commandToExecute not work as linux shell

I try to do a simple things with CustomScriptForLiunx.
Step 1 :install squid
Step 2 : start squid service
so I use this command in CustomScriptForlinux
$settings='{"commandToExecute":"yum install squid -y;service squid start"}'
after I run the powershell command
Set-AzureRmVMExtension -ResourceGroupName -VMName vmsquid -Publisher "Microsoft.OSTCExtensions" -TypeHandlerVersion "1.5" -ExtensionType "CustomScriptForLinux" -SettingString $settings -DisableAutoUpgradeMinorVersion -Name SSHExtension -Location "China North"

I didn't get an output a my expected:The squid installed and started.
I got a message as below:
----------------------------------------------------------------------------
---stdout---
Loaded plugins: fastestmirror, security
Setting up Install Process
Loading mirror speeds from cached hostfile
**No package squid;service available.**
**No package start available.**
Resolving Dependencies
--> Running transaction check
---> Package squid.x86_64 7:3.1.23-16.el6_8.6 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
-----------------------------------------------------------------
It work like the second command "service squid start" became a part of the first command "yum"
I also try to use command "yum install squid -y && service squid start" and yum install squid -y |service squid start"
But I got the same problem.
My VM is CentOS 6.6

Start at the CustomScriptForLinux command execution entry point and reproduce the CentOS 6.6 settings with the reported yum and service commands. Check how commandToExecute is passed to the shell and verify completion by confirming that squid installs and starts without treating the second command as a package name.

Written by the indexing model from the issue text.