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I would like to run few commands such as stop or start web server as a root user. How do I allow a normal user to run these commands as root?

You need to use sudo command which is use to execute a command as another user. It allows a permitted user to execute a command as the superuser or another user, as specified in the /etc/sudoers (config that defines or list of who can run what) file. i.e. the sudo command allows users to do tasks on a Linux system as another user.

sudo is more more secure then su command. By default it logs sudo usage, command and arguments in /var/log/secure (Red Hat/Fedora / CentOS Linux) or /var/log/auth.log (Ubuntu / Debian Linux).

If the invoking user is root or if the target user is the same as the invoking user, no password is required. Otherwise, sudo requires that users authenticate themselves with a password by default (NOTE: in the default configuration this is the user's password, not the root password). Once a user has been authenticated, a timestamp is updated and the user may then use sudo without a password for a short period of time (15 minutes unless overridden in sudoers).
/etc/sudoers Syntax

Following is general syntax used by /etc/sudoers file:
USER HOSTNAME=COMMAND
Where,

* USER: Name of normal user
* HOSTNAME: Where command is allowed to run. It is the hostname of the system where this rule applies. sudo is designed so you can use one sudoers file on all of your systems. This space allows you to set per-host rules.
* COMMAND: A simple filename allows the user to run the command with any arguments he/she wishes. However, you may also specify command line arguments (including wildcards). Alternately, you can specify "" to indicate that the command may only be run without command line arguments.

How do I use sudo?

For example, you want to give user rokcy access to halt/shutdown command and restart apache web server.
1) Login as root user

2) Use visudo command edit to edit the config file:
# visudo
3) Append the following lines to file:
rokcy localhost=/sbin/halt
rokcy dbserver=/etc/init.d/apache-perl restart
4) Save the file and exit to shell prompt.
5) Now rokcy user can restart apache server by typing the following command:
$ sudo /etc/init.d/apache-perl restart
Output:

Password:
Restarting apache-perl 1.3 web server....

The sudo command has logged the attempt to the log file /var/log/secure or /var/log/auth.log file:
# tail -f /var/log/auth.log
Output:

May 13 08:37:43 debian sudo: rokcy : TTY=pts/4 ; PWD=/home/rokcy ; USER=root ; COMMAND=/etc/init.d/apache-perl restart

If rokcy want to shutdown computer he needs to type command:
$ sudo /sbin/halt
Output:

Password:

Before running a command with sudo, users usually supply their password. Once authenticated, and if the /etc/sudoers configuration file permits the user access, then the command is run. sudo logs each command run and in some cases has completely supplanted the superuser login for administrative tasks.
More examples

a) Specify multiple commands for user jadmin:
jadmin ALL=/sbin/halt, /bin/kill, /etc/init.d/httpd
b) Allow user jadmin to run /sbin/halt without any password i.e. as root without authenticating himself:
jadmin ALL= NOPASSWD: /sbin/halt
c) Allow user charvi to run any command from /usr/bin directory on the system devl02:
charvi devl02 = /usr/bin/*

REFERENCE
http://www.cyberciti.biz/tips/allow-a-normal-user-to-run-commands-as-root.html