Some Linux System Commands

Kernal Parameters:

Kernel control parameters exist in  etc/sysctl.conf 

For changing any parameter or appending, after modifying the file for making it effected immediately:

/sbin/sysctl -p

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List of installed packages:  rpm -qa

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To get the RAM info: free -m

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To get the Hard drive info: df -h

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User Limits are defined in : /etc/security/limits.conf

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Making system Secured, Security Level config file: /etc/selinux/config