Question
Is it possible to set up a services or check_command with a dollar sign ($) in one of the arguments?
Answer
Yes, but some careful configuration is needed, in several steps:
- The arguments containing the $ character must be escaped with yet another $ character, so it looks like: $$
- The arguments in the check command lines must be properly quoted using single quotes.
Begin by looking at (editing) the check command used in this check. Look for the $ARGn$ macro corresponding to the argument in the service's check_command_args setting. The check command can be set up in three different ways, as outlined by the examples below.
$USER1$/check_plugin_example -H $HOSTADDRESS$ -u '$ARG1$' -d '$ARG2$'
- $ARG1$ and $ARG2$ are single quoted.
- The arguments defined in the service's check_command_args setting should be set without quoting, such as:
username!mysql$$instance
$USER1$/check_plugin_example -H $HOSTADDRESS$ -u $ARG1$ -d $ARG2$- $ARG1$ and $ARG2$ are not quoted at all.
- Add single quotes surrounding your $ARGn$ as seen in the first example.
- Set up the service's check_command_args setting as described in the first example.
$USER1$/check_plugin_example -H $HOSTADDRESS$ -u "$ARG1$" -d "$ARG2$"- $ARG1$ and $ARG2$ are double quoted.
- Change from double quotes to single quotes surrounding your $ARGn$.
- Set up the service's check_command_args setting as described in the first example.
Why $$ and single quotes, then?
When naemon generates the command line for a check, the first operation is to expand all macros. As $ is treated as a macro character, it must be type twice to end up as a single $ ? otherwise you get no dollar sign at all! This transformation occurs pre-execution of the command.
The command line to execute is then, in most cases, executed via the system's shell (such as /bin/sh). In many command line shells, $ characters marks the beginning of a variable string. Only within single quotes, shell variables are not expanded.
For an even better understanding of this issue, spawn a shell such as a bash. For instance, a simple way to do this is by logging on to your op5 Monitor server via SSH. Then run the commands below.my_var=worldecho Hello "$my_var"echo Hello '$my_var'
unset my_varecho Hello "$my_var"echo Hello '$my_var'
Do you spot the difference?
What if we use the same argument as in the check command?
echo "mssql$instance"echo 'mssql$instance'
And what if instance is a defined variable?
instance=ABC
echo "mssql$instance"echo 'mssql$instance'
As you can see, single quotes is the only safe option in this check command case.
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