014-04-07 05:31:41Z

1

You are using g flag in your sed command which means global substitution (will change all occurrences). Also, the variables although quoted are wrapped inside single quotes and hence are not interpolated.

Try this:

sed -i "s/^$user/$newuser/" /mypractice/myfiles/testfile

I have placed a ^ anchor in the substitution part which means only substitute if the word is at the beginning of the line. This protects you from making a change if the name of the user is not at the start but somewhere in the middle.

answered Apr 7, 2014 at 5:31

jaypal singh

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3 Comments

user1574726 Over a year ago

But i want to change only the name that match on the if condition.

jaypal singh Over a year ago

@user3464541 Right, just put this sed under your if condition. So if the condition returns true, you can make the change.

user1574726 Over a year ago

suppose i have methun, methun1, methun2, methun3. want to replace methun with jon. When i input methun and want to change name with jon then i found the result like jon jon1 jon2 jon3 But my expected result is like jon methun1 methun2 methun3.

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