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Title       : 	Check if passwordless sudo can be used in a bash script or nagios check
Author      : 	Remy van Elst
Date        : 	30-01-2014
URL         : 	https://raymii.org/s/articles/Check_if_passwordless_sudo_can_be_used_in_a_bash_script_or_nagios_check.html
Format      : 	Markdown/HTML
This is a simple trick to see if you can use passwordless sudo in a script. This
for example can be usefull in a Nagios plugin which requires sudo. Instead of
putting the sudo line in your README and otherwise having a NRPE Unable to parse
result error, you could just give a nice warning message plus the right sudo
configuration rule.

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The example below comes from a Nagios plugin which checks if an [OSSEC][2]
server has disconnected agents. The nagios user should have a special exception
in `/etc/sudoers` to allow calling the ossec command with elevated privileges.
If the sudo is not successful it gives a nice error plus the required config to
add to `/etc/sudoers`:
    AGENTS="$(sudo -n /var/ossec/bin/list_agents -n 2>&1)"
    if [[ ${?} != "0" ]]; then
        echo "UNKNOWN: Unable to execute list_agents. Is sudo configured?"
        echo "Add the following to /etc/sudoers USING VISUDO!:"
        echo -e "$(whoami)\tALL=NOPASSWD:\t${DIRECTORY}/bin/list_agents -n"
        exit 3
Instead of seeing a "Unable to parse output" error in Nagios we get a nice
UNKNOWN warning actually telling us what's wrong, like so:
    # sudo -u nagios  bash /etc/nagios-plugins/ossec-agents.sh
    UNKNOWN: Unable to execute list_agents. Is sudo configured?
    Add the following to /etc/sudoers USING VISUDO!:
    nagios  ALL=NOPASSWD:  /var/ossec/bin/list_agents -n
The trick is using the `-n` / non-interactive option with sudo. The [man
page][3] tells us the following:
    -n' The -n (non-interactive) option prevents sudo from prompting the user for a password. If a password is required for the command to run, sudo will display an error message and exit. 
Which is perfect to test passwordless login instead of letting it just fail.
   [1]: https://www.digitalocean.com/?refcode=7435ae6b8212
   [2]: http://www.ossec.net/
   [3]: http://linux.die.net/man/8/sudo
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