mjhjmtu

I am trying to learn bash scripting. I am working on a practical problem and at one point I need to add a line to a file which requires root permission to write.

The code looks like this:

# some code
echo "add this line to the code" >> fileName
# some code

Is it possible to somehow make the script ask for the root password, validate the password, and on successful authentication modify the file? The script should then return to the user mode and continue the command execution.

There's a tip in the sudo man page which explains how to do something like this. Here's my one-liner:

#!/usr/bin/bash
sudo sh -c "echo "add this line to the code" >> fileName"

Obviously, you'll first have to set up your user to have sudo privileges. The sh shell is used because of the redirection to the root-owned file. I also had to escape the quotes used for the echo command.

su is available on most unix systems and should work:

su root -c 'echo "add this line to the code" >> fileName'

You could use tee with sudo:

echo "add this line to the code" | sudo tee -a filename > /dev/null

echo's output is redirected with | (pipe) to sudo tee.
tee reads from standard input and writes to standard output any given file, in this case filename. -a (or --append) makes tee append to files, without it the files would be overwritten.
As tee is run with sudo it opens files with root-permissions. Finally, > /dev/null suppresses tee's output to standard output.

One advantage of using tee instead of just starting the whole command including redirection with su -c or sudo sh -c is, that you do not have to change the quoting of the initial command in any way (Quoting lines already containing quotes can get quite ugly at times).

Try this
This command is available on Unix and Linux.

sudo sh -c "echo 'add this line to the code' >> fileName"

Would do the trick:

ssh host "sudo su root -c 'echo "add this line to the code" >> /etc/hosts'"