visudo command

I am a new CentOS Linux system adminstrator. How do I use sudo command without a password on a CentOS Linux systems? I log in as marlena@my-cloud-server-ip and disabled root login for ssh. After login, I need to run some commands as root user. I am the only sysadmin using my CentOS Linux 7 server. How do I run or execute sudo command without a password for a user named marlena under CentOS Linux cloud server?
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I‘m a new Unix system user. How do I use sudo command without a password on a Linux or Unix-like systems? I log in as tom@my-cloud-server-ip and disabled root login for ssh. After login, I need to run some commands as root user. I am the only sysadmin using my server. How do I run or execute sudo command without a password for a user named Tom under Debian/Ubuntu/CentOS Linux cloud server?
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I use sudo to provide permissions and command execution rights to other users under UNIX and Linux operating systems. However, I noticed something strange about sudo. When executing sudo, it will prompt for the user’s password the first time, but sudo no longer requires a password anymore when re-executing it. Why? Is this is a security risk?
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