Given a service name, get its port number?

 To get the port number of a service by its name in a Unix/Linux environment, you can use various methods and commands. Here are some common approaches:


1. **Using `getent` and `/etc/services`**:

   You can use the `getent` command along with the `/etc/services` file to look up the port number of a service. For example, to find the port number for the SSH service (OpenSSH), you can run:


   ```bash

   getent services ssh

   ```


   This will return the port number associated with the SSH service, such as `22/tcp`.


2. **Using `nc` (netcat)**:

   You can use the `nc` (netcat) command to attempt a connection to the service by name, and it will display the port if successful. For example:


   ```bash

   nc -z -v -n -w 1 localhost 22

   ```


   This will attempt to connect to the SSH service (port 22) on the localhost, and if successful, it will display the port number.


3. **Using `ss` (Socket Statistics)**:

   The `ss` command can provide information about active sockets, which can include the service name and port number. For example, to list all listening sockets and their associated services:


   ```bash

   ss -tuln

   ```


   This will display a list of listening TCP and UDP sockets along with their associated services and port numbers.


4. **Using `netstat`**:

   The `netstat` command, although deprecated on some systems, can still be used to find the port associated with a service name. For example:


   ```bash

   netstat -nlt | grep <service_name>

   ```


   Replace `<service_name>` with the name of the service you want to find.


5. **Using `nmap`**:

   The `nmap` tool is commonly used for network discovery and security scanning. You can use it to discover the port number associated with a service on a remote host:


   ```bash

   nmap -p <service_name> <hostname_or_ip>

   ```


   Replace `<service_name>` with the service name and `<hostname_or_ip>` with the target host.


Note that the availability and behavior of these commands may vary depending on your Linux distribution and version. Additionally, some services may use non-standard port numbers or be defined in custom configuration files, so it's important to verify the information in your specific environment.

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